USS Pampanito 20mm Twin Gun Mount Restoration

gun mount on the pier
20 Apr 2021, Gun mount as removed from the boat, before restoration

NOTE: These are actual working notes as well as project documentation so there will be changes. There is no shame in skipping to the acknowledgments and photos near the end. There are probably omissions & errors to fix in this note, please let us know any problems you find.


USS Pampanito is a WW II submarine museum and memorial on Fisherman's Wharf in San Francisco. The boat is owned and operated by a non-profit and receives no government operating funds. Our preservation goal is to make the submarine as complete and accurate to our summer 1945 restoration date as possible. This note is a description of the project to overhaul a twin 20mm Oerlikon gun mount, and a heartfelt thank you note to the individuals and businesses that will help make it possible.

On 2 Aug 1945 Pampanito had a twin 20mm Oerlikon gun forward on the foundation that previously held a 4"-50 cal gun (and elsewhere a 5" 25 caliber wet mount gun, two Bofors 40-mm, x4 .50 cal. with 3 stowed in lockers on deck, and small arms stowed inside the boat.) Pampanito was donated to our non-profit by the Navy in 1978 with no guns. Over the years the museum has acquired replacements for all the large caliber weapons.

During the early 1980s, the museum acquired a single 20mm Mount Mk 10 from USS Silversides, an incomplete Mk 4 mod 0 gun from a federal agency, shoulder rests and a magazine from Jeremiah O'Brien, and a sight from a private collector. Together they were assembled into a single 20mm Mount Mk 10 that was displayed for many years. A second Mk 10 single mount without a gun was acquired during the same period. A second Mk 4 gun was acquired sometime in the early 1990s. Finally a third Mk 4 mod 1 gun without a barrel was acquired in the 2000s. The single mount was displayed forward on the conning tower with occasional removal of the gun for paint. The museum then traded with PT Boats Inc. the extra single mount that was then in storage without a gun for a twin 20mm mount without guns. Neither traded mount had any of the locking, depression stop, sights, shoulder rest, firing, cart. bag, belt, or cocking bar parts. Sometime after the twin mount arrived, the twin carriage and cradle was swapped with the single carriage and cradle on the boat and the two incomplete guns installed. When leaving for Pampanito's 2021 drydocking this mount was removed from the boat for restoration and eventual return to the correct location on main deck forward. As found in 2021 both guns were missing many parts, both magazines were rusty, one gun was set up as a single (Mod 0), the other has Mod 0 markings but with a Mod 1 (twin type) breech casing cap. The mount was missing many parts. Neither the mount or the guns had ever been properly restored.

The goal is restore a Mk 24 twin mount that is as complete and accurate as we can while preserving as much of the historic fabric as possible. The gun mechanisms are demilitarized (unable to load a cartridge in chamber, fire, or pass a projectile through the barrel). We are not planning to simulate firing.

Restoration will follow the U.S. Secretary of the Interior's Standards for Historic Vessel Preservation Projects wherever possible.


Gun Mount Mk 24 Mod 2 is a Mount Mk 10 modified to have two Mk 4 Mod 1 guns, simple site Mk 4, and an improved (quite different) cam depression stop mechanism. Mk 24 Mod 5 has other small changes in the cradle, carriage and stand. This includes omission of the sheave for cocking tackle and ball bearings in stand.

Catalog Data:
Mk 24 twin mount (our goal)
Mk 10 single mount (for comparison)

Gun Mount Mark 24 Mod 5 consists of:
Machine gun mechanism Mark 4 Mod 1 (x2)
Gun Barrel - Mark 4 or Mark 4 Mod 1 (x2)
Sight Mk 4 Mod 1
Magazine Mark 4 Right hand (same as single)
Magazine Mark 5 Mod 0 Left hand (grey zig-zag stripe)
Shoulder rests and handle bars Mark 8 Mod 0
Stand Mk 7 Mod 23
Carriage Mk 18 Mod 0
Cradle Mk 14 Mod 0

Submarine mounts do not have shields, or Mk 14 sight or sight adapter equipment Mk 2.

Mount assembled weight without shields ~1,000 lbs
Gun with barrel and mechanism (without magazine) 150 lbs
Barrel alone 46 lbs
Sight 13 lbs
Loaded with 60 rounds a magazine is 63 lbs, empty 31 lbs.

References to NOP are the Naval Ordnance Plant. The NOP in Center Line, Mich. operated by Westinghouse during the war made many of the 20mm guns. The Pontiac Motors division of General Motors also produced many guns. We do not know what manufacture used MOM shortcut, but it is on 3 of our magazines.


Ordnance Pamphlet No 1439 Twin mount
Ordnance Pamphlet No 911 Gun Mechanism, and very important changes and circular notices op-911-changes.pdf (2.1 MB)
Ordnance Pamphlet No 909 Single Mount
Ordnance Pamphlet No. 826, 20MM Antiaircraft Gun-Mark 4 and Mark 2, Extracts from OP 813, 1942.

Machine Gun Bulletin No 2 - 20mm Machine Gun, 1942. These are operating instructions for surface ships, with a good list of failures at the end. 20mm-mg-bulletin-no2.pdf (6.1 MB)

We have most of the drawings for the single and twin mounts. The single drawings are from a folio book in the collection of USS COD that we borrowed and scanned in 2010, and many drawings for the twin mount came from USS NORTH CAROLINA in 2022.

Sketch 138012 20mm Assembly No 314, Gun Mark 4 Mod 1 and Mount Mk 24 Mod 5, Index to List of Drawings and General Arrangements of Assembled Units
Sketch 138004 20mm Gun Mech Mk 4 Mod 1 Index to List of Drawings

OD 2929 at the beginning of the drawings page is an OE number to/from navy part number cross reference. Also see the back of the manuals for more cross references and parts illustrations.

Favorite cutaway and assembly drawings:
Color mount general arrangement OP 1439 Plate 2
Mount General Arrangement 492472 and 492473
Cradle General Arrangement 487474
Carriage General Arrangement 492476
Stand General Arrangement 487479
Working Circle 492496
Color Gun Mechanism General Arrangement OP 911 Plate 1
Gun Mechanism General Arrangement 299825 and 299826
Magazine Mk 4 RH, 299706
Magazine Mk 5 LH, 367739 and 367740

Youtube CAD generated animation of operation, by Vbbsmyt
Live Firing at Stone Mountain 2017, by Novectaner (not our gun)
Cocking the gun, by Novectaner (not our gun)

History of procurement of 20mm guns:
The Machine Gun, George M. Chinn, BUORD, 1951:

MK 7 projectile for Oerlikon is 20mm x 110mm RB (RB is rebated base, a rim that is significantly smaller in diameter than the base of the case)
Ordnance catalog pages for 20mm projectiles
Ammunition description in OP 911 manual
British handbook for 20 MM Small Arms Ammunition, 1943. gun20mm-26-pubs-5566-ammo.pdf (3.1 MB)
B.R. 932, Handbook on Ammunition, 1945. British Navy ammunition from WW II br932.pdf (21 MB PDF)

We have collected some general hardware 12-Z-xx-xx BUORD part numbers

During war time these were considered more likely to have jammed cartridges, misfires, and explosions than a 40mm or .50 Browning. Even so they were a very successful weapon and widely used before being replaced by longer range weapons. They were more reliable than the 20mm Hispano-Suiza that was popular in aircraft (the British tried these on ships and rejected them) or the very problematic 1.1 inch.

Clearing of Live Ammunition from Guns, NAVWEPS OP 1591, 1961, describes the safe methods of clearing live ammunition from guns.


The data plates are missing from our Mk 24 mount. Our carriage casting is 492480-1, is hand stamped 2225, and does not have a cocking sheave. The trunnion height is approximately 47" and stand lower bearing is ball type, so we think this a Mk 24 Mod 5.

The information below is on the data plates from the single mount, these were painted over at some point, but we have photos. The serial number (175094) is the same on the single carriage Mk. 7 and cradle Mk. 5 found in the warehouse, and the stand found on the boat. The paint colors, stand height, etc. all match. This is how we know that sometime after the twin mount Mk 24 arrived at Pampanito the carriage and cradles were swapped on the single and twin stands:

Below as found on the boat:
20mm Stand (found with twin carriage and cradle on it)
MK ?? Mod 1 (mark is not legible, should be Mk 7 stand)
WT 325 Serial 175094
USN Ord Plant Center Line Mich.
Westinghouse Elect. & Mech. Co.

Assembly No. 86
20mm Mount
Mk 10 Mod 1
USN Ord. Plant Center Line Mich.
Westinghouse Elect. & Mech. Co.

Note the stand (single) that was on the boat is missing one of the cleats. The missing cleat blocked a deck pressure proof locker when mounted on the conning tower so it may have been removed when installed as a single during the museum era. This would have been consistent with war time practice for this type of interference.

20mm M.G. Mech. Mk. 4, Ser. No. 109987 N.O.P. Center Line (was mounted on left), has C2453 stamped on the bottom.
20mm M.G. Mech. Mk. 4, Ser. No. 219775, G.M. Corp. (was mounted on right), has 50595 stamped on bottom of breech case.
20mm Barrel Mk 4 Mod 1, 141132, B17864, OE-1013-C
20mm Barrel Mk 4 Mod 1, 348030, OE-1013-K
20mm Magazine Mk 4, right hand, stamped NC
20mm Magazine Mk 5, left hand, stamped MOM
20mm M.G. Mech. Mk. 4 Mod-1, Ser. No. 130618, N.O.P. Center Line (found in storage)
Unmarked Mk 7 stand with the single carriage and cradle mentioned above in the warehouse.
Right hand magazine, marked MOM, loaded with donated dummy cartridges for interpretation.

Found in collection (FIC) parts not assigned to the project yet:

x2 299894-4 single mount style shoulder rest clamping levers. We may use these if they fit and we do not replace them with bolts for visitor proofing. If not, they will go with the single handle/rear cap.
x1 299666-2 OE-1308 securing bolts missing spring pins
x2 299666-4 damaged spring pins for OE-1308
x1 299676-1 OE-1058 barrel spring seating ring retaining catch
x1 29966-1 OE-1316, breech cotter
x1 299680 OE-1034 magazine interlock and ejector assy
Found in the same box with the 20mm parts above was a 9-3/4, 5/8" dia, .0xx", thin spring and a very short rusted piece of a similar spring. There was also a roller from a 40mm Bofors loader and a couple of screws. These are not 20mm parts but were mixed in the same box.
x1 Mk 4 Mod 1 sight complete in box
x2 empty magazines, right hand (ABC)
x1 empty magazine, left hand with a section cut out (Cont)
x?? 20 mm empty casings, shell 20mm mk 3 mod 2 342, with fuse mk 26 NC, links probably for Hispano-Suiza (CM)
x1 Single mount in the warehouse. This mount does not have most of lock cradle locking parts, or any of the depression stop parts except an uncut cam. It has the carriage and cradle from the stand that was under the twin carriage and cradle (same serial numbers). We found cradle and carriage locking parts in collection that look like they came from the single mount, but will be used in the twin mount. We removed the last locking plunger in single for use in the twin mount.


In general the work on the stand, carriage, and cradle are the first priorities so the mount can be installed on deck of the boat and interpreted at the first crane availability for loading. The guns can be hand carried aboard and installed later if needed. (This fell apart when we had delays getting the single mount out of the warehouse. The gun mechanisms were lower priority, but they got done first.)

We are restoring the two guns that were on the boat, but using the trigger handle from the Mk 4 Mod 1 gun found in collection on one of the display guns. The goal is a complete Mk 24 twin mount on the boat. When done we will have many of the parts needed for a single mount Mk 10 with gun. If opportunities arise to acquire the missing parts or make an extra part that is being made for the twin, we will get them to facilitate a future single Mk 10 mount/gun restoration. The single would be used for shore side interpretation and would probably be restored to war patrol configuration (with double loading stop and barrel spring center).

