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

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.