Muncie 4-speed Transmission (Group 4.000)

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.

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, 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

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 (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.

Even after 45 years, the Muncie four-speed is still a very popular transmission with Chevrolet muscle car enthusiasts, and it is easily adaptable to a wide range of GM and other non-GM vehicles. It debuted in 1963 Chevrolet full-size cars and Corvettes, and was originally available with two first-gear ratios: 2.20 to 1 and 2.56 to 1. The Muncie 2.56 to 1 ratio gearbox became the commonly used unit for four-speed applications beginning with the 1964 Chevelles and Novas. It was also used in non-Chevys, in the Pontiac Tempest and Oldsmobile F-85/Cutlass models. The easiest way to identify a Muncie transmission is by its side cover. Muncies have seven bolts, two bolts on the bottom, three on the top and one down each side. There are also 2-3/8-inch studs for the shifter levers protruding from the side cover. The Saginaw four-speed is similar in appearance; however, the Muncie's side cover is more irregularly shaped, the top of the cover is wider than the bottom. Another way of identifying the Muncie is the reverse lever. The Muncie reverse lever stud is in the extension housing, whereas the Saginaw has a third stud through the side cover for the reverse shift lever. All Muncie transmission cases are made of aluminum, and the identifying casting numbers are prominently stamped in the passenger side of the transmission case. Those stamping numbers are: 3831704 - 1963 only 3851325 - 1964-'65 3885010 - 1965-'67 3925660 - 1968-'70 3925661 - 1971-'74 The 2.20:1 ratio first gear is usually referred to as the Muncie M21 transmission; based on GM's production codes. The 2.56:1 ratio is referred to as the M20. Muncie also had a second close-ratio transmission coded M22 and known as the "rock crusher," which was used behind performance big-block engines and in some Corvettes. These M22 units are highly sought-after for performance applications. The term "rock crusher" refers to the noise made by the meshing of the strong-er helical gears in the M22. Later model 1971-'74 Muncies are also popular with racers, because these transmissions used a 32-spline output that is stronger than the early 27-spline, and the newer units seem to perform better in high-rpm and high-horsepower applications. One of these vehicles is where you are most likely to find a Muncie four-speed. Ten-spline input-shaft with 27-spline output-shaft units came in these vehicles: 1964-'70 Chevrolet Corvette and Pontiac Tempest 1967-'70 Buick Special, Firebird and Camaro 1969-'70 Grand Prix 1963-'69 Impala 1965-'70 Nova and Olds Cutlass/F-85 and Chevelle (except 70 w/454) 1970 Monte Carlo Twenty-six-spline input with 32-spline output shaft units came in these vehicles: 1970 Chevelle 454 1971 Grand Prix and Monte Carlo 1971-'72 Special and Tempest 1971-'73 Nova, F85 and Cutlass 1971-'74 Firebird, Camaro, El Camino, Corvette, Chevelle and Sprint 1973 Buick Century 1973-'74 Pontiac Le Mans The 27-spline output shafts have the same diameter and spline as the output shafts used on many GM automatic transmissions; however, the yokes are not always the same length. You can identify the various M20, M21 and M22 transmissions by the input shaft. GM marked these units in this way so production line laborers could identify them during installation. Here is a quick spline-groove identification chart: 1963-'65 M20 2.56:1 ratio 10-spline 7/8-inch shaft - no grooves- 27-spline output shaft 1966-'70 M20 2.52:1 ratio 10-spline 1-inch shaft - 2 grooves - 27-spline output shaft 1971-'74 M20 2.52:1 ratio 26-spline 1-inch shaft - 2 grooves - 32-spline output shaft 1963-'65 M21 2.20:1 ratio 10-spline 7/8-inch shaft - 1 groove - 27-spline output shaft 1966-'70 M21 2.20:1 ratio 10-spline 1-inch shaft - 1 groove - 27-spline output shaft 1971-'74 M21 2.20:1 ratio 26-spline 1-inch shaft - 1 groove - 32-spline output shaft 1965-'70 M22 2.20:1 ratio 10-spline 1-inch shaft - no groove - 27-spline output shaft 1971-'74 M22 2.20:1 ratio 26-spline 1-inch shaft - no groove - 32-spline output shaft

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