[20-mm Automatic Gun M1 and 20-mm Aircraft Automatic Gun An-M2]
[From the U.S. Government Publishing Office, www.gpo.gov]

™91227
Document -----------------------------
Reserve	WAR DEPARTMENT
TECHNICAL MANUAL
ORDNANCE MAINTENANCE
20-MM AUTOMATIC GUN Ml
AND
20-MM AIRCRAFT AUTOMATIC
GUN AN-M2
August 7, 1942
NON-CIRCULATING
NTSU LIBRARY
*TM 9-1227
TECHNICAL MANUAL!
No. 9-1227
WAR DEPARTMENT, Washington. August 7, 1942.
ORDNANCE MAINTENANCE
20-MM AUTOMATIC GUN Ml AND 20-MM AIRCRAFT
AUTOMATIC GUN AN-M2
Section I. General.	Paragraphs
Scope_______________________________________    1
Characteristics________________________________ 2
Differences in	models__________________________ 3
Data___________________________________________ 4
II. Disassembly, assembly, and inspection.
General____,___________________________________ 5
Tools__________________________________________ 6
Disassembly____________________________________ 7
Assembly-______________________________________ 8
Inspection_____________________________________ 9
III. Maintenance and repair.
Exterior of gun_______________________________ 10
Driving spring guide group____________________ 11
Rear buffer group_____________________________ 12
Breechblock assembly__________________________ 13
Sear cover plate group________________________ 14
Sear block group______________________________ 15
Magazine slide group__________________________ 16
Gas cylinder and sleeve group----------------- 17
Muzzle brake assembly_________________________ 18
Recoil spring and mounting sleeve assembly__	19
Tube and receiver groups______________________ 20
IV. Care and preservation.
General_______________________________________ 21
Cleaning______________________________________ 22
Lubrication___________________________________ 23
Page
Appendix. List of references_________________________________ 22
*This manual supersedes TM 9-1227, January 24, 1942.
1
TM 9-1227
1-4	ORDNANCE MAINTENANCE
Section I
GENERAL	:
Paragraph
Scope------------------------------------------------------------------------ 1
Characteristics____________________________________________________________   2
Differences in models-------------------------------------------------------- 3
Data_________________________________________________________________________ 4
1.	Scope.—This manual is published for the information and guidance of ordnance maintenance personnel. It contains detailed instructions for inspection, maintenan.ee. and repair of the 20-mm aircraft automatic gun AN-M2 and 20-mm automatic gun Ml. supplementary to those in the Field and Technical Manuals prepared for the using arm. Additional descriptive matter and illustrations are included to aid in providing a complete working knowledge of the materiel.
2.	Characteristics.—The 20-mm, 2,850-foot per second automatic gun is a combination blowback and gas-operated aircraft weapon. The gun is air-cooled and has a cyclic rate of fire of 600 to 700 rounds per minute. It is designed for mounting as a fixed gun in the wing or fuselage of an airplane. It may also be mounted to fire through the hub of the propeller.
3.	Differences in models. The differences between the Ml and AN-M2 guns are in manufacture only; they do not affect troop use or care, but are useful as means of identifying the different models. The guns are identical with respect to the construction of the tube and the working parts, the only differences being in the dimensions of some of the receiver parts. The AN-M2 receiver is 0.2 inch longer than the Ml. Each receiver slide of the AN-M2 gun has a projection which fits into a slot in the receiver side and the receiver slide bolts are locked by cotter pins. In the Ml gun each receiver slide has a head flange which overlaps the bottom face of the receiver side and the receiver slide bolts are locked by locking wire. In some Ml guns the receiver slides have no head flanges and the slides are riveted instead of bolted to the receiver sides. The shoulders on the bottom faces of the receiver sides serve as further means of identifying the Ml gun.
Note.—The AN-M2 gun is the new model designation for the M2 gun with which the using arm is already provided. The M2 and AN-M2 guns are identical.
4. Data.
Weight of Ml gun without magazine______________pounds__ 112
Weight of AN-M2 gun without magazine______________do___	118
Weight of tube____________________________________do____ 47. 5
Weight of magazine, 60-round. 20-mm. Ml __________do___	22
Weight of mechanism, feed, 20-mm. Ml______________do___	19
2 '
>—1227
4
TM
2 0-MM AUTOMATIC GUN Ml AND AIRCRAFT, AN-M2
|J .	..	;	.	.	.	. . .	-
RA F0 nlg9
Figure 1.—Right side of gun.