Priorities (these may change:)
DONE- Disassembly, coatings removal, rust removal of the mount, gun mech. and magazines. Complete the list of what is missing or broken. Identify parts to be repaired, vs bought commercial vs made in our shop vs CNC.
- Repair the twin stand, then swap the carriage/cradles back to their original stands.
- The carriage lock and cradle lock are most needed to safely display on the boat while still doing maintenance and interpretation. (UCCG 11?)
DONE- Shoulder rest support brackets. (RP)
DONE- Sight mounting hardware. (BC) We still need the eyecup.
DONE- The cocking bars and clips for the mount.(BSY)
DONE- Repair for display the two magazines that were installed.(LS)
- The external trigger handle, links, etc on carriage.(USCG 11?)
- The depression cam stop, we need help with these parts. (USCG 11?)
- Cover for winter and rain
DONE- Complete two sets of barrel springs.
DONE- The barrel lock handle assemblies for both display guns.
DONE- Trigger parts missing for display guns.
DONE- Test assemble on the bench both guns.
DONE - Overhauled then loaded an extra right magazine with the donated inert ammunition for interpretation.
- It would be nice to have the shoulder rest buckle and belt for demos.
- We need to research the cartridge bag use (or not) on submarines.
- Tools as needed for stripping/assembly/interpretation.
- On the boat items. Wood foundation bed, steel coatings on foundation. Missing pressure proof locker, sound powered phone outlet, etc.

Of course it would be great to find a more complete Mk 24 twin mount to start with...


- After the guns have been removed from the cradle, care should be taken when unlocking the cradle at the five-degree or 30-degree elevation positions. The cradle springs and the weight of the cradle will cause the rear of the cradle to spring down violently. (Also why we need to fix the cradle lock.)

- The guns and magazines have strong springs that need to be released under control.

- The assembled mount is top heavy, it is designed to be bolted to deck and will easily fall over if mishandled when off the foundation.

- Gun mechanisms are demilitarized.


- For all replica parts: Decide what will be mild steel and plated vs stainless? Determine what missing parts can be replaced with available commercial. Replace concave top self locking screws with available place bolts that work on similar principle. Document any changes in heat treatment or case hardening. Some of the tolerances can be relaxed. Document carefully any changes in replicas from the original drawings.

- Create visitor proofing plan. Document any changes i.e. set screws, sub of hex cap, thread locker, replacing springs with solids, stored in collection, paint, locks, etc.

DONE- Paint and coatings have been removed. We are done with disassembly, ultrasonic cleaning of small parts, and Evapo-Rust treatment of the small rusty parts.

- Drawings we are still looking for:
133221 assembly cover mk 3 mod 0 list of drawings
138001-138003 we are curious what these are
138004 machine gun mk 4 mod 1 list of drawings (our scan is incomplete)
138005 we are curious what this is
427097 assembly cover mk 3 mod 0 general arrangement
427102 assembly cover mk 3 mod 0 general arrangement
482515 screw for cheek plates on cradle (1/4-28 1/2" self locking)
482532 plunger washer from link assembly-trigger
487475 cheek plate
492372 shoulder rest and handle bars mk 8 mod 0 general arrangement
561496 lube chart surface
561497 lube chart submarine
OD 2929 if it is after 4-15-1943 so it might include the twin mount part numbers. It will be about 25 pages so worth checking dates
OD 2930 description of marks and modifications with the twin Mk 24 variants.
The handle bars that replaced shoulder rests on single mounts.
367727 part of stand weldment

- Look for a copy of NAVORD OP 3476, and 3476 Illustrated Parts Breakdown to see if it is just an excerpt of OP 911, or something new and useful. Also OSO-IPB 5350
- Find a copy of, "Manual for Instructors of Artillery Mechanics on 20 mm (Oerlikon) Anti-aircraft Gun and Mounts, To be Used with Naval Ordnance Pamphlets 911 & 909", General Motors Corp, Pontiac Motor Division, 1944
- Find a copy of BR 274 with amendments. "Handbook and Drill for 20mm Oerlikon Machine Gun Marks II, II USN and IV USN on Marks I, IIA, IV US, V US VIIA and VIIIA Mountings."
- Find OrdAlt 2294-20MM machine gun mechanisms MK IV modification 0 & 1. Installation of Magazine Mouthpiece Guide. Is this the fairing added to our FIC gun at the base of the magazine catch lever that is not on our other two guns?
- Check out updates in additive technology for applicability to the missing small castings, forgings, and the rubber parts (TPU). Or to create molds form castings. Our first attempt at an elastomeric parts did not have enough stretch to fit on this mounting. We need to find folks with access to other materials.
DONE- 3D model in Autodesk Inventor key parts.
DONE- Hone the rough holes in the guns, cradle/handle bars and carriage. We used ball hones and they worked great. There may be a bit more fitting during reassembly.


- Swap the single and twin carriage and cradles to return everything to its original stands after the twin stand is restored.

- The Stand 487479. Its weldment 367726-2 is 33-1/8" tall, which uses bottom ring 367531, legs&spider 367529, top 367530 and 367727-1. We think this along with 47" trunnion height and lower ball bearing verifies this is a mount version Mk 24 Mod 5, see pg 23 .
* Three of five stand mounting bolt holes have been crudely cut open with a torch, including the re-enforcement underneath. We need to crop and insert the 1/2" thick steel without distorting the ring. The five 1-5/32" holes need to be accurately centered, and square with the re-enforcement under the ring to be sure the long bolts through the stand, wood bed, and foundation will fit.
* The surface of the pivot housing that the felt gasket in the carriage runs on is rough and needs to be smoothed.
* The slot in the pivot housing that fits the draw stud was modified and this causes the draw stud to be pulled in deeper than it should before engaging. We are going to leave this rather than risk welding on the pivot housing. Maybe add a shim. A photo on the PT Boat shows the problem pre-dates the trade.
DONE * The bulk of coatings on the weldment have been removed by needle gun. The cement in the upper holes of the legs have been cleared.
DONE * We looked and did not find part numbers stamped on stand.
DONE * The 365905 pivot was frozen in the 492475 pivot housing. The 299963 pivot retainer holes were not aligned with in the pivot housing and was frozen in the pivot housing. We got the pivot retainer out with a custom spanner wrench. Everything was frozen because the pivot retainer was overtightened, the upper bearing locating lugs were installed outside the locating notches and the bearings were upside down (see pg 33), there was also quite a bit of grit. The misplaced bearing lugs were crushed by the bearing retainer.
DONE * About 1" of the pivot retainer screw was left about .8" deep in the pivot housing. It looked like it had been partially drilled out. We extracted the remaining piece and used a tap to clean the threads. Whatever happened here probably explains the scoring on the pivot housing surface that the pivot housing retainer mates with.
DONE * There was concrete in cam mounting holes that has been cleared out and the threaded ones have been cleaned with a thread repair tap. We cleaned dirt out of the pivot retainer lock screw hole, and the pivot housing cap screw holes and cleaned their threads.
DONE* One of the pivot housing lock screws was drilled out off center, and too deep. The other pivot housing lock screw hole was drilled straight through the pivot housing. We have picked out the remains of the old pins in the stand weldment and cleaned the threads with a tap. However the threads are now thinner and care should be taken not to cross thread when inserting the pins. The holes in the pivot housing that accept the pins were drilled, but remains of the old pin still needed to be extracted. So pretty much every hole in the pivot housing except one grease through hole needed some form of repair.
DONE * We ground off some crude weld off the top of stand weldment that matched the weld berries that where on the bottom of the carriage.
x1 367549-1 pivot housing cap gasket
x1 299962-3 OE-3506 bearing, one side of the two is rough from melting of Babbitt metal. One of the lugs is barely hanging on after straightening. We have not decided if we are going to fix the lug and redo the Babbitt.
DONE x2 367549a-3 pivot housing lock screw. We replicated in stainless with .064" instead of .109" slot.
DONE x1 299962-2 OE-3505 pivot retainer lock screw. We replicated in stainless.
DONE x1 12-Z-9-245 1/2"-20 nut is missing on 365905-3 draw stud. (bought commercial)
DONE x1 367676-5 pivot housing draw stud lock washer (1/2" split lockwasher commercial)
DONE x0 365905-3 Draw stud. Repaired the 1/2"-20 threads.
DONE x6 12-Z-46-221 pivot housing cap bolt (5/16"-24 7/8")
DONE x6 12-Z-22-252 pivot housing cap bolt lock washers (5/16" split lock)
DONE x2 299932 oiler, zerk grease fittings on pivot housing
DONE x1 Pipe plug 12-Z-329-57 in 367549-2 cap on bottom of the pivot housing (pg 21). This is not used on submarines, but we may put one in to reduce dirt in the stand. The commercial part is hex key drive instead of slotted.

- Bolts and shafts filled with concrete in cradle and carriage:
DONE- stand cam mounting holes, cam pin holes, grease fitting holes have been cleared. The threaded cam mount holes had the threads repaired.
DONE- Cocking bar mounting holes on both sides. Mostly cleared with air hammer. Then a dental pick. Then used a drill to clear inside .687" counterbore, and hone through .500" through hole.
DONE- Trigger shaft mounting pin hole on left handle bar, trigger mounting pin holes on back of carriage. Mostly cleared with air hammer. Then dental pick. Grease fittings cleared with thread repair tap.
DONE- x3 holes cocking bar clip holes in the front of the carriage. Mostly cleared with battery hammer drill. Then dental pick, then 3/8-16 thread repair tap.
DONE- Three grease fitting holes cleared with dental pick, then thread repair tool
DONE- Splinter shield mounting hole cleared of concrete and remains of old fastenings
Frozen or broken off:
DONE- 482478 Shaft-cam limit stop idler lever, on the left, lower carriage. Was pushed in about 1/2" too far and sheared its 1/4" groove pin when found. It has been extracted.
DONE - 299969-3 OE-3526 pivot retainer lock screw cover plug. The round slot was damaged on the plug, and it was stuck in carriage. It has been extracted.
DONE- Carriage lock lever housing set screw was stuck in its hole, it has been safely extracted.
DONE- x1 hole for cocking bar clips hole in the front of the carriage had a broken screw in it. 3/8-16. It has been extracted.
DONE- Removed 482472 Cam limit rear stop rod pin assembly and its grove pin without damage.
DONE- Removed the remains of fastenings from a splinter shield on the left side of the carriage.
DONE- Removed a broken off grease fitting on the carriage lock that also held the spring for the carriage lock ball detent. No sign of the ball.

- Gun securing bolt assembly locks the gun on the cradle 299794. There was lots of errant paint and crud to remove.
DONE x2 299932 oiler (zerk) on cradle, both were missing (stock)
DONE x2 299794-5 OE-2261 pin for 299794-1 gun securing bolt was found replaced with too long and too small diameter cotter pins. (bought .154" x .812" dowel pins)
x0 299794-2 OE-2190 hole for pin to securing bolt is oversized from abuse on one of these so we may do some repair.

