Muncie 4-speed Transmission (Group 4.000)

                               

Casting

Year(s)

Ratio

 

 

3831704

1963 Only

M20 2.56 1st

M21 2.20 1st 

 

3851325 (1)

1964 - 1965

M20 2.56 1st

M21 2.20 1st 

 

3885010

1966 - 1967

M20 2.52 1st

M21 2.20 1st

M22 2.20 1st

3925660

1968 - 1969

M20 2.52 1st

M21 2.20 1st

M22 2.20 1st

3925661

1970 - 1974

M20 2.52 1st

M21 2.20 1st

M22 2.20 1st

(1) The M22 case casting is unknown at this time

 

Years

Type

Ring(s)

Ratio

Reverse

1963 - 1965

M20

None

2.56 / 1.91 / 1.48 / 1.00

2.64

1966 - 1975

M20

Two

2.52 / 1.88 / 1.46 / 1.00

2.59

1964 - 1975

M21

One

2.20 / 1.64 / 1.28 / 1.00

2.26

1967 - 1972(2)

M22

None

2.20 / 1.64 / 1.28 / 1.00

2.26

 

 

 

 

 

Saginaw M20

 

M20

One

2.54 / 1.80 / 1.44 / 1.00

 

 

Ratio of drop between gears

Transmission

1st

2nd

% Reduction

3rd

% Reduction

4th

% Reduction

Reverse

M20 (63-65)

2.56

1.91

25.39%

1.48

22.51%

1.00

32.43%

2.64

M20 (66>)

2.52

1.88

25.40%

1.46

22.34%

1.00

31.51%

2.59

M21/M22

2.20

1.64

25.45%

1.28

21.95%

1.00

21.88%

2.26

 

Years

Type

Ring(s)/Groove(s)

Input Splines

Output Splines

1963 - 1965

M20

None

10

27

1966 - 1970

M20

Two

10

27

1971 - 1974

M20

Two

26

32

1963 - 1965

M21

One

10

27

1966 - 1970

M21

One

10

27

1971 - 1974

M21

One

26

32

1965 - 1970(1)

M22

None

10

27

1970 - 1972(2)

M22

None

26

32

 (1) All M22 applications except 454 engine.
 (2) All 454 Chevelle applications. The M22 was produced in 1973 and 1974 for other GM divisions.

All wide ratio transmissions are designated M20 and close ratio transmissions are designated M21 or M22 depending on the gear set.  From 1963 to 1965 the close ratio M21 was ordered under the M20 RPO code and the factory decided whether to install a wide ratio M20 or close ratio M21 depending on the rear axle ratio specified. From 1966 on, the customer could decide the transmission selection.

The M20 and M21 have the same cut to the gears, the M22 is a straighter cut resulting in a greater strength. Since the M20 and M21 look the same one way to tell them apart (aside from the grooves on the input shaft) is the tooth count on the cluster gear.

1963-1965:
M21 tooth count - 27, 22, 19, 17
M20 tooth count - 29, 22, 19, 17

1966-1974:
M21 tooth count - 27, 22, 19, 17
M20 tooth count - 25, 22, 19, 17

There are 7 different input shaft possibilities on a Muncie. All 26 spline inputs came with 32 spline output shafts and all 10 spline inputs came with 27 spline output shafts. It is commonly mistaken that all "fine spline" 26 spline input shafts are M22 transmissions. The M22 transmission has a 20 degree helix angle on the gear set as opposed to a 45 degree angle found on the M20 and M21 and were made with a higher nickel alloy. The straighter angle was designed to produce less end loading of the gear train and less heat but created more noise, thus the nickname "Rock Crusher" was born. Another misconception is if you have a drain plug you have a M22. This was only true when the first M22 boxes were created; all 3925661 castings (1970-1974) had drain plugs.

Partial or "concealed/confidential" VIN Stamping

Most transmissions since 1962 will also have a CON VIN (concealed or confidential VIN, whichever term one likes) stamped in the case generally along the top ridge or even on a mounting surface such as the flange mounting the tail shaft to the main case noting the original car it was installed in. The stamped CON VIN is the same that is stamped on the engine pad in all 1968 and later Chevelles, and generally 1966 & 1967 SS396 Chevelles with an alphanumeric set of characters like 7K123456; model year, assembly plant, and vehicle sequence number. The engine and transmission were stamped at the same time by the final assembly plant

Most  plants stamped the CON VIN on the top while some stamped it on the side near the date code. Characteristics of the VIN stamp, including the location, type size of the stamp, and the method (individual or 'gang' stamped) was chosen by the individual assembly plant.