3
TM 9-1227
4-7	ORDNANCE MAINTENANCE
Over-all length of gun__________________________inches— 100. 6
Length of tube__________________________________do----- 67. 5
Muzzle velocity_______________________feet per second__2, 850
Number of grooves______________________________________ 9
Rifling is uniform with right-hand twist, slope 7°.
Section II
DISASSEMBLY, ASSEMBLY, AND INSPECTION
Paragraph
General________________________:--------------------------------------- 5
Tools___________________________________________________________________ 6
Disassembly____________________________________________________________  7
Assembly_________________________________________________________________ 8
Inspection______________________________________________________________ 9
5. General.—a. Disassembly.—Since the using arm is authorized to disassemble this gun almost completely, disassembly instructions will be found in TM 9-227. Disassembly instructions in this manual pertain only to ordnance maintenance personnel.
b. Inspection.— (1) The purpose of inspection is to determine the condition of the materiel and the repairs or adjustments necessary to maintain the materiel in serviceable condition.
(2) Administrative procedure to be followed relative to inspection and maintenance is covered in TM 9-1100.
6.	Tools.—Ordnance maintenance personnel will use the tools described in TM 9-227 in addition to those listed below.
a.	Tube locking pin tool.—This tool is used to remove the tube locking pin as described in paragraph 7e.
b.	Receiver vise (D36367).—This tool is used to hold the receiver when unscrewing the gun tube. Complete information is not available at the present time.
c.	Tube clamp (070632).—This tool is used to obtain a firm grip on the gun tube when unscrewing it from the receiver. Complete information is not available at the present time.
7.	Disassembly.—a. Muzzle brake assembly (fig. 2).— (1) Remove muzzle brake assembly from the gun tube.
(2)	Clamp muzzle brake body assembly in a vise which has soft jaws.
(3)	Use an end wrench on the hexagon shoulder of the front muzzle brake ferrule and unscrew it from the muzzle brake sleeve.
(4)	Straighten the edges of muzzle brake washer which are bent over the locking flats on muzzle brake sleeve and rear muzzle brake ferrule.
4
TM 9-1227
20-MM AUTOMATIC GUN Ml AND AIRCRAFT, AN—M2	7
MUZZLE BRAKE WASHER-1 MUZZLE BRAKE BODY ASSEMBLY-1 MUZZLE BRAKE FRONT FERRULE—]
ySigfc	ffljlOfi; ■ |	| B B B B B III fl fl M 3 Ipjilh I :	kJ
:	■ K J, <	: •	< •;; • <
f —-■	'	'	-
B ^.****4***^A*? %.	\ jAL-wxi '	? % *	'	W * '1®	All	®-	I
V	RW“«
I—MUZZLE BRAKE REAR FERRULE	RA PD 17188
j—THREAD PROTECTOR I
U-ATT ' ;	' j|
Figure 2.—Muzzle brake assembly and thread protector.
3
TM 9-1227
7-8	ORDNANCE MAINTENANCE
(5)	Fit muzzle brake wrench over the splines on rear muzzle brake ferrule and unscrew rear ferrule from muzzle brake body assembly.
(6)	Remove washer and straighten it.
1).	Mounting sleeve group (fig. 3).— (1) Remove recoil spring and two sleeves, as described in TM 9-227.
(2)	Remove mounting sleeve group.
(3)	Unscrew lubricant fitting.
(4)	Unscrew mounting body and remove mounting sleeve nut.
(5)	Remove dashpot cylinder bushing.
(6)	Remove dashpot piston and washer from dashpot cylinder.
c.	Breechblock pin.— (1) Remove breechblock pin taper pin.
(2)	Remove breechblock pin.
d.	Sear cover insert.—Tap out insert by inserting a drift through the two holes in bottom side of sear cover plate.
e.	Tube assembly.— (1) Remove gas cylinder sleeve as described in TM 9-227.
(2)	Remove cotter pin from lower front part of the receiver.
(3)	Screw tube locking pin tool into tube locking pin and remove the pin by turning the tool (fig. 4).
(4)	Unscrew tube assembly from the receiver.
/. Ejector assembly.— (1) Drive out ejector stud pin which locks the ejector stud (fig. 10) into the ejector.
(2)	Unscrew ejector stud.