- Cradle Springs. The left spring was in backwards. See fig 17 pg 40. The right case cover was not installed flush. Left side cradle spring locating screw was not fully seated. This was probably paint or rust build up before last installation since it fits well now that everything has been cleaned. We made a crude version of the spring case installation tool 367543-1.
x4 299791-9 lock washers on spring cover screws 299799-7. Four of eight washers were found on the mount. (1/4" split lock washer)

- 299798-1 OE-2160 Trunnion pins are badly scored. One has damaged threads. It looks like they might have been removed or installed without relieving the weight of the cradle. We touched up the high spots with a file and chased the damage thread.
The list on page 96 for mount mk 5 shows two of 299798-1 OE-2160. This looks like what was installed. Mount mk 2, page 82 uses different l.h. pin 487511. Our trunnion pins have two holes for a spanner pin wrench not shown in the drawing for 299798-1. These holes were rusty, but close to 5/16" at 1.26" diameter. We need to consider why these might be needed with trunnions that have keys locking them to the cradle, and why bigger than on the single left trunnion.
DONE x2 367676-6 cotter pins, 1/8" x 2" (1-3/4" from stock)
DONE x2 299932 - oilers on both sides (zerk, stock)

- Carriage lock lever housing assembly, training, is missing 482490, & pg 34 drawing, Plate 1 see section D-D. The hole in the bottom of the carriage is damaged, we may need to fix this if the cradle lock has too much play during test assembly.
x1 454131 carriage lock lever housing. We made a replica, but hope to get an original from USCG 11.
x1 299968-3 OE-3519 carriage lock lever seal (rubber boot is same as carriage lock). SS JEREMIAH O'BRIEN gave us a real example, but in poor condition. We had a 3D printed replica made that is accurate in dimensions, but the material chosen is very had to stretch. We need to try heating it to see if we can fit it over the housing. ***
DONE x1 482491 carriage lock lever housing screw was frozen in the cradle and has been extracted without damage.
DONE x1 299966-3 OE-3511 carriage lock ball, from single mount (5/16" diameter)
DONE x1 299968-2 OE-3512 carriage lock ball spring (bought stainless).
DONE x1 299932 - oiler (zerk, stock)
FIC x1 299966-5 OE-3514 carriage lock cover plug. (7/8", similar to a casting plug)
FIC x1 299966-2 OE-3512 carriage lock lever (handle)
FIC x1 299966-4 OE-3515 carriage lock lever pin
FIC x2 299969-2 OE-3518 carriage lock lever spacer
FIC x1 299966-1 OE-3515 carriage lock plunger

- Cradle lock, elevation. 482489, & Plate 1 see section B-B
x1 487480 cradle lock lever housing ***
x1 299968-3 OE-3519 cradle lock lever seal (rubber boot same as on cradle). See notes for same part above. ***
DONE x1 299969-2 OE-3518 cradle lock lever spacer, two needed, only one FIC. Replica is 304 SS.
DONE x1 299943-5 cradle lock plunger, from the single mount
DONE x2 482517 carriage lock lever housing and cam limit stop cam shield screw holds housing on. 5/16-18 3/4 self locking hex head. (Using donated commercial place bolts)
DONE x1 299966-3 OE-3511 cradle lock ball (5/16" diameter, bought)
DONE x0 299968-2 OE-3512 carriage lock ball spring. Found in carriage.
DONE x1 299932 - oiler (zerk, bought)
FIC x1 299966-2 OE-3512 carriage lock lever (handle)
FIC x1 299966-4 OE-3513 cradle lock lever pivot pin

- Shoulder rests
492488 Right hand shoulder rest assembly.
492489 Left hand should rest assembly.
138008-1, 138008-2 Shoulder Rest & Handle Bars Mk 8 Mod 0 list of drawings
pg 110 of OP 1439 list of parts.
x2? 454141 shoulder rest support lever. We have a pair of single mount 299894-4 levers FIC, but I think it will need the different lengths to clear the guns in the twin. We may replace these with cap head screw for visitor proofing.
DONE x2 487489 shoulder rest frame bracket. We made replicas in 304 stainless.
DONE x2 299886-5 OE-3558 washer, 21/32" ID, 1-5/16" OD, 3/32" thick (commercial 5/8" washer)
DONE x2 482531 3/16" x 7/8" Woodruff key (#11 Woodruff key commercial, #607 cutter)
DONE x2 482524 nut 5/8-18 N.F. 2 hex lock nut (commercial thin flex top lock nut)
FIC x2 299897-3 OE-1718 support pin, was missing on bottom of both shoulder rest and support assemblies (.247" x 1")
FIC x1 367647-1, 367647-2 shoulder rest and support assembly RH.
FIC x1 367646-1, 367646-2 shoulder rest and support assembly LH.
FIC x2 299894-3 OE-1712 shoulder rest support bushing.
FIC x2 299894-5 OE-1713 shoulder rest support bushing screw.

FIC x1 Sight Mk 4 Mod 1. 299821
x1 299705-1 OE-1198 eye cup is UV damaged. The material in our first attempt at 3D printing may not be elastic enough to mount on the sight. SS JEREMIAH O'BRIEN gave us a real example, but in poor condition. ***
DONE x3 299704-8 OE-1192 clamping screws for 299705-3, 6 (donated)
DONE x2 299705-6 OE-1190 dovetail piece for 299700-1 (donated)
DONE x1 299705-3 OE-1190 stop piece for 299700-1 (donated)

- Both cocking bars, clips, etc.
pg 101 of OP 1439 list of parts. The hole in which they are mounted is shown in section F-F, of drawing 492482.
DONE x2 482497 cocking bar pivot pin (replicas are 304 stainless)
DONE x2 482498 cocking bar pivot pin retaining pin (we are using a 1/8" x 3/4" cotter pin instead of straight pin)
DONE x2 482499 cocking bar pivot spring (316 stainless commercial)
DONE x2 454138 cocking bar clip Bay Ship and Yacht replicated them. Maybe plate these.
DONE x4 482533 cocking bar clip screw, 3/8-16 1-3/4" self locking hex head. We are using donated commercial place bolt.
DONE x1 455385 cocking bar right hand. Bay Ship and Yacht replicated. Maybe plate this.
DONE x1 455386 cocking bar left hand, Bay Ship and Yacht replicated. Maybe plate this.
DONE x2 482500 cocking bar pivot spring washer (cup washer, HK Metalcraft donated CUP-2000-6400 that are 1")

- Cradle packing is missing. See Fig 13, pg 34. And assembly text pg 35. Parts pg 100. It fits in cradle 10.250"-10.260", see section A-A 492483
x1 487481 carriage packing retainer is missing
DONE x1 299966-6 OE-3522 carriage packing (We cut down purchased F-13, 3/8" thick, 1" wide felt to 5/8".)

- Cradle trigger mechanism is missing. pg 39 of OP 1439 illustration, pg 95 of OP 1439 list of mount trigger parts, pg 113 of OP 1439 list of handle bar parts. 492479 handle bar assembly left hand is intact. See also section drawing B-B on 487474
454133 trigger connecting yoke assy
x1 1. 454140 Trigger on handle bar ***
x1 2. 482539 Pin-trigger axis (clevis pin .309" x 1.5")
stock x1 3. 12-Z-48-821 Pin-cotter for 482539 (3/32" x 3/4"?)
x1 4. 482538 Pin-trigger to rod
x1 5. 454139 Rod-trigger
x1 6. 482521 Nut-clevis lock (1/4"-28 hex nut)
x1 7. 482509 Clevis-trigger rod
x3 8. & 14. 482530 Pin-clevis (.245" x .796")
stock x2 9. & 15. 12-Z-48-811 Pin-cotter for 482530. (1/16" x 3/4"?)
x1 10. 454137 Lever-trigger intermediate ***
x1 11. 482525 Pin-groove (3/16 x 3/4" type A.)
x1 12. 482534 Lever Assembly-trigger
  482536 handle bar trigger link lever ***
  482535 handle bar trigger intermediate shaft
stock x1 13. 299932 Oiler-pressure (zerk).
x1 16. 482537 Link-trigger to idler lever
x1 17. 482496 Pin-trigger idler lever to link
x1 18. 454132 Lever-trigger idler
x1 19. 482492 Lever and shaft assembly-idler and yoke
  482493 trigger idler lever and yoke shaft
   482526 groove pin (.186"-.198" x 1")
  482494 trigger idler lever and yoke shaft lever
stock x1 20. 299932 Oiler-pressure. (zerk)
x1 21. 487482 Yoke-trigger ***
x1 22. 482495 Washer-yoke shaft (.475 ID, .8125 OD, .062 thick, a 7/16 nominal washer)
DONE x1 23. 299665-4 OE-1346 Ring-yoke shaft, snap (.435 ID snap ring, in gun drawings, 12mm nominal is 11mm = .435" ID bought stainless)
x1 24. 482507 Pin-trigger yoke axis (clevis pin .433" x 2.031")
stock x1 25. 299932 Oiler-pressure. (zerk)
stock x1 26. 367676-2 Pin-cotter for 482507 (3/32" x 3/4")
x2 27. 482503 Link Assembly-trigger
  454134 trigger link ***
  482504 trigger link plunger
  DONE 482505 trigger link plunger spring (5/32" OD, .0256" wire, 9/16" bought close in stainless 1986K49)
  482532 trigger link plunger washer, no drawing (.3175" dia, why is this called a washer instead of plug? Note it is peened into the trigger link and holds plunger spring.)
x2 28. 482502 Pin-trigger link ***
x2 29. 482506 Retainer-trigger link pin ***

- All the cam limit-stop parts are missing. pg 36 illustration in OP 1439. The sequential numbers in the list below are from this illustration.
454126 Cam Stop Limit Assembly, parts are below
482481 Cam limit stop roller shaft assy parts below
x1 1. 492474 Lever-cam limit stop. CNC instead of forging ***
x2 2. & 18. 482525 Pin-cam limit stop ball (.155"-.165" x 3/4")
donated x1 3. 482520 Screw-cam limit stop lever, 3/4-16 1-1/2, full thread self locking hex head. Sub. commercial place bolts.
x1 4. 482485 Washer-cam limit stop lever
x1 5. 482482 Shaft-cam limit stop roller
x5 6, 12, 19, 21, 27 299932 OE-2259 Oiler-pressure. (zerk)
x1 7. 482484 Roller-cam limit stop
x2 8. & 15. 482483 Retainer-cam limit stop shaft
donated x2 9. 482516 Screw-shaft retainer, 5/16-24 1/2" full thread self locking hex head. Sub. commercial place-bolt.
x2 10. & 19. 482480 Ball-cam limit stop rod ***
x6 11.& 20. & 22. 482476 Pin-cam limit stop rod
x1 13. 454125 Clevis-cam limit stop rod ***
x1 14. 482483 Retainer-cam limit stop pin
x1 16. 482522 Nut-clevis lock
x1 17. 454127 Rod-cam limit stop-front ***
x1 24. 482486 Retainer-cam limit stop rod pin
DONE x2 25. 482516 Screw-pin retainer (5/16-24 1/2" full thread self locking hex head)
DONE x1 26. 482478 Shaft-cam limit stop idler lever. Extracted the one that was in the carriage.
x1 28. 482528 Pin-cam limit stop idler lever shaft. Original groove pin was sheared off in the hole. (.249"-.262" x 2-1/4")
x1 29. 487476 Lever-cam limit stop rod idler ***
x1 30. 454128 Rod-cam limit stop-rear ***
DONE x1 31. 482472 Pin Assembly-cam limit rear stop rod. We extracted the one stuck in the cradle without damage.
DONE x1 32. 482527 Pin-cam limit rear stop rod pin. Groove pin. See Section E-E 492487 for through hole. (.259"-.262" x 1-3/4")

And the actual cams. pg 34 of OP 1439 has an illustration
x1 487472 right hand cam. ***
x1 487473 left hand cam. ***
x8 482479 cam limit stop screws (1/2"-20 7/8" socket head cap screw)
x2 482616 cam dowel (1/2" x 1.125">

Cam limit stop shield on carriage keep the cartridge bag clear of deflection cam, it is shown in 492476 Carriage General Arrangement. A sheet metal part.
Also see LCS-102 photos IMG_5844.jpg, IMG_5886.jpg:
x1 487477 cam limit stop cam shield
x3 482610 screw for shield (5/16"-18 x 3/4" hex head locking)
x3 12-Z-22-852 lock washer (5/16" nominal)

- Shoulder rest buckle and belt assemblies, pg 111 OP 1439
x1 299898-5 buckle and strap assembly
299898 OE-1705 shoulder rest buckle assembly
299899 OE-1707 shoulder rest strap

- Cartridge bag fittings on carriage, bag, weight bands and weights are all missing. We need to look through more photos to see if these were used on submarines in the war zone. On the one hand they protect the crew from burns and tripping, on the other hand it would take time to install and remove. We have a photo of single mount on another boat in port with the bag installed, but also the only firing underway video we have seen does not have a bag installed.
x2 299800-2, OE-2177 pivot bolts
x2 299800-3, OE-2178 pivot nuts
x2 12-Z-48-812 cotter pin
x2 367776-1 weight band assembly
  367776-2 weight strap
  299806-2 OE-2183 hinge
  367776-4 weight band
  367776-5 rivet for weight strap
  299800-1 OE-2179 cartridge bag weight
  367776-3 weight strap
x2 492491 cartridge bag and frame assembly
  492493 20mm cradle mk 14 mod 0, cartridge bag assy
  299800-6 OE-2185 copper washer for rivet
  299801-2 OE-2265 rivet for bag assy
  454151 cartridge bag plate and hinge assembly
  492492 cartridge bag assembly
  299807-5 cartridge bag detail
  299805-6 OE-2181 link bolt
  299804-5 OE-2180 washer
x2 482546 cartridge bag bolt
x4 299957-6 OE-2805 cartridge bag bolt
12-Z-48-825 cartridge bag cotter pin


- As found the gun mechanisms on the twin mount were a mess. Magazines had significant rust. Mod 0 style breech casing cap with shoulder rest frames and hand grip was wrong on the left gun, it is not used on the twin where the shoulder rests mount on the cradle. The triggers, and most of the internal parts were missing from both guns. Barrel springs were not correct. Neither gun was in battery. The guns were swapped/misassembled so the cocking bar studs on the breech bars interfered with each other. Lots of mis-assemble parts. Many missing or badly improvised parts. The breech blocks, breech bolts, trigger casings and a few small parts were found in collection in decent condition.

