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chevy four speed transmissions

Muncie (left) Saginaw (center) Borg Warner (right)

To quickly finger what you have, notice in the above picture:
The muncie reverse lever is in the extention housing, and the cover has 7 bolts.
The Saginaw reverse shift lever is on the cover, and the cover has 7 bolts.
The Borg Warner has a 9 bolt cover.
Muncies are the hot tip, since they usually were used with high performance engines. Again on the muncie, and looking at the cover, 3rd/4th gear lever is on the left. 1st/2nd gear lever is on the right.

Further Muncie identification
3 Muncie 4 speeds were offered. The M-20 wide ratio, the M-21 close ratio, and the "Extra heavy duty close ratio" M-22, also known as the "Rock Crusher". The M-22 was generally installed behind severely powerful (high torque) big-block engines. The M-22 is distinguished from an M-21 by the angle of the gear teeth. The M-22 has "straight cut" gears. Due to this, the box tends to be noiser (whines or howls almost like a blower, which is why it's called a "rock crusher") than other boxes. The M-21 has a higher pitch angle on its gear teeth.

The close ratio gearboxs generally came with 3.73+ gear ratio'ed cars. 3.55 geared and higher (numerically lower) cars used the wide ratio gearbox.

Due to the wide combination of id's, stampings, numbers, etc... varied and even GM couldn't keep track of what was what, it's best to ID your gearbox visually, taking what is stamped on the box FWIW. The best way to ID the transmission is to count teeth.

Muncie 4 speed spline, groove and tooth count characteristics.

Input Spline

Input Groove

Input Gear
Tooth Count

Cluster Gear
Tooth Count

Output Spline

Gear Ratio

M20: 1963 - 65 10 none 24 29-22-19-17 27 Note 1
M20: 1966 - 70 10 2 21 25-22-19-17 27 Note 2
M20: 1971 - 74 26 2 21 25-22-19-17 32 Note 2
M21: 1963 - 70 10 1 26 27-22-19-17 27 Note 3
M21: 1971 - 74 26 1 26 27-22-19-17 32 Note 3
M22: 1965 - 70 10 none 26 27-22-19-17 27 Note 3
M22: 1971 - 72 26 none 26 27-22-19-17 32 Note 3

Note 1: M-20 gear ratio: 1st 2.56:1, 2nd 1.91:1, 3rd 1.48:1, 4th direct
Note 2: M-20 gear ratio: 1st 2.52:1, 2nd 1.88:1, 3rd 1.46:1, 4th direct, Rev 2.59:1.
Note 3: M-21/22 gear ratio: 1st 2.20:1, 2nd 1.64:1, 3rd 1.28:1, 4th direct, Rev 2.27:1

Effective October 21, 1968, an additional letter was added to the plant prefix number to help identify the gear ratios in Muncie transmissions. The additional letter codes as follows:

Muncie 3 speed Manual   Muncie 4 speed manual transmissions
Suffix   1st gear ratio
A   3.03:1
B   2.42:1
Suffix   1st gear ratio
A   2.52:1 wide range
B   2.20:1 close range
C   2.20:1 Rock Crusher

Of course like engines, transmissions come and go over time, so below is the (as complete as I can make it) transmission plant prefix list:

Plant Type Prefix   Plant Type Prefix
Cleveland Manual Powerglide A   Saginaw overdrive O
Cleveland Turbo Hydramatic B   Warner Gear 3 & 4 speed P
Cleveland Powerglide C   Muncie 4 speed P
Hydramatic Turbo Hydramatic CA   Muncie 4 speed R
Saginaw Overdrive D   Saginaw 4 speed R
McKinnon Powerglide E   Muncie 3 speed S
Muncie 3 speed H   Saginaw 3 speed S
McKinnon 3 speed K   Toledo Powerglide T
GM of Canada Turbo Hydramatic L   Cleveland Turbo Hydramatic X
Muncie 3 speed and Overdrive M   Toledo Turbo Hydramatic Y
Muncie 4 speed N        

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Last updated: 10/20/2011
Author: MadMike Maciolek - the most comprehensive numbers decoding site on the net
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