8.	Assembly.—For the most part, assembly is carried out in the reverse order of disassembly.
a.	Muzzle brake assembly.— (1) When assembled, the locking flats on the rear ferrule and the muzzle brake must not line up doser than inch.
(2)	Bend muzzle brake washer over the locking flats on rear ferrule and muzzle brake body.
(3)	Stake front ferrule to muzzle brake body.
b.	Mounting sleeve group.—When assembling, make sure that the lubrication fitting holes in the dashpot cylinder, mounting body, and dashpot cylinder bushing are in line for the lubrication fitting.
c.	Sear cover insert.—Assemble with tapered edge toward the rear of the sear cover plate.
d.	Tube assembly.—Coat ground threads with antiseize compound before assembling the tube assembly to the receiver.
6
1-1227
8
TM
2 0-MM AUTOMATIC GUN Ml AND AIRCRAFT, AN—M2
DASHPOT CYLINDER-	r— MOUNTING SLEEVE PLUG
।— DASHPOT PISTON RING	I FLUSH TYPE LUBRICATING FITTING
I	MOUNTING SLEEVE DETENT
DASHPOT WASHER—I	>
MOUNTING SLEEVE NUT ----1	MOUNTING BODY __
DASHPOT PISTON-^	_
DASHPOT CYLINDER BUSHING -
RA PD 17175
Figure 3.-—Mounting sleeve group.
7
TM 9-1227
9
ORDNANCE MAINTENANCE
Figure 4.—Removing tube locking pin.
9. Inspection.—a. Exterior of gun.— (1) Check and record the serial number which wil] be found on the receiver. The number on the tube is not the serial number of the gun.
(2) Examine exterior of gun for rust, defects, and damages.
b.	Muzzle brake assembly.—Remove and disassemble muzzle brake assembly and examine for wear, rust, carbon, and broken or missing parts.
c.	Mounting sleeve assembly.— (1). Examine interior of dashpot cylinder for wear, rough surfaces, and foreign matter.
(2)	Examine fibre washer for cracks or a charred condition.
(3)	Examine piston ring to see that it is free of foreign matter and excessive wear.
(4)	Note that the ring has a tight fit in the dashpot cylinder.
(5)	The piston ring should move freely in its groove.
(6)	With the ring assembled in the dashpot, clearance between the ends of the ring should be not less than 0.002 inch and not more than 0.005 inch.
(7)	When the ring is free, clearance between the ends should be approximately 0.028 inch.
d.	Tube.— (1) Check tube for looseness in the receiver.
(2)	Inspect tube for condition of threads and splines at forward end of the tube.
8
RA PD 10813
TM 9-1227
20-MM AUTOMATIC GUN All AND AIRCRAFT, AN-M2 9-11
(3)	Examine bore for excessive wear of the lands. A bore gage is now being developed and will be issued when available.
Section III
MAINTENANCE AND REPAIR
Paragraph
Exterior of gun__________________________________________________________ 10
Driving spring guide group______________________________.________________ H
Rear buffer group________________________________________________________ 12
Breechblock assembly____________________________________________________  13
Sear cover plate group___________________________________________________ 14
Sear block group_________________________________________________________ 15
Magazine slide group_____________________________________________________ 10
Gas cylinder and sleeve group____________________________________________ 17
Muzzle brake assembly__________________________________________________
Recoil spring and mounting sleeve assembly_______________________________ 19
Tube and receiver groups_________________________________________________ 20
10.	Exterior of gun.—Remove all burs and refinish rough surfaces on exterior of gun. Remove rust or corrosion with steel wool and then coat with oil, lubricating, preservative, light. Paint surfaces, which were originally painted, with lusterless olive-drab synthetic enamel.
11.	Driving spring guide group (fig. 5).—a. Chase out threads in the rear buffer and guide head, if necessary. The threads are 25-mm-1.5-mm pitch, international form.
b.	If necessary, bevel the mouth of guide tube at the forward end to insure free movement of plunger.
c.	The guide tube should be perfectly straight and form a 90° angle with the forward face of guide head. Dents or bends will interfere with functioning of the weapon. A bent tube can be straightened by revolving it in a lathe and applying pressure against the bend. Deep dents are difficult to remove. If shallow dents cannot be removed by use of a %6-inch expansion reamer on the interior of the tube, replace the tube.
d.	The guide tube and head are assembled by sweating. The procedure is as follows: Thoroughly clean the threads to be sweat together on both the driving spring guide tube and the guide head. Heat to solder melting temperature, 475° F. Tin both threads with resin core string solder or 50/50 solder, using soldering paste as a flux. Assemble while solder tinning is at a free melting temperature. Assemble the two parts so that the locking screw openings are alined. Allow parts to cool to room temperature. Chase out locking screw threads with a No. 8-32NC-2 bottoming tap to insure proper fit of the locking screw. Tighten screw securely and stake in position.
