Muncie 4-speed Transmission
| Casting | Year(s) | Ratio | ||
| 3831704 | 1963 Only | M20 2.56 1st | M21 2.20 1st | |
| 3851325 | 1964 - 1965 | M20 2.56 1st | M21 2.20 1st | |
| 3885010 | 1965 - 1967 | M20 2.52 1st | M21 2.20 1st | M22 2.20 1st |
| 3925660 | 1968 - 1970 | 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 |
| Years | Type | Ring(s) | Ratio | Reverse |
| 1963 - 1965 | M20 | None | 2.56 / 1.91 / 1.48 / 1.00 | 3.16 |
| 1966 - 1974 | M20 | Two | 2.52 / 1.88 / 1.46 / 1.00 | 3.11 |
| 1963 - 1974 | M21 | One | 2.20 / 1.64 / 1.28 / 1.00 | 2.27 |
| 1967 - 1974 | M22 | None | 2.20 / 1.64 / 1.28 / 1.00 | 2.27 |
|
Ratio of drop between gears |
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| 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% | 3.16 |
| M20 (66>) | 2.52 | 1.88 | 25.40% | 1.46 | 22.34% | 1.00 | 31.51% | 3.11 |
| M21/M22 | 2.20 | 1.64 | 25.45% | 1.28 | 21.95% | 1.00 | 21.88% | 2.27 |
| Years | Type | Ring(s) | Spline |
Input Shaft Tooth Count |
| 1963 - 1965 | M20 | None | 10 | 24 |
| 1966 - 1974 | M20 | Two | 26 | 21 |
| 1970 - 1974 | M21 | One | 26 | 26 |
| 1969 - 1974 | M22 | None | 26 | 26 |
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.
The M22 was only produced in a close ratio version. It would not have been sold with a 3.31 or higher (lower numerically) rear end ratio unless it has been changed by an owner somewhere along the line.
The date code indicates the particular year the transmission was built for.
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. P0101 indicates January (first 01) 1st day (second 01).
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, P0B02C, is
for a 1970 M22 dated February 2.
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.
M20/M21 vs. M22 gear sets
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