1963 through 1966

Date codes from 1963 to 1966 included only the month & day; such as P0110 indicates January (01) 10th day (10).

1967 and 1968

In 1967 and 1968, the date code got a year designator and a letter designator for the month such as P8A01, meaning 1968 (8), January (A) 1st (01). Note that a Muncie dated with a September to December build date was actually built the prior calendar year. An example would be the date code P8T13, meaning 1968 (8), December (T), 13th. The transmission was assembled December 13, 1967 for the 1968 model year. Likewise a P8S28 stamp means 1968 (8), September (S), 28th of the calendar year 1967 for a 1968 Chevelle. See second example below.

1969 through 1974

The 1969 to 1974 Muncie got a ratio designating letter at the end of the date code. An example would be P2C25B. This decodes to 1972 (2), March (C), 25th, M21 (B).

Note: The ratio designation letter didn't appear until around October 1, 1968 for the 1969 model year so any Muncie in a 1969 Chevelle dated August (M) or September (P) would probably not have the A, B, or C designation.

Date codes:

A ~ January

B ~ February

C ~ March

D ~ April

E ~ May

H ~ June

K ~ July

M ~ August

P ~ September

R ~ October

S ~ November

T ~ December

Type Codes: A ~ M20, B ~ M21, C ~ M22

Muncie 4-Speed Casting Numbers

Main Case

3831704

1963 Only small 6207NR Front Bearing ,Pat. Pending, 7/8" Bore

3839606

1964 Regular Bearing Pat. Pending, 7/8" Bore

3864_____

1964 Milled off last 3 digits 7/8" Bore Patent Pending

3851325

1964 - 1965 7/8" Bore Patent Pending Mostly 1964

3851325

1964 - 1965 7/8" Bore Patent Number

3885010

1966 - 1967 1" Bore Patent Number

3925660 (3)

1968 - 1969 1" Bore Patent Number

3925661

1970 - 1974 (some early 1975 cars) 1" Bore Patent Number

(3) Colvin's "by the numbers" book notes that Muncie data shows 1968 and 1969 M22 transmissions received main case #3925661. All documented M22 transmissions have the #3925660 main case. It is unclear why there is the discrepancy.

Tail Housing

3831731

1963 "Thin Fin Tail" Driver speedometer 27 spline

3846429

1964 - 1970 "Thin Fin Tail" Driver speedometer 27 spline

9779246

1964 - 1968 Pontiac & Oldsmobile Long Tail Driver Speedo 27 Spline

3857584

1966 - 1970 Passenger side speedometer, 27 spline (all except Chevelle)

3978764

1970 - 1974 passenger speedometer, 32 spline output (Chevelle only)

Side Cover

3831707

1963-1965 Early side cover stud type shift shafts

3884685

1966-1968 Used with 3857584 tail stud type shift shafts

3950306

1969 Short boss with bolt on type shift shafts - no switches

3952642

Long boss bolt on type w/ TCS switch on 3-4 (not verified)

3952648

1970-74 Short boss bolt on type w/ TCS switch on 3-4

335308

Long boss bolt on type with neutral safety switch. Some have a boss for the switch that is cast but not machined on 1-2.



