Muncie 4-speed Transmission (Group 4.000) - Casting Numbers

CHEVELLESTUFF
 

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.

There is strong evidence the Saginaw M20 wide ratio transmission did not appear before February/March of 1966, so all M20 4-speeds are assumed to be Muncie before this date. If anyone as documented proof, such as a warranty card (Protect-O-Plate) showing a Saginaw coded M20 (R) prior to February/March of 1966 is encouraged to send a readable image of the warranty card for verification.

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, partial VIN, or VIN derivative - 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

1964

CHEVELLESTUFF
Here is an example from a 1964 Chevelle from Fremont, CA. Note the H103065 stamped on the flange and the date is shown by the P1030 stamping in the upper right of this photo. The VIN derivative stamping positively identifies this as a 1964 Fremont Chevelle because the letter "H" was only used in 1964 for Fremont.

CHEVELLESTUFF
Another Fremont example as noted by the leading "H" in the VIN derivative.

1966

CHEVELLESTUFF
Note 1966 date format, February 24

1967

CHEVELLESTUFF
1967 Muncie coded P7S29 for November 29, 1966.*

CHEVELLESTUFF
1967 Muncie coded P7T21 for December 21, 1966.*

* Note the extra smaller "7" or possibly "v" after the dates. The exact meaning of this character is not known; there are suspicions, but no facts. If you have a Muncie with a similar character after the date, can get a photograph of it and KNOW whether it's an M20, M21, or M22, please pass it along.

1968

CHEVELLESTUFF
1968 Muncie transmission tag #3924706. Date stamped P8P27 for Muncie, Septermber 27, 1967 and partial VIN of 18K116891. Reported to be M21 but cannot find tag number to verify.

CHEVELLESTUFF
1968 Muncie coded P8S28 for November 28, 1967.

CHEVELLESTUFF
1968 Muncie coded P8S15 for November 15, 1967

CHEVELLESTUFF
Same transmission above with partial VIN stamping.

CHEVELLESTUFF
1968 Muncie coded P8C07 for March 27, 1968.

CHEVELLESTUFF
Same transmission above with partial VIN stamping.

1969

03/21/2010
Note early 1969 Muncie (September 4, 1968) stamping without the type (A, B, or C) designation. The ratio designation letter didn't appear until around October 1, 1968 for the 1969 model year.

CHEVELLESTUFF
1969 Muncie P9S11 ~ November 11, 1968 - M20

CHEVELLESTUFF
1969 Muncie P9T04A ~ December 4, 1968 - M20

CHEVELLESTUFF
1969 Muncie P9A31C ~January 31, 1969 - M22 with partial VIN of 19K405888.

09/23/2014|
1969 Muncie P9C15A ~ March 15, 1969 - M20

1970

CHEVELLESTUFF
P0R23A ~ M20 dated October 23, 1969

CHEVELLESTUFF
P0R29A ~ M20 dated October 29, 1969. Partial VIN for Oshawa, ON Chevelle.

09/30/2013
P0T30C ~ M22 dated December 30, 1969.

CHEVELLESTUFF
P0B02C ~ M22 dated February 2, 1970.

04/30/2010
P0C17A ~ M20 dated March 17, 1970

CHEVELLESTUFF
Warranty transmission. Note CT0 (Chevrolet, Transmission, 1970) and 42862 serial number.

1972

08/19/2012
P2P28A ~ M20 dated September 28, 1971.

Courtesy Rick Nelson
WO stamp found on a 1970 Flint LS6 M22.

Transmission Identification

CHEVELLESTUFF  CHEVELLESTUFF
1964 to 1968 Muncie 4-speed transmissions can be identified from later models by the way the transmission arms attach. Early Muncie side covers had a stud and the shift arm attached with a nut on the stud while the 1969 and later model side covers had a hole and the shift arms attached with a bolt.

CHEVELLESTUFF
The Saginaw 4-speed transmission was also used and shown for "M20 PROD V-8 ENG & R.P.O. L30". Easily identified by the reverse gear being on the main case instead of the tail housing.
Saginaw 4-speed

CHEVELLESTUFF
Early (64-68) side plate. Note the shift arms are held in place by a stud/nut arrangement.

CHEVELLESTUFF
Late (69 >) side plate. Note the shift arms are held in place by bolts.

Part Number Tags

CHEVELLESTUFF
1966 L34/L35 M20

CHEVELLESTUFF
Found on 1966 L78, M21

CHEVELLESTUFF
Found on 1968 L35, M20

CHEVELLESTUFF
Found on 1969 L78, M21

CHEVELLESTUFF
1970 L34/LS3/L48/L78, M20

CHEVELLESTUFF
1970 L34/LS3/L48/L78, M21

CHEVELLESTUFF
1970 LS6/LS6, M22

08/19/2012
Found on 1972 LS3, M20

If you're lucky enough to find one of these attached to your Muncie 4-speed it indicates the GM part number of the transmission assembly aiding you further in identifying the transmission. Be forewarned these tags are being reproduced, see example below, so be wary of the tag's condition before accepting the fact it is the correct transmission for your application.  The WB and WO suffix in the two examples above also correspond to the broadcast codes on the build sheet.

CHEVELLESTUFF
Reproduction transmission tag.

Some of these tag numbers/codes may apply to other Chevrolets in these years.

1964 M20 3841435
1965 M20 3857577
1965 M20 3857578 Z16
1966 M20 3870357 L34, L35
1966 M20 3890534 L78
1966 M21 3877459 ALL
1966 M22 3879993 L78
1967 M20 3870357 L34, L35, L79
1967 M21 3877459 ALL
1967 M22 3879993 L78
1968 M20 3915079 L34, L35, L78, L79
1968 M21 3915085 L34, L35, L78, L79
1968 M22 3925626 L78
1969 M20 3950301 L48, L35
1969 M20 3946797 L34, L78
1969 M21 3946798 ALL
1969 M22 3950318 L78, COPO L72
1970 M20 3952659 WB
1970 M21 3968012 (disc.3-11-70)
1970 M21 3968011 WL
1970 M22 3978766 WO
1971 M20 3974484 WN
1971 M22 3978766 WO
1972 M20 3974484 WN
1972 M22 6271517 WJ

M20/M21 vs. M22 gear sets & input shaft

04/30/2010

The input shaft above is a 10-spline unit while the one below is a 26-spline unit.  Note the much finer splines on the one below.

04/30/2010

Another 10-spline versus 26 spline comparison
04/30/2010  04/30/2010

CHEVELLESTUFF
Note thicker teeth on the M22 input shaft as well as the straighter cut gears.

CHEVELLESTUFF
RPO M20/M21 gear set - compare the angle cut of the gears to the M22 below.

CHEVELLESTUFF
RPO M22 gear set

Warranty Replacement

Transmissions were also subject to replacment, be it just the case or the entire transmission. These will have a stamp something like "CC" for Chevrolet Case or "CT" for Chevrolet Transmission. The same information about partial VINs on engine blocks also applies to transmissions.

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 (1970 M20/M21 only)
3978764 1970 - 1974 Passenger side speedometer, 32 spline output (1970 M22, all 1971 thru 1975)

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.

It should be noted that there are many different websites and publications with Muncie transmission information that seems to be in conflict as to various casting numbers, input/output shaft splines, etc. I've tried to weed through as much of this as I can and bring you the consensus of what's correct. If you have some reference or documentation that contradicts anything here I'd love to hear from you.