Guns have been removed from the cradle and field stripped. We used a chemical stripper, but also lots of hand scrapping to remove all the old coatings, paint and most rust (oops, we missed the barrel springs). Small parts that were frozen were removed, cleaned and lubricated. All the missing parts have been found or acquired. The guns have been test fit together. They are almost entirely historic fabric with only a few small springs and pins replicated. External coatings have been removed from the magazines and sheet metal repaired. The high risk work on the gun mechanisms is completed. We still need to create and implement both preservation and visitor proofing plans.

Found In Collection (FIC) is next to parts we found in storage, not on the guns.

- NOP Used Gun (was on left side of mount as found):
20mm M.G. Mech. Mk. 4, SER No. 109987 N.O.P. Center Line
x1 299672-6 OE-1265 Pin on trigger (.1166" d x .828" dowel pin, bought 3mm 52100 steel), OP 911, Addendum 1, 12 July 1943 allows substituting a 1/8" x 1" cotter pin.
x1 299672-6 OE-1265 Pin on barrel locking handle, same as on trigger, ), OP 911, Addendum 1, 12 July 1943 allows substituting a 1/8" x 1" cotter pin.
x1 299675-3 OE-1081 double loading stop guide bushing lock washer
DONE x1 300048-1 breech cap to replace the single style with handles that was installed. (We traded a single style with handles. It required some gentle fitting.)
DONE x1 367534-4 barrel spring center spacer. Donated, then we found note OP 911, Change 1, 2 Dec 1944 that removes the center spacer on round wire springs
FIC 367534-3 rear barrel spring pilot
FIC 367533-1 rear spring. Be sure to get paint on correct end.
DONE x1 367533-2 front spring, be sure to get yellow paint on the correct end. (donated)
DONE x1 299698-6 trigger with shorter handle for twin mount. (donated)
DONE x1 299698-4 OE-1221 trigger retaining bolt (donated)
DONE x1 299673-2 OE-1079 barrel locking handle shaft (donated)
DONE x1 299672 OE-1039 barrel locking handle assembly (donated)
DONE x0 299683-4 OE-1065 hand grips retaining catch plunger. This, and the spring, and pin were in the retaining catch but stuck with rust. Cleaned and reinstalled with new spring. (breech casing cap lock)
DONE x1 299683-5 OE-1335 spring for hand grips, breach casing cap lock retaining catch plunger was rusted out. (bought stainless steel spring)
DONE x0 299683-6 OE-1258 pin that holds OE-1065 (breech casing cap lock) was cleaned and re-installed.
DONE x1 299698-2 OE-1218 trigger lever intermediate-left (donated)
FIC x1 299691 OE-1246 trigger pawl and holder assembly
FIC x2 299666-2 OE-1323 securing bolts
DONE x2 299666-4 spring pins for securing bolts.(donated)
FIC x1 299659 OE-1302, breech bolt assembly
FIC x1 299651 OE-1301 OE-1303 breech block assembly
DONE x1 299653-1 OE-1307 striker pin was missing. (Bought on eBay)
DONE x1 299654-1 OE-1308 recoiling mass hammer for striker pin (donated)
DONE x1 299653-2 OE-1309 bolt axis recoiling mass hammer for 299654-1 (donated)
FIC x1 299684 OE-1248 & OE-1245 trigger casing assembly
DONE x1 299698-1 OE-1223 trigger crank-left. Holds OE-1246 trigger pawl assembly. (donated)
DONE x0 Breech casing barrel stop pin 299671-5 OE-1052, 299671-1 OE-1072, 299671-2 OE-1331, 299671-3 OE-1059 came out and have been cleaned and re-installed.
DONE x1 299695-5 OE-1262 Fire/Safe handle pin was found sheared off. The three bits were extracted. Replica pin .1166" x .562", 1022 steel installed.
DONE x0 299696-3 OE-1235 detent plunger on fire/safe handle was frozen in place. It was cleaned and lubricated with 299696-1 OE-1328 Detent spring and 299690-7 Pin for detent spring.
DONE x0 Breech casing, hammer plate securing spring (OE-1064) and hammer plate (OE-1060). Modified and welded in place.

- GM Used Gun (was on right side of mount as found):
20mm M.G. Mech. Mk. 4, Ser. No. 219775, G.M. Corp. Has a Mod-1 breech cap, i.e no handles.
x1 299672-6 OE-1265 Pin on trigger (.1166" d x .828" dowel pin), OP 911, Addendum 1, 12 July 1943 allows substituting a 1/8" x 1" cotter pin
x1 299672-6 OE-1265 Pin on barrel locking handle, same as on trigger, OP 911, Addendum 1, 12 July 1943 allows substituting a 1/8" x 1" cotter pin
DONE x1 299671-4 OE-1263 pin in breech casing that holds 299692 OE-1242 trigger pawl tripping bolt spring case (.1166" d x .680" l)
DONE x1 299698-6 trigger handle, twin mount style. Moved from FIC gun.
DONE x1 299698-4 OE-1221 trigger retaining bolt (donated)
DONE x1 299692-4 OE-1338 spring was replaced with stainless
DONE x1 367534-4 barrel spring center spacer (donated). Then we found note OP 911, Change 1, 2 Dec 1944 that removes the center spacer on round wire springs.
DONE x1 367534-3 rear barrel spring pilot (donated)
FIC 367533-1 rear spring. Be sure to get paint on correct end.
DONE x1 367533-2 front spring, be sure to get yellow paint on the correct end. (donated)
DONE x1 299672 OE-1039 barrel locking handle assembly, pg 169 (donated)
DONE x1 299678 OE-1035 magazine catch lever axis assembly to replace the threaded bolt that was installed. (donated)
DONE x1 299673-3 OE-1070 plunger for spring on barrel stop OE-1078. This was stuck in the hole, we drilled, tapped and used a 4-40 screw to jack it out. The old one can be repaired, but we received donation of a NOS (new old stock) one.
DONE x1 299673-4 OE-1339 spring on barrel stop OE-1078. Was badly rusted. (replaced with new stainless spring)
FIC x1 299691 OE-1246 trigger pawl and holder assembly
FIC x2 299666-2 OE-1323 securing bolts
DONE 299666-4 spring pins for securing bolts. These stay in the securing bolts when removed, just the end squeeze in. Attempts to remove them from the securing bolts probably explains why so many of these are found broken. (donated)
FIC x1 299659 OE-1302 breech bolt assembly.
FIC x1 299651 OE-1301 OE-1303 breech block assembly
FIC x1 299684 OE-1248 & OE-1245 trigger casing assembly
DONE x1 299697-6 OE-1326 one of the recoil buffer springs was missing. (donated)
FIC x1 299676-1 OE-1058 barrel spring seating ring retaining catch pg 167
FIC x1 299676-3 OE-1269 retaining pin for OE-1058
FIC x1 299676-2 OE-1337 spring for OE-1058
DONE x0 299676-5 OE-1056 barrel spring seating ring was not installed correctly, the hole for the lock was not lined up with the ring. This is probably why 299676-1 OE-1058 lock for the seating ring that we found in collection was not installed. It was been removed, all the parts cleaned and re-installed. It took quite a bit of work to remove the springs that were rusting in the 20mm blind holes in the breech casing.
DONE x1 299698-1 OE-1223 trigger crank-left that holds OE-1246 trigger pawl assembly (donated)
DONE x0 Breech casing barrel stop pin 299671-5 OE-1052, 299671-1 OE-1072, 299671-2 OE-1331, 299671-3 OE-1059 where frozen, but all removed, cleaned, re-installed.
DONE x0 Fire/Safe handle was frozen. Its shaft was disassembled and re-assembled with anti-seize grease. Detent plunger just had oil put into its air/oil hole then started to work.
DONE x0 Breech casing, hammer plate securing spring (OE-1064) and hammer plate (OE-1060) were modified and welded in place.

- FIC Gun was in good condition. It has not be surveyed as closely as the other two, we are not restoring it in this project:
20mm M.G. Mech. Mk. 4 Mod-1, Ser. No. 130618, N.O.P. Center Line (FIC)
No barrel, barrel springs, spring pilot & separator, or double loading stop assembly. Had correct twin gun breech casing cap (300048-K-1) and trigger handle (299698-6). Note Mk 4 Mod 1 is for the twin mount. It is the same as a Mk 4 Mod 0 except for the breech casing cap and the trigger handle. See pg 65 of twin manual.
x1 299672-6 OE-1265 Pin on trigger bolt (.1166" d x .828" dowel pin, using 3mm dowel pin), OP 911, Addendum 1, 12 July 1943 allows substituting a 1/8" x 1" cotter pin
x1 299672-6 OE-1265 Pin on barrel locking handle, same as on trigger, OP 911, Addendum 1, 12 July 1943 allows substituting a 1/8" x 1" cotter pin
x1 There is a long cotter pin on magazine catch lever axis clevis pin. The clevis pin looks real, with the guard instead of the spring loaded shorter pin, but the cotter pin is wonky. We do not have a drawing of this style axis clevis, so our best guess is just a better fitting cotter pin.
x1 367533-1 rear spring was donated, but it is cut in half. Make a sleeve and silver solder, this will not be repeatedly cycled, just for assembly, display, and maintenance. Apply paint to center end.
x1 299901-1, OE-1013-K, 20MM Mk 4 Mod 1 Barrel without the cooling ribs was donated. It is cut with gas torch in multiple locations that needs to be reassembled, welded. It has demilitarization installed in addition to cuts.
DONE x1 367533-2 front spring, be sure to get yellow paint on the center end. (donated)
DONE x1 299698-3 OE-1220 trigger with single style handle (long) to replace the dual style trigger found on this gun that is now on the GM gun on display. (donated)
DONE x1 367534-3 rear barrel spring pilot (donated)
FIC Breech casing cap with handles and shoulder mounts (Mod 0, single gun type). Missing one of the set screws, and both locking levers. It will be needed if this gun is installed on our single mount. We may have the locking levers if they are not used on the twin.
DONE x0 Breech casing, hammer plate securing spring (OE-1064) and hammer plate (OE-1060). Modified and welded in place.