Even after 45 years, the Muncie four-speed is still a very popular transmission with Chevrolet muscle car enthusiasts, and it is easily adaptable to a wide range of GM and other non-GM vehicles. It debuted in 1963 Chevrolet full-size cars and Corvettes, and was originally available with two first-gear ratios: 2.20 to 1 and 2.56 to 1. The Muncie 2.56 to 1 ratio gearbox became the commonly used unit for four-speed applications beginning with the 1964 Chevelles and Novas. It was also used in non-Chevys, in the Pontiac Tempest and Oldsmobile F-85/Cutlass models. The easiest way to identify a Muncie transmission is by its side cover. Muncies have seven bolts, two bolts on the bottom, three on the top and one down each side. There are also 2-3/8-inch studs for the shifter levers protruding from the side cover. The Saginaw four-speed is similar in appearance; however, the Muncie's side cover is more irregularly shaped, the top of the cover is wider than the bottom. Another way of identifying the Muncie is the reverse lever. The Muncie reverse lever stud is in the extension housing, whereas the Saginaw has a third stud through the side cover for the reverse shift lever. All Muncie transmission cases are made of aluminum, and the identifying casting numbers are prominently stamped in the passenger side of the transmission case.
Muncie Four Speed casting numbers identify all aluminum Muncie main cases:
3831704 - 1963 only
3851325 - 1964-'65
3885010 - 1965-'67
3925660 - 1968-'70
3925661 - 1971-'74
The 2.20:1 ratio first gear is usually referred to as the Muncie M21 transmission; based on GM's production codes. The 2.56:1 ratio is referred to as the M20. Muncie also had a second close-ratio transmission coded M22 and known as the "rock crusher," which was used behind performance big-block engines and in some Corvettes. These M22 units are highly sought-after for performance applications. The term "rock crusher" refers to the noise made by the meshing of the strong-er helical gears in the M22. Later model 1971-'74 Muncies are also popular with racers, because these transmissions used a 32-spline output that is stronger than the early 27-spline, and the newer units seem to perform better in high-rpm and high-horsepower applications. One of these vehicles is where you are most likely to find a Muncie four-speed. Ten-spline input-shaft with 27-spline output-shaft units came in these vehicles:
1964-'70 Chevrolet Corvette and Pontiac Tempest
1967-'70 Buick Special, Firebird and Camaro
1969-'70 Grand Prix
1963-'69 Impala
1965-'70 Nova and Olds Cutlass/F-85 and Chevelle (except 70 w/454)
1970 Monte Carlo
Twenty-six-spline input with 32-spline output shaft units came in these vehicles:
1970 Chevelle 454
1971 Grand Prix and Monte Carlo
1971-'72 Special and Tempest
1971-'73 Nova, F85 and Cutlass
1971-'74 Firebird, Camaro, El Camino, Corvette, Chevelle and Sprint
1973 Buick Century
1973-'74 Pontiac Le Mans
The 27-spline output shafts have the same diameter and spline as the output shafts used on many GM automatic transmissions; however, the yokes are not always the same length. You can identify the various M20, M21 and M22 transmissions by the input shaft. GM marked these units in this way so production line laborers could identify them during installation. Here is a quick spline-groove identification chart:
1963-'65 M20 2.56:1 ratio 10-spline 7/8-inch shaft - no grooves- 27-spline output shaft
1966-'70 M20 2.52:1 ratio 10-spline 1-inch shaft - 2 grooves - 27-spline output shaft
1971-'74 M20 2.52:1 ratio 26-spline 1-inch shaft - 2 grooves - 32-spline output shaft
1963-'65 M21 2.20:1 ratio 10-spline 7/8-inch shaft - 1 groove - 27-spline output shaft
1966-'70 M21 2.20:1 ratio 10-spline 1-inch shaft - 1 groove - 27-spline output shaft
1971-'74 M21 2.20:1 ratio 26-spline 1-inch shaft - 1 groove - 32-spline output shaft
1965-'70 M22 2.20:1 ratio 10-spline 1-inch shaft - no groove - 27-spline output shaft
1971-'74 M22 2.20:1 ratio 26-spline 1-inch shaft - no groove - 32-spline output shaft