473665°—42---2
9
9-1227
11
TM
ORDNANCE MAINTENANCE
r DRIVING SPRING GUIDE HEAD
X,. [- MACHINE SCREW
j— DRIVING SPRING GUIDE PLUNGER
L— DRIVING SPRING	RA FD 17173
Figure 5.—Driving spring guide group.
10
TM 9-1227
2 0-MM AUTOMATIC GUN Ml AND AIRCRAFT, AN—M2 12-13
12.	Rear buffer group (fig. 6).—®. The mating threads in rear end of housing and guide assembly are 25-mm-1.5-mm pitch, international form. These may be chased out when necessary.
b.	1 breads in front end of housing and in sleeve are 52-mm—1.5-mm pitch, international form. These threads may require cleaning or chasing out due to infrequent disassembly. Stake threaded sleeve into rear buffer housing in four places before gun is put into operation to insure against the unscrewing of the threaded sleeve during the firing of the gun.
c.	If necessary, file or stone the dovetail sides of the housing to make a good push fit to the receiver. If fit is too tight, it will tend to spiead the receiver 5 if too loose, it will cause the housing to hammer the receiver. Remove all sharp corners.
d.	Remove all burs from the lock plunger but take off no more metal than is absolutely necessary. Removal of excess metal will cause the plunger to fit loosely and allow the rear buffer group to move vertically in the receiver. A crushed or dented rear buffer lock bushing will hinder free operation of the plunger and should be replaced.
13.	Breechblock assembly (fig. 7).—a. Polish bolt assembly with an oilstone or crocus cloth to remove all burs and rough surfaces.
b.	Remove all burs from the inertia blocks and Straighten the guides if necessary.	7 •, ■ y
c.	Remove left breechblock slide first. Remove all burs and smooth cams on rear with a fine oilstone. Test plunger for movement. If plunger movement is sluggish, remove, clean, and refit. Next remove right slide with key and treat exactly as left slide. If key fits very loosely, remove slide key taper pin and replace the pin.
d.	Remove lock from bolt. Remove all burs and smooth the locking surface. Finish rough spots on the cam surfaces with a fine grain sharpening stone.
e.	Withdraw firing pin and remove rough surfaces around key opening. Smooth point of pin with an oilstone and remove all sharp edges or corners.
/• If firing pin hole is enlarged sufficiently to cause blown primers, replace and refit the bolt. If necessary, finish the passage in top of bolt for clearance of the ejector.
11
TM 9-1227
13	ORDNANCE MAINTENANCE
-RETAINER ASSEMBLY	. r- REAR BUFFER HOUSING
-/	£	I- REAR BUFFER SPRING
<"	’ fee r J /, f r f
iMf t.Wpp p ffr A jf^Cw. jf WnA m S. IlL s ■ I ■ l ff I I’I 1 j p 1.';	»
REAR li ri" L0CK PLUNCER	— THREADED SLEEVE
-- REAR BUFFER LOCK SPRING	*— REAR BUFFER WASHER
— REAR BUFFER LOCK BUSHING
’— REAR BUFFER LOCK COLLAR
— REAR BUFFER LOCK PLUNGER PIN
RA PD 17174
Figure 6.—Rear buffer group.
12
TM 9-1227
20-MM AUTOMATIC GUN Ml AND AIRCRAFT, AN-M2 14
j	RA PD 17180
1.	Firing pin.
2.	Bolt. body.
3.	Left breechblock slide.
4.	Left inertia block.
5.	Inertia block plunger retaining pin.
6.	Inertia block plunger spring.
7.	Inertia block plunger.
8.	Extractor pin.
9.	Breechblock slide spring guide.
10.	Breechblock slide spring.
11.	Extractor spring.
12.	Extractor.
13.	Breechblock slide key.
14.	Breechblock slide key taper pin.
15.	Breechblock pin.
16.	Breechblock pin taper pin.
17.	Breechblock lock.
1 8. Hight breechblock slide.