- Notes on double loading stop:
Double loading stop parts are missing from all three guns. pg 34. These were of great interest to us until we found OP 911 Change 2, 23 July 1945 that shows the double loading stop was removed from the guns. This explains why all the barrels we have seen have the double loading stop hole filled with a custom screw peened in place and the hardware on the breech casing is missing from almost all of the guns we have seen.
We would still like to get one set for the FIC gun that will go on a single mount interpreting war patrol configuration:
x1 299674-5 OE-1053 double loading stop lever
x1 299675-1 OE-1063 axis bolt for double loading stop lever OE-1069
x1 299665-4 OE-1346 securing spring for axis bolt OE-1063 (snap ring)
x1 299674-1 OE-1054 double loading stop
x2 299674-2 OE-1071 double loading stop plunger
x2 299674-3 OE-1336 spring for double loading stop plunger OE-1071
x1 299675-4 OE-1069 double loading stop plunger
x1 299674-4 OE-1080 double loading stop plunger-upper
x0 299675-3 OE-1081 lockwasher for 299675-2 OE-1055 double loading stop guide bushing on NOP gun
x0 299675-2 OE-1055 double loading stop guide bushing (bushing installed on NOP gun, hole without bushing is drilled in the other two breech cases so this is not used)
x1 299827 OE-1001 A-Double loading stop plunger-lower assembly in barrel. (299827-1 OE-1011 and 299827-2 OE-1012 for Barrel Mod 0. -OR- OE-1014, OE-1015, OE1016, OE-1012 for Barrel Mod 1 see pg 163)

- Notes on barrel springs:
367534-1 spring assembly rear and pilot
367534-2 spring assembly front and pilot
367534-3 rear barrel spring pilot
367534-4 front barrel spring pilot, barrel spring center spacer, OP 911, Change 1, 2 Dec 1944 shows deleting the center spacer from service on round wire springs.
367533-1 rear spring, longer, round wire
367533-2 front spring, shorter, round wire

Note the yellow ends on both springs go to the center and the springs twist in opposite directions. The longer spring with rear pilot is closest to the breech casing. The pilot should be taped to the rear spring with single piece of 1/4" tape (367534). pg 148 Cotter goes in with the "handle" on the left side. Breech bars are installed with cocking studs are on opposite sides of each gun, then guns mounted with them outboard. pg 63

Both guns as found on the boat had only one round wire spring installed. Only one had a rear pilot. The FIC gun had no springs or pilots. The found springs have 21-1/2 coils, .331" dia, free length 20" and 21" (should be 20-13/16"), pitch not even, ground on both ends, right hand coil. They are likely were real, but a bit damaged. Gun FIC had no springs or pilots.

There is about 25-3/4" front of the spring case to breech casing when the gun is in battery which means the springs will have to compress about 31-5/8" free length - 25-3/4" case space - 1/4" rear pilot = approx. 5-5/8" for installation. Each spring at 20% compression is approx. 130 lbs. The gun mechanism manual says one man should be able to push in while another puts the cotter in place. This will work if everything fits smoothly, but that is a lot to hold. On the mount, the twin manual says the cocking bar can be engaged, then gun elevated and locked in the 5 degree position to compress the springs and allow removal/installation of the cotter. pg 28 We used a small come-a-long on the spring case to compress the springs.
Front sleeve 299667-2 OE-1318 is not needed on our guns. Spring cases on all our three guns are 299883 OE-1105 welded style they do not use a front sleeve.

- Magazines that were on display were cleaned, coatings removed, stripped and surveyed. They had significant rust and the sheet metal cases required significant repair. Some of the case nuts were wasted away. We did not find a universal (left and right hand) ratcheting loading lever, so we made a simple, functional wrench that works on left magazines. We were able to buy a British right hand magazine lever. It is an open question as to whether we should leave the magazines on the guns for normal display. During normal patrol they would only have come out only when the gun was in use. If we do not put the magazines on display, we should replicate transit/weather covers that fit over the hole in the top of the breech casing.

The magazines were originally parkerized, but this finish does not provide adequate protection from corrosion in our environment. We coated the interior of the cases and sides with acrylic enamel, and then grease. The exterior will get our normal epoxy primer and polyurethane top coats. We are considering storing these with the cartridge feeder all the way in to ease disassembly during the next overhaul.

299706 Magazine Mark 4 general arrangement right hand (also used on single mounts). How to strip the right hand magazine. This has a couple of errors in the printed text. Page 72 of OP 911, relieving magazine spring tension, step 1 the handle should be on the ratchet for the spring, i.e. Fig 58 instead of 63. Also in step 5, it should repeat step 2 instead of 3. Step 10 leaves out the removal of the screw, retaining ring, spring and sleeve. Note the instructions on how to tension, load, unload, and strip a left hand magazine are slightly different (clockwise vs counter-clockwise) from a right hand magazine. However, they are better instructions so read them before attempting a right hand magazine.
367739 Magazine Mark 5 general arrangement left hand. We need to be sure to paint the zig-zag stripe on the case and the handles grey after final coatings, see (pg 70 and 367751). How to strip the left hand magazine is in the twin mount manual.
Normally the case is not fully disassembled, however if you need to, the trick is to remove the screws and cotter 299726-1 OE-1560 on the mouthpiece before trying to separate the mouthpiece from the side pieces and casing.
Side bolts and nuts need to be flush on front (top when loading). Easiest is to insert bolt from front of magazine so nuts are on the back.

DONE x1 299724-1 OE-1544 Sheet metal casing on Mk 4, right hand magazine. Fits on 299708 and 299711. This was replicated by Luke Stevens. It came out 1/8" short but works.
DONE x0 367751 Sheet metal casing on Mk 5, left hand magazine was repaired with welded inserts by Luke Stevens. Fits on 367742 and 367745.
DONE x6 299724-5 OE-1291 Nuts. 1 replaced on right magazine casing bolts, 5 on the left. (10-32 nut stock)
DONE x0 299724-2 OE-1556 magazine casing bolts (10-24 8-1/4"). We chased the threads.
DONE x1 299727-7 OE-1593 Round head, slot, shoulder bolt, 3/8"-24, .965" l, .312" shoulder, for mouth piece of right magazine that has a rusty hex head now. We used a commercial hex head bolt instead of making this.
DONE x1 299728-8 OE-1298 castle nut for 299727-7 above (3/8"-24)
DONE x6 299721-7 OE-1297 cotter pins in both magazines for articulating bolts 299576-5 and 299720-2 swivel bolt. (1/16" x 3/4" stock cotter pin).


- Foundation preparation:
The basic 4" 50 cal gun foundation drawing:
The bolt hole pattern, five 1-5/32" holes on 26-3/8" dia. circle (15.503" chord) is in the working circle drawing:
A drawing of a 4" or 5" gun foundation forward, compare/contrast and some easier to read details that are the same:

We knocked out five mostly rotted wood plugs in the 20mm mounting holes in forward gun foundation bed. There is rot in the 1945 wood foundation bed and rust in the steel. We considered new plugs in all but the five 20mm mount holes, and several crop and inserts of the mushy wood. However we do not think it would last long enough. We will remove the old wood, prep and paint the steel top and bottom, and put in a replica wood bed.

Note when installing the 20mm mount, there is only one bolt hole in the gun mount stand base ring which is adjacent to a foot cleat. Normally the foundation should be mounted on the deck with this bolt hole lined up with the forward hole in the deck for proper alignment. Our foundation hole pattern is lined up for this to point aft. Because the 20mm was first mounted here in Aug 1945 we can guess this was to make it easier to avoid interferences with foundation gussets facing aft rather than forward. Functionally it would not matter as long as the mount is consistently mounted with the same hole on centerline because the depression stop cam is profiled after installation to match the ship.

Note the outer bolt hole pattern on the foundation (50-1/2" dia., 6.559" chord, x24 1-1/2" dia. holes) matches a 5" 25 cal. gun stand. The wood bed is about 55" which is close to the 54-3/4" OD of a 5" 25 cal. stand (ID of the 5" 25 cal stand is 36-1/2"). It is also thicker (4-1/2") than any of the foundation drawings show (3" or 4", which is 1/4" more than the steel coaming). The center core is cylindrical like the 4" foundation drawings, not the truncated cone of the 5" aft or dual purpose forward drawings. There are additions/reinforcements added to the diagonal framing and support of the cylinder under the deck that also support the enlarged bed. So we think the foundation was built for the 4" 50 cal. as shown in the drawing, and carried a 4" 50 cal for the 6 war patrols. We think it was modified during the 1945 refit to be able to hold a 5" 25 cal. or twin 20mm. We do not know if this was pre-planned improvement for two 5" guns, or they tried a gun forward before building the foundation aft. We have a photo of USS Grouper at Mare Island during July 1945 with a twin twenty mounted and 5" 25 cal bolt hole pattern so this was not a unique modification. The 4" 50 cal. bolt holes found on the mount are all too small diameter (1" instead of required 1-1/2") and are not visible in the 1945 and early 1980s photos so we think these were added during the museum period.

Notes on the wood foundation bed:
The 1-1/8" bolt hole pattern for the stand should be drilled from the drawing before install, or at least accurately marked. These should be drilled 1-1/16" for tight fit of the 1-1/8" bolts in the wood. (The holes in the foundation steel and stand are 1-5/32".) The tight fit is needed to minimize water, but will require accurate, squared to foundation holes to fit.
See section F-F of four inch foundation drawing for the countersink of the 5/8" bed bolts, we need to measure the bolts during demolition. We will drill from underneath on the boat to match the existing holes in the steel foundation bed after the wood bed is installed. There are fewer than 22 wood bed mounting holes on the mount, and if we use wide boards like the 5" we can also use fewer. We installed the 5" gun without any of the bed bolts.
Most accurate would be to drill the outer 24 bolt 5" 25 cal bolt hole pattern in the photos, but for best wood preservation we will not drill these. We can paint where the bolt holes would have been for interpretation if desired. If we use a half circle stencil repainting will be easy.
Beds on foundations are usually laid with the grain and seams parallel to ships centerline.
Note when looking at the drawing, 30.6 lb steel plate is 3/4"
x? Teak 4" thick on the drawing, but what is installed is 4-1/2". All the drawings we have call for 1/4" taller than the coaming and what is installed is much taller. We will try to match the existing wood thickness but will accept 1/4" above coaming minimum. The wood needs to be clear (no knots.) Wider is better, but sized to avoid seams over or close to the bolt holes. Original widths are roughly 10", 9-1/2", 7" and 5-1/2" not too different than the drawings. The OD is ~55", ID ~21-1/4" but we need to verify these after removing old wood and needle gunning the steel. The 4" gun foundation forward drawing shows 48-3/4" OD and 22.5" ID, the combined 4" and 5" gun foundation forward drawing has 55-5/8" OD and 22-3/8" ID. We are probably going to substitute douglas fir based on availability.

x5 1-1/8"-7 hex head bolts. We need to measure up the length for the forward gun foundation once a decision about the wood bed is made. The ones removed from bridge forward are too short for to be reused.
x5 1-1/8"-7 jam nuts
x5 1-1/8"-7 nuts

x22? 5/8"-11 4-5/8" long, std. hex head bolts, for bed bolts. This may change after we measure the holes in the foundation top and extract some original bolts. Maybe small diameter, probably fewer. We will also look at one of the originals to see if national course or fine threads.
x22? 5/8"-11 std. hex nut, or whatever we find for bed bolts
x22? 5/8" std. flat washer, or whatever we find for bed bolts
Marine glue (pitch) to seal the bed bolt countersink
Bedding compound

There are other details on deck near the foundation that are missing, but not part of this project. The fittings for the torpedo derrick stays have been cut back just forward of the foundation and to starboard. There was a training circle for the four inch gun that was left on top of the wood deck in the July photos. The salvage valve deck plates are missing.

- Cover for winter/rain
133221 assembly cover mk 3 mod 0 list of drawings, no drawing
427097 assembly cover mk 3 mod 0 general arrangement, no drawing
427102 assembly cover mk 3 mod 0 general arrangement, no drawing
if we do not find the drawings above, use 300005, OE-2328, gun cover assembly that is for a single gun as inspiration for a twin size cover
x2 299881-2, muzzle cover

DONE- Interpretation magazine. A donor supplied x60 20mm inert cartridges, and we had an extra right hand magazine in collection. So we overhauled and loaded the magazine with the cartridges. The total weight is 56.2 lbs instead of 63 lbs because there is no propellent or explosives in the drill cartridges. We still want to do more research on what was the appropriate right cartridge load Pentolite vs Tetryl, tracer every 3 or 4, etc. for 1945. Some references:
US Explosive Ordnance pg 66
colors are at:
OP 911 pg 104
OP 911 Addendum 2
Ordnance magazine drawing.
The ordnance magazine drawing we have shows at one point carried 1/3 armor piercing (APT Black), 1/3 high explosive (HET Light Grey), & 1/3 incendiary (HEI Red) and a smaller number of blind loaded and plugged (BL &P Dark Gray Green) cartridges. The BL&P are to clear obstructions, for example a muzzle cover accidentally left installed.
A guess from OP 911 would be mostly "HB" high explosive without tracer (loaded with Pentolite, Yellow), and every 4th "HL" high explosive with tracer (loaded with Pentolite, Blue). The last two cartridges (first two to fire) should be blind loaded and plugged rounds (Dark Gray Green).
Note the cartridges that were donated in the cardboard sleeves have the correct rebated base (RB) extractor lip, are drilled to show they are empty, and painted brown (a.k.a. inert, dummy, drill, training cartridges).