STRONGEST Direct Replacement M22 With NEW Muncie AUTO GEAR SUPER CASE With The Strongest M22w WIDE RATIO or Close Ratio Gears for a 4 SPEED TRANSMISSION. A GREAT HEAVY DUTY ROCKCRUSHER FOR YOUR CAMARO, NOVA, CHEVELLE, CORVETTE, GTO,442. THE MAIN CASE IS THE STRONGEST MADE with fill and drain plugs. AG Steel Mid-Plate. AG Roller Side Cover. SUPER TAIL HOUSING IS THE 27 SPLINE (TH-350) (passenger side speedo) OUTPUT. COMPLETE PROFESSIONALLY BUILT MUNCIE. (ZERO MILES SINCE BUILT). WILL WORK IN ALL 63-74 GM CARS WITH 10 SPLINE DISK. NEW 10spl INPUT NO RINGS 21 TOOTH INPUT SHAFT. 27 SPLINE OUTPUT SHAFT. NEW CLUSTER GEAR, TOOTH COUNT=25-22-19-17.GEAR RATIO = 2.56/1.75/1.36/1.00, ALL NEW FORWARD GEARS, 12% STRONGER THAN OEM. 3.55 SPEEDO GEAR. NEW STEEL MID-PLATE. NEW I” CLUSTER PIN, NEW CLUSTER NEEDLE BEARINGS AND TUBE, NEW FRONT 307SL SHIELDED BEARING, NEW HD MAIN SUPPORT BEARING, NEW SYNCRO HUBS AND SLEEVES, NEW HIGH PERFORMANCE BRASS SYNHRONIZER RINGS, KEYS AND SPRINGS, NEW WASHERS, SHIMS , BUSHINGS, PERFORMANCE GASKETS, CAGE BEARINGS AND SEALS


Gears t Inapect all gears and replace all that are worn or ‘ ‘ g damaged. ’ y h V Reverse Idler _ "-’ . l l. The bushings used in the idler gear are pressed into g}! t V .. , l the gear then peened into holes in the bores, and are “.2 .: "‘51"; '2 N v bored in place. This lnsures the positive alignment ”if?“ Wank‘“ of the bushings and their shafts, as well as prOper ' ‘ '5?“ . g5, ‘ I meshing o! the gears. Because of the high degree oi 1431‘ . i “’9‘ fl}; «I. accuracy to which these parts are machined, the ugh»). “an is (g up“? ‘7 _) :t bushings are not serviced separately. I l. £1 1; -' 'Il, ~ r *' 3; .. t
2. Check bushings for excessive wear by using a nar- gag: 73:5? I, , "CLWH "F" row feeler gauge between the shaft and the bushing - Z'If/fi"._a‘ l \‘ KEV f or use a micrometer. The prOper clearance is from ..fi'-7,:'-~.-t-~.~‘., ‘ ’ ‘ p, 5-; 003" to .005". Ed‘s-39$}... v ‘9' ~ “Mfg Repairs ‘ - .l .' f . l , ; X; Reverse Shifter Shah and Seal Replacement - r r -~ - -_-m, '._, _ ,_ A. l. With case extension removed from transmission the " J“ ‘ " "J reverse shifter shaft lock pin will already be re- moved‘ (See Step 4 under Disassembly.) Fug. l2M-lnstollmg Synchronizmg ng
2' Remove Shift fork . mission lubricant, then install new oil seal using Tool
3- carefully drive Shifter Shaft mt" case ”ammo“, J-5154. Prelubricate seal between lips with cup grease. allowing ball detent to drop into case. Remove shaft and ban detent spring. Clutch Keys and Springs Replacement
4. Place ball detent spring into detent Spring hole and ”on: The clutch hubs and sliding sleeves are grom inside emens‘o" install Shift" Shaft mu" into a select fit assembly and should be kept tOgether its opening until the detent plate is butted against in- as originally assembled, but the ”wee keys and Side or ”tension housmg‘ two springs may be replaced if worn or broken.
5. Place detent ball on spring (fig. 11M) and, holding ban down with yo‘nv thumb or a stumble tool push 1. PUBb the Mb from the Sliding Sleeve. Th8 keys Win the shifter shaft back in away irom case until it is {an free and the Sprinfis may be 83811)! removed: directly over the ban and tum um“ the ban drops 2. Place the two springs in position (one on each side into detent on the shah detent plate. of the hub). so all three keys are encased by both 6. Install shift fork. springs. Place the keys in position and, holding them
in place, slide the hub into the sleeve. NOTE: Do not drive the shifter shaft lock pin into place until the extension has been installed No75: The Sleeve With the (3%mede edge on the transmission case. opposite the fork groove identifies