19. Right inertia block.
Figure 7.—Breechblock assembly.
14. Sear cover plate group (fig. 8).—a. Replace any missing or damaged screws which hold sear cover plate to the bottom of receiver. Receiver threads may require chasing out. These threads are 7-mm-1-mm pitch, international form.
b.	Replace weak sear spring. Do not stretch it except in an emergency.
c.	Check condition of thread (16-mm-1.5-mm pitch, international form) in plate. If sear spring housing is damaged beyond repair, replace it.
d.	Withdraw safety lever pin and safety lever. Remove burs from bottom of safety lever and, if worn, replace and refit to safety pin. Check free movement of safety pin and action of positioning ball. With an oilstone or fine file remove rough, bruised, burred, or binding surfaces and then refit.
e.	Withdraw bowden connection nut. If it binds or sticks on bowden connection shaft, polish the nut and shaft. If it is cracked or broken, replace it.
13
TM 9-1227
14-15
ORDNANCE MAINTENANCE
/. Grasp bowden connection shaft and actuate by hand to check for binding and spring tension. If spring is weak, replace it. If shaft binds, remove burs and gum and then polish. Use an oilstone to smooth the cam on upper end of shaft. If it is bent, straighten it or replace the shaft.
g. The threads on both nuts are 20-mm-1.5-mm pitch, international form.
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■u	- wTZ’Jki
3
!	13	12	11 to $	8	|
RA PO 17178
1.	Sear cover plate.
2.	Sear cover plate screw.
3.	Safety trigger pin.
4.	Safety lever.
5.	Safety lever pin.
6.	Trigger locking pin ball spring.
7.	Sear cover plate lock washer.
8.	Bowden connection shaft.
9.	Bowden connection shaft spring.
10.	Bowden shaft housing nut.
11.	Outer bowden connection bushing.
12.	Inner bowden connection bushing.
13.	Bowden connection nut.
14.	Sear spring housing.
15.	Sear spring.
16.	Sear spring plunger.
17.	Ball.
Figure 8.—Sear cover plate group.
15.	Sear block group (fig. 9).—a. Stone the sear to shape if burred or wavy. If wear is excessive, replace sear. If extensions on rear of sear show indications of fracture, replace it. Stone cams on rear of sear to fit bowden connection shaft.
b.	Sear should pivot freely on sear block with very little or no side play. Check free movement of plungers, then invert plungers, and check movement again. If plungers bind, stone them to fit without binding.
c.	Replace buffer blocks if chipped or cracked. Replace fiber block at least once a year, and more often if necessary.
14
TM 9-1227
20-MM AUTOMATIC GUN Ml AND AIRCRAFT, AN-M2 15
REAR SEAR BUFFER BLOCK
--r— SEAR BUFFER SPRING r- SEAR BLOCK
■I J a -ft	......i
W ' >	•Aimnv d' ’	'V; v.	/*/
S3® ■	..a„*
9F<*	®	*—sear
wF w T	W
---1- SEAR BUFFER SPRING PLUNGER	W%
।— FRONT SEAR BUFFER BLOCK
SEAR PIN-
— ‘	RA PD 17172
Figure 9.—Sear block group.
15
TM 9-1227
16-17
ORDNANCE MAINTENANCE
16.	Magazine slide group (fig. 10).—a. The group should move freely in its ways on the receiver. If possible, remove any binding due to warping, burs, or deformation of parts.
Z>. If arm fits loosely in its seat, tighten, peen, or spread the projecting rib and refit. If arm is bent or distorted beyond repair, replace it and refit, making certain key fits properly.
c.	Examine ejector for cracks or flaws and, if necessary, replace it. If ejector springs are weak, replace them. Remove all burs, bruises, and binding from ejector or its seat. If necessary, chase out threads (7-mm-l-mm pitch, international form) on the ejector stud. If stud is broken, replace it or the whole ejector assembly. If fiber washer on stud is cracked or crushed, replace it.
d.	Check free movement of latch, magazine slide lever, springs, and pins. If there is any binding, remove and clean or stone to fit. Replace broken or excessively worn parts.
e.	Try magazine on slide. If there is excessive end play, it may be necessary to replace the slide. If there is excessive end play between latch and magazine, replace the latch or repair the magazine, as the condition requires.
