Muncie 4-speed Transmission - Casting Numbers

| 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 |
| 1963 - 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 |
|
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 gearset. 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 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" VIN Stamping
Most transmissions since 1962 will also have a CON VIN 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. An example of the 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.
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.
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).
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
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 CONVIN stamping positively identifies
this as a 1964 Fremont Chevelle because the letter "H" was only used
in 1964 for Fremont.

This example, P9T04A, is for a 1969 M20 dated
December 4.


Note early 1969 Muncie (September 4) stamping without the type (A, B,
or C) designation.
This example, P0B02C, is for
a 1970 M22 dated February 2.

This example, P0C17A, is for
a 1970 M20 dated March 17.

Transmission Identification

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.

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.

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

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

1966 L34/L35 M20

1970 L34/:S3/:48/L79 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.

Reproduction transmission tag.
M20/M21 vs. M22 gear sets & input shaft

Both of these are M20. The one above is a 10-spline unit while the one below is a 26-spline unit. Note the much finer splines on the one below.

Another 10-spline versus 26 spline comparison


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

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

RPO M22 gear set
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) Calvin'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 & OLong 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. |
It should be noted that there are many different web sites 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.