DONE * Stripped, cleaned, greased, reassembled the extra right hand magazine. It was built by MOM.
DONE * Made replica wooden magazine cartridge loading tool, shell pusher. This does help when installing to prevent leaving too much space and thereby jamming cartridges.
DONE * Acquired a magazine loading lever. This or a replica is need to tension/un-tension a magazine.
DONE * We decided to leave the cartridges painted for drill/inert cartridge, Brown. Only the last two are visible once loaded and leaving the color coding will avoid confusion with live ammunition even though they are drilled.
DONE * Cartridges were coated with a thin coat of grease and loaded in the magazine.
DONE * Visitor proofing to prevent easy removal of the cartridges, or insertion into a gun. In this case it is mostly for docents since visitors will not have unfettered access.

- Gun captain sound powered phone outlet. It is the same as is described in the notes for the five inch gun. Its cable is currently coiled up on an athwartship bulkhead aft of the forward gun foundation.

DONE- 299728-1 Magazine loading frame (holds magazine down while pushing cartridges into the magazine) mounting details in the crew's mess. One of the four tables (forward, centerline) in our crew's mess has the nuts needed to mount this welded in place with holes in the steel table top. However the replacement linoleum on top covered the holes. We cut the three holes in the modern linoleum. Note that there were only two mess tables (including this one) in 1980s, the other two are replacements that were found later. This may explain why two of the four tables have the fold up stanchion (fwd, outboard and aft, centerline), and only one the 20mm loading frame mounting holes. See crews mess table drawing. Note that there was a modification available to provide a folding mess table in the crew's mess in way of the ammunition scuttle (BuShips Ltr. SS/S34(515) 6 Feb 1945, and included in an Aug 1945 list of authorized alterations to Pampanito. Two of our mess tables have the folding stanchion but none are mounted to the bulkhead with quick removal hardware or a hinge.
See also the magazine stowage details. This drawing also has the list of small arms ammunition stowage, magazines, x3 loading frames & levers, magazine loader, spare parts & tools.
Note that the deck fitting drawings show a loading frame mount on the handrails fore and aft when there where 20mm guns on the conning tower. Other ships seem to always have had a loading frame near the guns on deck. It would be handy to clear jams even if not used for loading. So there is a good chance that they had a loading frame near the twin mount as well, but we have not found it in drawings or photos.

- Pressure Proof Lockers for ready ammunition storage. Did they modify the 4" ready ammunition locker in conning tower fairing at deck level forward to fit 20mm magazines? We need to open all the pressure proof lockers on the boat to see what, if anything, is in each. Also replace the handwheels and locking hardware that is missing from some of these.

* Aft main deck level there are 2 large lockers set in the conning tower fairing. One has to have been used for 5 inch shells. The other could be either more 5 inch, or more likely 40 mm ammunition for the gun above it and/or .50 cal. ammunition. The one on port has re-enforcement patches on it, and there is a note from the 1980s about finding one of these below and putting it back in place. Although just a guess, this might be the one and it might have been the one missing forward of the conning tower.
* Port and Starboard aft at deck level set in the conning tower there are two smaller diameter lockers that held .50 cal machine guns (they have a small diameter extension for the barrel length, gun overall with barrel is 66" long).
* Forward deck level set in the conning tower is a large locker that had 4" shells, and was repurposed for 20mm, or 40mm, or .50 cal. We do not know which.
* Forward set in the conning tower is a small diameter locker that is long enough for a 40mm barrel (barrel is 99" with flash arrestor), and may have had 20mm barrels (barrel 58").
* Forward outside the conning tower are the foundations for a large locker athwartships that is missing, see below. This might have had 40mm or 20mm magazines.
* Aft, bridge level there is a small locker that is set vertically in the deck, it originally held 20mm magazines when a single 20mm was here, but my guess is .50 or 40mm ammunition in 1945 because of its location.
* Aft, bridge level there are two deck lockers that probably had 40mm ammunition, but could have had some .50 cal or 20mm.
* Between the periscopes, bridge level, there is a small diameter locker. It could have had signal flares, or other materials. It has some wood dividers that might provide some hints but has not been documented yet.
* Forward, bridge level, there is a small locker that is set vertically in the deck. It was built to hold three 20mm magazines when a 20mm single was here (it is in the drawings.) Then it was modified to hold a .50 cal. machine gun (it has the long extension for the barrel).

x1 Large pressure proof locker. There are 1945 photos that show a second large pressure proof locker mounted athwartship just forward of the conning tower. There are two cut off pipes in the deck in this area that were its supports. The wood deck has been replaced twice in this area, but the remnants of the other two supports outboard, port side, are still in place under the deck. The supports are spaced roughly 23" x 50" which would fit nicely a locker the size of the ones aft, and is consistent with the photos. Note one of the two large lockers forward might also have been used to hold 40mm ammo for the gun just above it, but my guess is 20mm magazines. We have a drawing of a typical pressure proof locker (ignore the 5" shell inserts), but not one that shows a locker mounted in this location.

We do not know where spare 20mm barrels were stored on the boat. The smaller locker forward in the conning tower fairing is long enough to hold a spare 40mm barrel. We need to open it to see what partitions are in it to see if it has partitions that might provide clues. Or they could have been stored inside the boat, we do not know.

- We found the dip pot, i.e. hot barrel cooling fixture. It is a vertical pipe open on top, closed on the bottom accessed through the wood deck just forward of the conning tower. The see dimensions in this drawing from when a single was mounted on the bridge deck level. It is half full of scuzzy water, we should put in a non-historic hole tapped for pipe plug in the bottom that is left open most of the time. It looks like there was something down there that was plated over as some point. We need to measure the depth to compare with the drawing.


Gun Tools Pg 130 of OP 911
Mount Tools Pg 107 of OP 1439
299814 OE-1603 gun mechanism toll roll list of tools, 299820-1 OE-1604 is the tool roll w/tools, 367532 tool roll
367565 20 mm machine gun mechanism mark 4, spare parts box list of contents
299872 20 mm machine gun mechanism mark 4, spare parts bag list of content
482557 - Mk 24 Mod 5 Mount tool bag and tools

DONE x1 299814-3 OE-1621 barrel spring seating ring spanner (also rear cap). We made this wrench to prevent damage to the barrel spring seating ring. It would still be nice to find a real one.
x1 367720-1 barrel wrench that folds for second gun and can grip gun to pull out. We used a bearing locknut hook wrench without problem, but this custom wrench would make is easier to pull the barrel out after turning. USS TEXAS sent the drawing of a 3 foot long wrench they made.
DONE x1 367543-1 cradle spring housing spanner wrench. We made a working replica from scraps, not hardened. It would still be nice to find a real one.
x1 299831-2 OE-3159, pin spanner wrench for single left trunnion.

DONE x1 367770-1 Universal version of magazine lever works on both left and right magazines. We made a simple non-ratcheting wrench for left magazine. It must be lifted off at the end of each move, but the coupling sleeve needs to be locked to remove tension at the end of each move anyway. It would still be great to find an original (ratcheting) universal magazine lever.
DONE x1 British 20 MM OE Mk I-II N 1943 magazine lever. This is very similar to 299717, OE-1535, 299718 lever details that works on right magazines, but does not work on left magazines. (bought on eBay)
DONE - x1 299712-7 Magazine loading tool, wood cartridge pusher. We made a replica.
x1 367524 lever type magazine loading tool. We would take if we could find one, but will not replicate it.
DONE x1 299728-1 Magazine loading frame (holds magazine down while loading). We esp. want one of these since we found the original mounting hardware under one of the crew's mess tables. There were three of these aboard during the war. We made a functional replica from .242" thick steel as weldment with no flange instead of a .150 stamping. We are still hoping to find an original. We cleared the holes in the modern linoleum on top of the WW II table.
DONE x1 Wedge for magazine loading frame. We had no drawing, only illustration Figure 53. So we guessed 1" x 1" x 10" long, 4-1/4" wedge 3/8" tip to full width. The hole in our replica is larger and farther down because it was already in the scrap of oak we used to make this. We need to add the lanyard.
DONE x3 5/8"-11, 2-3/4" bolts for the loading frame.

More tools for research and interpretation:

x1 454152-R.H. cam generating sights
  482548, check the sight parts we got from SBRF
  482547, check the sight parts we got from SBRF
  487494 cam generating sight base-r.h.
x1 454153-L.H. cam generating sights
  482548, check the sight parts we got from SBRF
  482547, check the sight parts we got from SBRF
  487495 cam generating sight base-l.h.
x2 482560 cam generating sight screws
x1 454154 cam scriber assembly

x1 299881-3 bore plug gauge assembly
x1 299815-1 OE-1619 breech face piece spanner
x1 299815 OE-1622 cartridge case extractor tool assembly
x1 299817-2 OE-1620 double loading stop bushing spanner
x1 299818-5 OE-1608 pliers external, normally closed, teeth filed on outside
x1 299818-6 OE-1609 pliers internal, knurled ends
x1 299819-1 OE-1635 special screw driver for double loading stop plunger
x1 299819-2 OE-1614 special punch for removing hammer plate
x1 367516-1 cleaning rod and handle assembly, or 299816
x1 367516-3 breech face securing spring (pick)
x1 367548 bore erosion gauge assembly
x1 299831-1 OE-2904, open end wrench for 1-1/4" and 2-3/16"
DONE x1 299704-7 OE-1189 sight universal tool. We bought on eBay. This version replaced 367711-1 and covers both types of ring sights.
DONE x1 367517 barrel bore inspection mirror assembly. (donated)
DONE x1 367562 gun mechanism spare parts box with insert, but not the parts. Marked LST 1129, Box 41, Spare Parts for Mk 4 Mech., Comp. C-304A. (donated)


We appreciate the advice, help, donations and discounted products and services from individuals and companies along the way. We had the help of an very talented team. We could not have succeeded without the incredible generosity of these people and companies:


Charlie Butcher
Barry Cox
Jake Roulstone
Luke Stevens
Submarine USS COD Museum loaned us drawings that we scanned,
USS NORTH CAROLINA Museum crew, Mary Ames Booker & Paula Grange, scanned many twin mount drawings,
Volunteers and staff of USS Pampanito

Rich Pekelney, Pampanito Volunteer was the project manager.


Autodesk, Inventor CAD software,
Barry Cox, parts
Bay Ship & Yacht, machined replica parts,
Bayshore Metals, steel,
Brush Research, ball hones,
G.L. Huyett, pins,
Gyrodyne Historical Foundation, parts,
Harris International Labs, Evapo-Rust,
LPS Laboratories, coatings and lubricants,,
PT Boats Inc., mount trade,
Ultimaker, 3D printed parts,
USS Siliversides, mount,
Wilson-Garner Company, place bolts,
Richard Pekelney, numerous, San Francisco, CA

The many donors that provided the machine shop and welding shop infrastructure that made this restoration possible.


posed photo on Pampanito
This photo was taken during Pampanito's summer of 1945 refit. It shows a single 20mm machine gun mount Mk 10-1 on a bridge level "cigarette deck" before replacement with a Bofors 40mm. During the first four war patrols she had a 4" 50 cal. Mk 12-44 on the main deck forward, two single 20mm Mk 10-1 mounts on the bridge level (one forward, one aft), two spare 20mm machine guns stowed, and two removable cal. .30 M1919A4 Browning machine guns. The fifth war patrol report has a note that the aft 20mm was replaced with a 40mm.
The 20mm gun's shoulder rests are replaced with handle bars to control the carriage. The double-loading stop is installed on the gun. A cam deflection stop has replaced a pipe frame deflection stop.