17.	Gas cylinder and sleeve group (fig. 11).—a. Binding or sluggish movement of the gas piston may be due to a number of causes, such as excessive carbon on the gas cylinder vent plug, a bent sleeve, damaged guide, damaged cylinder, or burred piston head. Remove all carbon and any other foreign matter. If sleeve is bent, repair or replace it. Remove burs on piston, if it is not burred excessively. If it is excessively burred, replace gas cylinder piston assembly. Remove dents in guide and burs or binding in rear end of sleeve. The guide should have a medium close fit in rear end of gas cylinder sleeve. If spring is short or kinked, replace it.
1). The gas cylinder bracket and plugs require very few repairs, except the chasing out of threads on plugs or in forward end of cylinder. Threads of the vent plug are 14-mm-1.55-mm pitch, international form. Threads of the bracket plug are 10-mm-l-mm pitch, international form.
16
TM 9-1227
20-MM AUTOMATIC GUN Ml AND AIRCRAFT, AN—M2	17
WIL4
1	3	5	6
„	I	I	I
fjllfc	hi i —-	L. 1
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•v-	.z________
4 ‘	~—-—.—' ' ”—jjfepk®
1	U A
■*--10
JlWr! 1-------16	1. M agazine slide.	10. Cotter pin.
/" A‘ I 1------17	2. Ejector.	11. Magazine slide back plate.
3. Ejector stud.	12. Magazine latch spring.
I-----18	4. Magazine slide lever pin bushing.	13. Magazine latch.
5.	Magazine slide lever.	14.	Magazine slide lever pin.
6.	Magazine slide back plate screw.	15.	Ejector spring.
7.	Ejector stud washer.	16.	Magazine slide securing arm screw.
8.	Ejector stud nut washer.	17.	Magazine slide arm screw lock washer.
9.	Ejector stud nut.	18.	Magazine slide securing arm.	RA	PD	17179
Figure 10.—Magazine slide group.
17
9-1227
17
ORDNANCE
MAINTENANCE
TM
A	R
'	C D
■j. sjKa	■>^'f-\ss ;>><• ■ rfjrtin’fl’fAnni	x ' ■ • -
? / T " /K	C\_t
■Afy*“*/	.I f e
"'"<:*:5'l
RA PD 17186
A.	Gas	cylinder	sleeve spring.	E.	Tube.	.1.	Gas cylinder taper	pin.
I>.	Gqs	cylinder	sleeve.	F.	Gas	cylinder	vent plug.	K.	Gas cylinder guide.
C.	Gas	cylinder	piston.	G.	Gas	cylinder	bracket plug.	L.	Split cotter pin.
D.	Gas	cylinder.	H.	Gas	cylinder	bracket.	M.	Gas cylinder guide	lock washer.
Figure 11.—Gas cylinder and sleeve group.
18
TEXAS STATE COLLEGE FOR WOLTM 9-1227
2 0-MM AUTOMATIC GUN AK- AND ALRC^A'bY, AN-M2 18-20
18.	Muzzle brake assembly (fig. 2).—a. Chase out threads if burred or dirty.
b. If threads of front or rear ferrule, or of the thread protector, are loose, replace the necessary parts. If threads are damaged, chase them out. Ferrule threads which mate with the muzzle brake body are 38-mm-l-mm pitch, international form. Threads of thread protector are 36-mm-2.5-mm pitch, international form.
19.	Recoil spring and mounting sleeve assembly (fig. 3).— a. Remove any burs or dents to obtain smooth fit of forward sleeve on tube.
b.	If recoil spring is distorted or short of free length, replace it.
c.	Remove burs from bearing surface of mounting sleeve nut. Check for crossed or toi n threads. Threads are 78-mm-2-mm pitch, international form.
d.	When removing plug from mounting body, clean oil opening with a No. 47 drill. Examine oil fitting in top,.of plug. If threads in mounting body are damaged, chase them out with a tap of 10-mm-1.25-mm pitch, international form.
e.	Remove all burs or rough surfaces from mounting body, dashpot mechanism, and bushing. Polish interior of dashpot with crocus cloth and remove dents, or replace dashpot.
/. Remove all burs from outer surface of piston with an oilstone. Remove all foreign matter from undersurface of piston ring.
g. If fiber washer in dashpot is cracked or badly crushed, replace it.
