ChevelleWorld Decode Links

Manual Muncie Four-Speed Transmission - M20/M21/M22 (Group 4.000)

Case Casting	Year(s)	Ratio/M20	Ratio/M21	Ratio/M22
3831704	1963 Only	M20 2.56 1st	M21 2.20 1st	n/a
3851325 ⁽¹⁾	1964 - 1965	M20 2.56 1st	M21 2.20 1st	n/a
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.

Muncie 4-Speed Casting Numbers

Main Case

3831704	1963 Only small 6207NR Front Bearing, Patent Pending, 7/8" Bore
3839606	1963 - 1964 Regular Bearing, Patent Pending, 7/8" Bore
3864_____	1964 Milled off last 3 digits 7/8" Bore, Patent Number (late 1964 with Patent Number 3088336)
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-side speedometer 27-spline output shaft
3846429	1963 "Thin Fin Tail" driver-side speedometer 27-spline output shaft 1964-1965 - Regular thick lower web, driver-side speedometer 27-spline output shaft
384362	1964 Long tail, driver-side speedometer, 27-spline output shaft (fairly rare in Chevelles)
9779246	1964 - 1968 Pontiac & Oldsmobile Long Tail drive-sider Speedo 27-spline output shaft
3857584	1966 - 1970 passenger-side speedometer, 27-spline output shaft (1970 M20/M21 only)
3978764	1970 - 1974 passenger-side speedometer, 32-spline output shaft (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.

To see just what four-speed manual transmissions were used behind which specific 6- and 8-cylinder engines, see my Chevelle Facts Guide CD.

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/M22 tooth count - 27, 22, 19, 17
M20 tooth count - 29, 22, 19, 17

1966-1974:
M21/M22 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 SS 396 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 P0C25B. This decodes to 1970 (0), March (C), 25th, M21 (B).

Note: By the same token a transmission with a date such as P0P29C could be 1970 (0), September (P0 29th (29), M22 (C). The second character denotes the model year and not the calendar year; so September 29 would be calendar year 1969 build for a 1970 model year car. The ratio designation letter did not 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

© ChevelleWorld
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.

1965

Currently I have no 1965 examples.

1966

1967

* 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

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

1969

© ChevelleWorld
Note early 1969 Muncie (September 4, 1968) stamping without the type (A, B, or C) designation. The ratio designation letter did not appear until around October 1, 1968 for the 1969 model year.

1970

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

1971

1972

Hurst Competion Plus™ Shifter & Installation Kit

Hurst Street Super Shifter™ & Installation Kit

The Hurst Super Shifter will NOT work with a factory console. The shifter is mounted higher on the transmission to allow for straight shifter arms.

Transmission Side Cover Identification

© ChevelleWorld
Note: 1964 and 1965 have the speedometer cable exiting on the driver's side while 1966 and later exit on the passenger's side of the tail housing. This does not affect the Hurst shifter mounting.

© ChevelleWorld
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. See examples below.

Those installing a 1969 or later Muncie into 1964-1967 and 1968 Chevelle and using a Hurst Competition Plus shifter must get the proper shifter tower assembly for the 1964-1967 and 1968 Chevelle and the installation kit for the 1969-1972 Muncie due to the shift arms bolting onto the transmission differently. The same is true if you are going to install a 1964-1967 Muncie transmission into a 1968-1972 Chevelle. The Hurst Competition Plus Shifter fits both with and without console.

The Saginaw (iron case) 4-speed uses different shift tower and installation kits. Make sure you know which make 4-speed you have.

Year Chevelle	Muncie Type	Shifter Number	Installation Kit Number
1964-1967	Muncie (451)	3917308	3733163
1968	Muncie (451)	3918014	3733163
1969-1972	Muncie (452/453)	3918014	3737834
1973-1974	Muncie (453)	3916848	3737897

© ChevelleWorld
The Saginaw 4-speed transmission is shown for "M20 PROD V8 ENG & R.P.O. L30". Easily identified by the reverse gear being on the main case instead of the tail housing in 1966 and later Assembly Instruction Manuals.

Part Number Tags

Some of these tag numbers/codes may apply to other Chevrolet models (not just Chevelles) in these years.
Those listed in red have sample images.
Those listed in orange have been found but numbers do not appear in any Chevelle Assembly Manual.

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 3924703 L35
1968 M20 3915079 L34, L35, L78, L79
1968 M21 3915085 L34, L35, L78, L79
1968 M21 3924706 L78
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 3271516 WD
1972 M20 3974484 WN
1972 M22 6271517 WJ

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.

M20/M21 vs. M22 gear sets & input shaft

From left to right:
No grooves - 1963-1965 M20 and 1965-1972 M22
Two grooves - 1966-1970 M20 (1971-1974 M20 has 26 splines as opposed to the 10 spline early unit shown here)
One groove - 1963-1970 M21 (1971-1974 M20 has 26 splines as opposed to the 10 spline early unit shown here)

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.

Another 10-spline versus 26 spline comparison.

Warranty Replacement

Transmissions were also subject to replacement, 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.

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.