Pampanito at the pier
Pampanito July 1945 at Hunters Point after refit. Twin 20mm mount forward on main deck (on the right in the photo) and Bofors 40mm guns on the bridge level both forward and aft. There was a 5" 25 cal. gun aft on main deck that not visible in this photo, and the cal. .30 machine guns were replaced with four cal. .50 machine guns. There were also quite a few small arms stored inside the boat.

mount without guns
Crop from photo above showing 20mm mount without the guns installed. Note the Measure 32 paint, black from above, grey from the sides. Also the outer bolt hole circle for a 5" 25 cal, but not for the 4" 50 cal that was mounted here during the six war patrols. The forward foundation was modified to be capable of supporting a 5" 25 cal including a new wood bed during summer 1945.

crop showing lockers
Crop from photo above show the back of a now missing pressure proof locker on port, with pipe leading to deck supporting it.

pamp at the pier
July 1945.

crop of photo above
July 1945. Crop of the photo above shows both the 20mm mount, and the missing pressure proof locker.

mount without guns
Pampanito July 1945 at Hunters Point with the 20mm gun mount.

mount without guns
Crop of 20mm mount from the photo above. The pressure proof locker is visible just forward of the conning tower fairing, mounted athwartship. It is mounted above the smaller, longer locker in the conning tower fairing that is fore and aft. Note the measure 32 paint black from above, grey from the side.

photo of Pampanito in drydock
Nov 1945. Taken from starboard, left in the photo is forward.

crop of just the area with the locker.
Nov 1945. Crop from the photo above. There is clearly a pressure proof locker that was gone before Pampanito was donated as a museum.

photo of main deck and front of conning tower with numbers identifying features.
2 Jun 2022. Some features forward on the conning tower, just aft of the gun foundation:

1 - Dipping pot for cooling 20mm gun barrels.
2 - Pressure proof ready ammunition locker that originally held 4" 50 shells. Maybe re-purposed to hold 20mm magazines and/or 40mm for gun above. The locking and hinge hardware should be painted all grey.
3 - Pressure proof locker. This is the only one long enough for a spare 40mm barrel. We need to open this to see if it has partions to hold more than the barrel.
4 - Two cut off support pipes for a missing pressure proof locker, these extend to the pressure hull to hold the weight.
5 - Salvage air valve stem and fitting, both missing the deck deck identification plate. Near here are two pipes that lead from plates under the deck to the pressure hull. They are hidden under the newly replaced deck, but will be extended upward to support the missing pressure proof locker.
6 - Ship's horn. It works, it is loud.
7 - Ugly modern deck lights for safety at night.

photo of area on port with truncated supports for the missing locker
2021. This photo from drydock 2021 shows the under deck portion of the stanchions that extended through the deck to support the missing pressure proof locker. This area was decked over with normal wood battons because we did not know about the missing locker.

photo from under deck of the supports
2022. This photo of the same area, under the deck

1 - The pressure proof locker supports
2 - Fuel filling pipe.
3 - Salvage pipe.
4 - Reach rod to salvage valve.

modifications to the 4 inch mount under the superstructure
17 Aug 2022. The re-enforcement of the forward foundation to support a 5 inch gun can be seen under the superstructure. She never carried a five inch gun forward on patrol.

foundation without mount
Foundation on the boat early in the 1980s. The hole pattern for a 4" 50 cal gun is not visible (because of bed replacement during summer 1945 or bunged), the outer bolt hole pattern matches a 5" 25 cal. The inner five hole pattern is for the 20mm twin mount. This is the restoration goal. This photo is taken from forward looking aft.

foundation bed in 2022 with lots of extra holes.
22 Jul 2022. Extra holes were either cleared, or drilled during the museum era for two different five inch guns and one four inch gun. The original five 20mm gun mount holes are shown with arrows. The extra holes need to be plugged. A lot of the wood is degraded and there are signs of rust scale expansion under bed. This photo is taken from forward looking aft.

drawing of bolt holes
Stand bolt hole pattern and notes on bolts. The bolt hole nearest the cleat (bottom of drawing) normally points forward when mounted on the foundation. Our hole pattern does not have the single hole forward, but rather aft.

submarine characterics card
Ship's Characteristics Card, Aug 1945.

document from 1945
This 7 Sep 1945 message documents the new gun configuration. It also lists the pressure proof lockers for ready ammunition and .50 cal gun storage, but leaves out the 20mm ammunition and does not specify which lockers hold what.

cable coiled and mounted on bulkhead
The gun captain sound powered phone cable is mounted on a bulkhead. Probably put here during preparation for storage at Mare Island.

single mount on conning tower
Single 20mm mount on the conning tower in the 1980s. Uncut single mount cam is in place, but none of the rest of the depression stop hardware.

gun mounted on the boat
Twin mount in 2007. Shoulder rests are mounted wrong without support brackets, no cradle or carriage locks (wood block holds it up), depression stop cam and linkages missing, trigger handle and linkages missing, cocking bars and clips missing, Mk 4 Mod 0 wrong gun on left, no cartridge bag hardware, no belt, springs painted full length yellow.

gun mounted on the boat
Twin mount in 2014, sight and shoulder rests are no longer mounted.

gun mount on the pier
20 Apr 2021. Gun mount as removed from the boat, before restoration.

mount in warehouse
2013. Single carriage and cradle on twin stand in warehouse 2013. Note the different color paint between the stand and carriage/cradle.

mount without guns
16 Jan 2022. Photo of a similar twin mount at Note it has the depression stop hardware, hand trigger, cradle lock, cocking bars, bag weights, and shoulder rest supports missing from our mount.

mount without guns
16 Jan 2022. Photo of a similar mount at It shows depression stop details we are missing.

photo of loading frame with lots of paint on it
Apr 2022. Loading frame mounted on a deck locker on LCS-102 take in 2022. This is what mounts on the galley table when loading magazines.

two loading frames bolted to a table
1942. Loading plates on Battleship North Carolina in 1942 showing alternate construction.

photo under mess table showing three nuts welded to table.
31 May 2022. This photo underneath one of the tables in the crew's mess shows the holes and three nuts welded to the table to mount a 299728-1 20mm magazine loading frame. The modern linoleum on top blocked the holes and hid this feature until we found the drawings of the table.

loading frame bolted to table.
16 Aug 2022. After cutting three holes in the modern linoleum, and cleaning the WW II nuts in the table, our replica loading frame is mounted. The replica was made in our shop as a weldment instead of the stamping. We would still like to find an original.

replica wedge on the loading frame
16 Aug 2022. Replica magazine loading frame wedge.

magazine in the loading frame in crew's mess
16 Aug 2022. Magazine mounted and ready for loading.

three types of loading tools
Photo from web of three types of cartridge pushers, magazine loading tools.

replica loading tool
16 Jul 2022. Replica magazine loading tool. Made from a file handle and wood scrap.

photo of top of gun with double loading stop
Photo from the web showing the double loading stop. We determined these were probably not installed in Aug 1945, but we would like to find one set of parts for the single gun.

photo of magazine handle
Photo of FIC gun, note the protective guide that is not on our other two guns. We think this might be OrdAlt 2294, magazine mouthpiece guide.

first magazine coming out
15 May 2021. First magazine coming out.

both magazines out
15 May 2021. Both magazines removed.

both magazines out
13 Dec 2021. Both magazines removed. Wrong breech cover, missing triggers, threaded bolts replacing pins, carriage springs not holding, no locks on carriage and cradle, lots of missing parts.

both guns with magazines removed
13 Dec 2021. Sight found in collection. Whoever removed it in 2014 did not replace it on the boat, the mounting hardware is missing.

bottom of sight with mouting hardware.
13 Mar 2022. Donated special clamping screws, dovetail pieces and stop piece test fit in the bottom of the sight.

special sight wrench
30 Jan 2022. Sight wrench bought on eBay.

two shoulder rests in storage
13 Dec 2021. Shoulder rests were found in collection in decent condition. But the locking sleeves should be on them and held in place with a missing pin in the bottom hole. Note the right hand one has the strap anchor to hold the webbing of the strap (bottom in photo above). Left hand just the anchor button for the clip.

barrels, springs removed
23 Jan 2022. Barrels out. Barrel locking handles on both, and barrel locking shaft missing on left.

gun lock shafts lowered and resting on pin without the handle
30 Jan 2022. Gun locks withdrawn. The handles were removed while working them free. Note the missing grease fittings. Also missing spring nut lock on right gun.

gun locks removed
13 Mar 2022. Later, after the guns out we removed the gun locks from the cradle.

rusty and missing most of the parts
30 Jan 2022. Inside the right gun breech casing as found. Note the loose part that was left inside. Rust was mostly superficial.

using hammer to remove guns
2 Feb 2022. After a lot of gentle attempts, it took some serious force to remove the guns.

empty cradle
2 Feb 2022. Guns removed.

parts laid out on workbench
10 Feb 2022. Guns partially stripped. Parts found in collection matched to guns.

special wrench on drawing
2 Mar 2022. Made a 299814 OE-1621 wrench replica. It would still be nice to find an original.

wrench in breech casing front
2 Mar 2022. Wrench is used for this locking nut, and the handle for the breach casing cap. It is not pretty, but is a good fit.

gun casing mounted on welding table
9 Mar 2022. When oil, heat and tender loving care does not work, bigger hammers.

locknut on table
9 Mar 2022. Removed. The other springs are still stuck in case.

view into the case with the stuck springs
9 Mar 2022. It will take more work to remove the rusted in springs.

cradle and carriage with paint removed
5 Mar 2022. First pass removing paint on the carriage was done with a needle gun. There are holes blocked with concrete and broken fastenings. At least one shaft stuck in the carriage.

cradle with coatings removed.
5 Mar 2022. First pass removing paint.

concrete in hole.
6 Mar 2022. Concrete in trigger linkage holes on handle bar and carriage. This was odd.

concrete in hole.
6 Mar 2022. Concrete in cocking bar pivot pin hole. It is on both sides.

concrete in hole.
6 Mar 2022. Broken screw on top hole, concrete in bottom cocking bar pivot pin hole.

screw extracted
13 Mar 2022. Broken screw extracted. Bottom hole mostly clear of concrete, but the threads still need repair.

springs and spring casing cover on bench.
6 Mar 2022. The left spring was installed backwards in the case. Neither case was clocked to setup the springs.

badly scored trunnion pins.
6 Mar 2022. Badly scored trunnion pins.

rear of spring case removed.
13 Mar 2022. Removed back of trunnion spring cases

cradle supported with straps and chain hoist.
13 Mar 2022. Cradle supported during trunnion removal.

trunnions that look the same on the table
13 Mar 2022. Cradle trunnion pins removed.

damage threads
3 July 2022. One of the cradle trunnion pins had damaged threads.

carriage without cradle.
13 Mar 2022. Carriage after cradle removed.

empty box
12 Mar 2022. Empty spare parts box was donated.

pin with broken locking pins next to it.
16 Mar 2022. Cam limit lever shaft removed. The grove pin was sheared off in the hole when found, we extracted the two of three pieces of the pin on the right.

top of carriage on the bench
16 Mar 2022. Carriage after removal of coatings, broken fittings, stuck pieces and concrete.

bottom of carriage
16 Mar 2022. Bottom of carriage after removal of coatings, broken fittings, stuck pieces. There is no sign of the carriage packing or retainer.

carriage on the bench
19 Mar 2022. Carriage after paint removal.

carriage on the bench
19 Mar 2022. Carriage after paint removal.

stand with carriage removed.
16 Mar 2022. Stand that was on the boat (originally from single) with cradle removed. Note the single mount style uncut cam in place.

side of breech housing with barrel lock removed
28 Mar 2022. Barrel lock removed from GM gun.

barrel lock parts
28 Mar 2022. Barrel lock parts from GM gun.

parts on the table
28 Mar 2022. Magazine catch and interlock gear removed from GM gun. The common bolt was replaced with the proper pin during reassembly.