20. Tube and receiver groups (fig. 12).—a. Stone all scuffed surfaces to remove burs.
b. If tube is loose, use shims made of brass shim stock to tighten it.
e. If receiver has cracks or other defects that cannot be repaired to insure complete serviceability of the gun, forward entire gun to a higher echelon for disposition of receiver and parts.
d. If chamber has pits, rust, or scratches, remove these defects with a fine oilstone or crocus cloth, as the condition requires.
19
TM 9-1227
20
ORDNANCE M A INTENANCE
mwrr™ breechblock locking key
'K . Wi<
■	j~ RECEIVER slide bolt nut
Wjjlfe/.	p RECEIVER slide
- X > "X <- -%^	IWWI^W^ >
LOCK WASHER^ |	/ 1 f | f \
BREECHBLOCK LOCKING KEY PLATE SCREW-J	/	. B	M L0CK WASHER
BREECHBLOCK LOCKING KEY PLATE—*	SPLIT COTTER PIN	RECEIVER SLIDE BOLT	RA PD 10807
Figure 12.—Receiver group.
20
TM 9-1227
20-MM AUTOMATIC GUN Ml AND AIRCRAFT, AN-M2 21-23
Section IV
CARE AND PRESERVATION
Paragraph
General______________________________________;_______________ 21
Gleaning_____________________________________________________ 22
Lubrication_____ ____ _______________________________________ 23
21.	General.—Because of the extremely low temperatures prevailing at all times at high altitudes, special care must be taken to see that aircraft gun lubricants are kept free flowing. Grease, dried oil, and gum must be very carefully removed before the gun is released for service.
22.	Cleaning.—a. The gun must be disassembled for thorough cleaning and lubrication. In cleaning the bore, use cleaner, rifle-bore, applied with a sponge. If this is not available, use a strong solution of issue soap and hot water. Continue swabbing the bore until a clean flannel patch picks up no foreign matter.
b.	Clean all moving parts with solvent, dry-cleaning, and wipe dry with a firm cloth. In cleaning oil cups, oil boles, and sliding surfaces, do the necessary wiping with a firm doth. Do not allow any lint to remain in orifices or on sliding parts.
c.	Clean gas cylinder bracket and charging cylinder with a brush and then with a flannel patch. Be sure to clean the gas port in the tube and the hole in gas cylinder vent plug with wire #16 American wire gage (AWG—0.050 8-inch diameter).
d.	Examine all cleaned parts for wear, scores, burs, and cracks; then oil and assemble them.
23.	Lubrication.—a. The life of the gun depends to a great extent on proper lubrication. Give particular attention to lubrication of sliding surfaces of operating mechanism of the gun and other bearing surfaces that do not contain oil holes, plugs, or lubricating fittings.
b.	Keep grit out of the lubricant and lubricating openings.
c.	Lubricate bore, chamber, receiver, and other working parts with oil. lubricating, preservative, light. If this is not available, use oil, lubricating, for aircraft instruments and machine guns. However, when the latter lubricant is used, inspection and lubrication must be made at intervals of not more than 24 hours, as this oil has almost no preservative qualities.
d.	Always lubricate very lightly. Excess oil impairs operation at low temperatures.
21
TM 9-1227
ORDNANCE MAINTENANCE
Appendix
LIST OF REFERENCES
1. Standard Nomenclature Lists.
Ammunition, fixed, all types, for pack, light, and medium field artillery-----------------------------------------------SNLR-1
Cleaning, preserving, and lubricating materials, recoil fluids, special oils, and similar items of issue___________________SNL K-l
Gun, automatic, 20-mm, Ml and M2___________________SNL A-47
Soldering, brazing, and welding material, gases and related items----------------------------------------------SNL K-2
Truck, small arms repair, Ml_______________________SNL G-72
Current Standard Nomenclature Lists are as tabulated here. An up-to-date list of SNL’s is maintained as the Ordnance Publications for Supply Index_______________________________________OPSI
2. Technical Manuals.
Ammunition, general________________________________TM 9-1900
Cleaning, preserving, lubricating, and welding materials and similar items issued by the Ordnance Department____________TM 9-850
Ordnance maintenance procedure — materiel inspection and repair---------------------------------------------TM 9-1100
20-mm automatic gun Ml and 20-mm aircraft automatic gun AN-M2----------------------------------------------TM 9-227
[A. G. 062.11 (7-1-42). |
By order oe the Secretary of War :
G. C. MARSHALL.
67C