the snap ring further than is necessary for _-L assembly. ,. , , l 9 l 6. Install the third speed gear (hub to front of trans- mission) and the third speed gear synchronizing ring ‘ I
(notches to front of transmission). x :- ,‘ i 7. Install the third and fourth speed gear clutch as- ' r ' 5‘ "WV” sembly (hub and sliding sleeve) with both sleeve F“: taper and hub toward the front. making sure the keys 5' ‘ ‘ //////,.fj ‘1 in the hub correspond to the notches in the third f‘. speed gear synchronizing ring. -1, , A;
8. Install snap ring in the groove in mainshaft in front = ‘ f I ll of the third and fourth speed clutch assembly, with 3.. 7 ends oi‘ snap ring seated behind spline teeth. - . 3
9. Install the rear bearing retainer (fig. 1oM). Spread 3'- the snap ring in the plate to allow the snap ring to F5 drOp around the rear bearing and press on the end :3? , ._ _ 7 of the mainshaft until the snap ring engages the if, ._ ,1 § i groove in the rear bearing. . ‘ a - r - W,
10. Install the reverse gear (shift collar to rear).
ll. Press speedometer drive gear onto the mainshaft using a suitable press plate such as J-1453. Position F'g' MM-Immmng Specdom'er Dmc Gem the speedometer gear to get a measurement of 4-7/8" from the forward


CAUTION: Be sure main drive ear 8. Install main drive gear and pilot bearings through both the COWWW the side cover opening and into position in trans- W mission front bore.
9. Place gasket in position on front face of rear bearing 13. With the guide pin in rear bearing retainer aligned retainer. with hole in rear of case, tap rear bearing retainer
10. Install the fourth speed synchronizing ring on main into position with a soft hammer. drive gear with the notches toward the rear of the 14. From the rear of the case. insert the rear reverse transmission. idler gear, engaging the splines with the portion of
11. Position the reverse idler gear thrust washer the front gear inside the case. (tinged) on the machined face of the ear 088* in the 15. Using heavy grease, place gasket in position on rear case for the reverse idler shaft and hold with heavy face of rear bearing retainer. grease. Position the front reverse idler sear next to 16. Install the remaining flat thrust washer on reverse the thrust washer, With the hub ”1011‘s toward rear 0‘ idler shaft. If new idler shaft is being used. drive the case. out the roll pin and press it into new shaft.
17. Install reverse idler shaft roll pin, and thrust CAUTION: Before attempting to install main- shaft assembly to case, snag-{63W- synchro- washer infto gears afihfrontt has: of case. Make sure n__i_zi clutch sleeve' Forward into fourth speed to pickup ront tange rus was er. detent position (fighgfifi. NOTE: Roll pin should be in a vertical position.
12. Lower the mainshaft assembly into the case making 18. Pull reverse shifter shaft to left side of extension certain the notches on the fourth speed synchronizing and rotate shaft to bring reverse shift fork forward ring correspond to the keys in the clutch assembly (fig. 18M). [-fi


in extension (reverse detent position). Start the ex- bearing fully seats against shoulder on gear. Torque tension onto the transmission case (fig. 19M), while retaining nut to 40 ft. lbs. and lock in place by slowly pushing in on the shifter shaft to engage the staking securely into main drive gear shaft hole with shift fork with the reverse gear shift collar. Then a center punch. Care must be used to avoid damaging pilot the reverse idler shaft into the extension the threads on the shaft. housing permitting the extension to slide onto the 23. Install the main drive gear bearing retainer, gasket, transmission case. four attaching bolts and (2) strip bolt lock re- I. Install 6 extension and retainer-to- case attaching tainers, using a suitable sealer on bolts. Torque to bolts. Torque upper 3 bolts and lower 3 bolts to specification. specification. , 24. Shift mainshaft 3-4 sliding clutch sleeve into neutral i. Push or pull reverse shifter shaft to line up groove position and 1-2 sliding clutch sleeve into second in the shaft with the holes in the boss and drive in gear (forward) detent position. Shift side cover 3-4 the lock pin. Install shifter lever. shifter lever into neutral detent and 1-2 shifter lever Press bearing onto main drive gear (snap ring into second gear detent position. groove to front) and into 0888 until several main 25. Install side cover gasket and carefully position side drive gear retaining nut threads are mad. cover into place. There is a dowel pin in the cover 2. Lock transmission up by shifting into two gears. In- to assure proper alignment with the case. Install at- stall main drive 8933‘ retaining nut on the gear shaft taching bolts and tighten evenly to avoid side cover and draw it up tight using Tool J-0933. Be sure distortion. Torque to specification.