hammer plate on the welding table
29 Mar 2022. Hammer plate removed from breech casing of GM gun.

barrel springs next to breech casing
29 Mar 2022. All the barrel springs removed from the GM gun. These fit in blind holes that filed with rust. We picked out the rust from the center of the springs that did not come out easily. Then heated. Then multiple rounds of Evapo-Rust, and finally we got them all out without damage.

parallelogram and spring on the table
29 Mar 2022. Parallelogram and spring removed from GM gun.

parts removed from NOP gun on the workbench
30 Mar 2022. Started disassembly of NOP gun. This was much easier than the GM gun. Only one of the barrel springs remained stuck in the breech casing. We got it out about 3 weeks later.

parts remove from NOP gun
6 Apr 2022. More parts out of NOP gun.

rusty bolt with concave head in mount
12 Apr 2022. The self locking screws in the drawings are interesting and have a concave hole in the center of the head. This is a surviving bolt on the mount.

modern bolt with concave head
12 Apr 2022. Here is a photo of a modern, military spec. self locking concave head screw that looks pretty much the same as the WW II drawings. We did not find stock of these.

modern place bolt
12 Apr 2022. This is a Wilson Garner Place Bolt we are using a substitute for the concave head self locking bolts. It is a modern design that works on a similar principle, but with the slots and asymmetry.

ultrasonic cleaner on welding table
Small parts are first cleaned in this Ultrasonic Power Corp. cleaner with Brulin 815GD fluid.

parts basket above bucket of Evapo-Rust
26 Apr 2022. Small parts that were rusty are treated with Evapo-Rust chelating agent after cleaning to remove the rust. Be sure to rinse the parts in water after treatment. The Dip and Drain parts basket's feet fold in to allow lowering the basket into the standard 5 gallon bucket.

guns on the welding table
30 Apr 2022. Almost all the paint is removed. We used Evapo-Rust Safe-Erase paint remover for the first time. It worked and was nicer to use than methylene chloride based products we have used in the past.

weld berries under the carriage.
3 May 2022. It was odd to find weld slag on the bottom of the carriage with no sign of welding nearby. When we got the stand later it was clear that this was a bad attempt to lock the carriage. We ground/filed this off.

underside of carriage
3 May 2022. Underside of carriage after cleaning.

handle shaft, three bits of broken retaining pin and lever sitting in the cover
8 May 2022. Safe/Fire handle pin that was sheared off was removed from the NOP gun. Not shown, but we got the spring out of the detent in the handle. Everything is reassembled with a new pin and works.

mount parts on bench, no paint
13 May 2022. Carriage and cradle parts after paint removal and cleaning.

barrel lock installed on gun
16 May 2022. Barrel lock test fit on the NOP gun, shown in the open position. We had not yet received the pin for the handle. The donated handles, and one shaft fit with our original parts without problems in both guns. One spring was replaced with stainless steel.

barrel stop installed in GM gun
16 May 2022. The barrel stops were test fit re-installed with all the original parts, no problems.

spring retainer catch installed.
16 May 2022. The barrel spring seating ring retaining catch with its spring and pin that were found in collection were installed with no problem.

rear cap latch installed.
16 May 2022. Hand grips retaining catch plunger, i.e. the rear cap latch is installed with a replacement stainless steel spring. All the other parts are original.

two guns on bench just the barrel lock area shown
18 May 2022. Another view of the barrel locks and handles installed. The barrel lock on the left is locked, on the right open. The NOP gun is missing the lock washer on the double loading stop bushing.

two guns on the bench shown with barrels installed.
18 May 2022. Magazine catch gear and interlock gear installed on both guns. Barrel spring seating ring retaining catches, buffer, buffer springs, and barrel spring seating ring installed on both guns. Care should be taken not to cross thread the catch ring, we think this is why one of the guns was found without its barrel spring seating ring retaining catch installed. The barrel will not fit if the retaining catch ring latch is not seated properly. Barrels were test fit by hand and the barrel locks engaged without problem.

photo showing trigger parts install in breech casing.
20 May 2022. Trigger parts, trigger casing, breech block, breech bolt are test fit in both guns.

photo of gun on the bench
20 May 2022. Both guns mostly test fit assembled for the first time. Both are demilitarized. We had not test fit the spring casing, springs, magazines, and the pins in the two handles.

spring with yellot paint on two coils
22 May 2022. We think the yellow paint is original on this front spring.

gun on table with tackle compressing spring.
22 May 2022. The small come-a-long made it easy to compress the springs without tiring out someone pushing them in.

photo of  both guns on the bench
22 May 2022. Fully test fit assembled. Oops, I put the cotter in from the right side instead of the correct left side.

photo of  both guns on the bench
22 May 2022. Fully test fit assembled. Cotter handle on wrong side.

photo of  both guns on the bench
22 May 2022. Fully test fit assembled. Cotter handle on wrong side. Note the different revision barrels are inserted correctly but the slots for a wrench are in different orientations.

magazines left and right on the workbench
29 May 2022. First layers of paint removed from the magazines. They will need a lot more work to remove the old coatings. There is a lot of rust. Note the left and right versions.

right magazine on bench
2 Jun 2022. Right magazine after coatings removal. Some wastage in the side plate. Hex head screw instead of round head. There was some kind of RTV and body putty in addition to paint.

large area of wasted, lacy metal
2 Jun 2022. Right magazine had a lot more lacy, wasted, sheet metal than the left.

another area of wasted metal
2 Jun 2022. Right magazine, more wastage. There is more, this photo is typical.

nut 90% eaten away by rust. 2 Jun 2022. Right magazine, wasted nut.

spring coupling held by small 4 jaw chuck
4 Jun 2022. Even after stripping paint, induction heating, penetrating oil, tapping, etc. the spring coupling sleeves were frozen. We eventually used a small 4 jaw chuck to move a little, then gentle up and down until it came straight up. By then it was loose enough to easily lift and rotate into the locked position by hand on both magazines.

magazine lever on magazine
4 Jun 2022. Test fit of magazine lever we bought on eBay. Fit on the feed axis on top as well.

two wrenches on the bench
29 Jun 2022. The simple shop made wrench on the bottom will work with either left or right hand magazines. The original ratcheting magazine lever above only works on right magazines.

right magazine in parts
5 Jun 2022. Normally the magazine casing does not come apart. However, both the feed axis (center shaft) and the spring axis were hard frozen in the right hand magazine. We also knew the sheet metal case needed to be removed for repair or replacement. The photo does not show about a pound of rust and paint that came out of the interior.

rt mag apart
8 Jun 2022. Right magazine apart, cleaned.

rt mag parts
8 Jun 2022. Right magazine parts cleaned.

rt mag parts
8 Jun 2022. Right magazine parts cleaned.

photo of magazine on bench
2 Jun 2022. Left magazine after coatings removal. We had a lot of mechanical prep by hand and some needle gun work to remove the coatings after the chemical stripper.

rusted through sheet metal
2 Jun 2022. Wasted sheet metal in left magazine.

left magazine apart
9 Jun 2022. Left magazine apart. Other than more damaged nuts on the cross case bolts the left magazine was in better condition than the right magazine. This photo was before cleaning and de-rusting. The trick to getting the casing apart after stripping, (not normally needed,) is to remove the cross bolts, then remove the cotter on the mouthpiece before attempting to slide the mouthpiece out and separate the sides, casing sheet metal and mouthpiece.

wrench on a spring casing
2 Jul 2022. Replica cradle spring housing spanner wrench. Just a left over scrap of bar a bit narrow and short (functional, not an accurate replica).

replica shoulder rest bracket on the bench
12 Jul 2022. Replica shoulder rest frame brackets.

replica shoulder brackets installed.
12 Jul 2022. Replica shoulder rest frame brackets with original shoulder rests, bushings, bushing screw, and single mount style locking lever are test fit. Left and right shoulder rests are swapped in this photo.

stripped magazine
16 Jul 2022. Spare right hand magazine stripped and cleaned before lubrication and reassembly.

60 cartridges layed out next to the magazine.
19 Jul 2022. Spare right hand magazine overhauled, and 60 greased drill cartridges ready for loading.

shell pusher in use
19 Jul 2022. There is little force needed to load the shells. The shell pusher is used to avoid pushing in too far, using it makes it less likely to accidentally capsize a cartridge in the magazine.

bottom of loaded magazine
19 Jul 2022. The cartridges can be seen from the bottom on the loaded magazine. It weighs 56.2 lbs, close enough to the 63 lbs it would have with live ammunition for interpretation with visitors.

cocking bar parts on the bench
29 Jul 2022. Replica cocking bar parts.

cocking bars installed on carriage on bench
31 Jul 2022. Replica cocking bars test fit.

magazine with a chunk cut out of the side.
5 Aug 2022. Left magazine found in collection. I have no idea why someone would make a cut like this.

top of stand
11 Aug 2022. The pivot retainer is frozen in the pivot housing. The pivot is frozen in the pivot housing. The color of the machined surfaces in the photo is oil, not rust. There are crude tool marks on the pivot retainer, probably from using a drift to attempt to rotate it instead of using the carriage.

carriage lock hole with damage
11 Aug 2022. The carriage lock pin hole shows damage from previous attempts to rotate the pivot retainer (in both directions.) The lock housing was missing so they probably used an undersized pin.

pin spanner wrench held over the pivot retainer
13 Aug 2022. We made a custom pivot retainer spanner wrench to get more leverage and avoid any damage to the pivot retainer. Distributing the shock load across six 1/2" pins prevented damage to the pivot retainer. The overall L shape was not an engineered solution, just the shape of the piece of scrap metal that was handy.

pivot retain next to stand showing lots of dirt inside bearing surface
18 Aug 2022. Pivot retainer removed. Pivot could be turned by hand once the pivot retainer was out. The pivot retainer was over-tightened, upper bearing locating lugs were installed outside the pivot housing locating notches and the bearings were upside down. Once the pivot retainer was tightened it distorted the lugs and froze all the parts.

pivot hanging over the stand
18 Aug 2022. Once the pivot retainer was off, the top babbit bearing and the pivot came out by hand.

empty pivot housing
18 Aug 2022. The top of the pivot housing shows scoring from some previous pivot retainer removal or install. On top you can see the pivot retain lock screw hole. About 1" of the pivot retainer lock screw was left in here about .8" deep in the hole. Someone must have drill this out, but did not go the full length of the pin or extract the leftover piece. Both pivot housing lock screw holes on the side of the stand show signs that the pins were drilled out (poorly.) One of the pivot housing lock screw holes on the side of the stand was drilled all the way through the pivot housing. Both holes were too big and deep.

parts layed out on a table
18 Aug 2022. Pivot, pivot retainer, upper babbit bearing, lower bearing, and housing lockscrew are out. One side of the upper babbit bearing is pitted and a bit melted, the locating tabs are damaged on both. The top race of the lower bearing is still on the pivot.

pivot housing suspended above stand weldment
18 Aug 2022. Pivot housing out of stand weldment.

foundation bolt holes cut with torch
21 Aug 2022. Someone torch cut 3 of the 5 stand mounting holes.

bottom of stand
25 Aug 2022. Bottom of the stand showing the torch damage on the re-enforcement

foundation with wood out.
7 Sep 2022. Rotted wood bed on forward foundation removed. Original was 4-1/2" thick, widest piece 10-1/2", teak.

screws on a drawing
7 Sep 2022. Replica pivot retainer lock screw and pivot housing lock screws.

replica pivot retain lock screw
7 Sep 2022. Replica pivot retainer lock screw test fit after extracting the remains of the old screw.

screws in stand weldment.
10 Sep 2022. Replica pivot housing lock screws test fit. The threads in the stand weldment needed repair, and parts of the old screws removed from the pivot housing.

two magazines on the bench, not painted.
17 Sep 2022. Magazine sheet metal cases repaired with an insert on left magazine and full replica on the right. Both still need external coatings (including under handles.)

replica lock housing with original handle installed.
23 Sep 2022. Replica carriage lock housing with real handle, spacers, shaft test fit.


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