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How to Identify Muncie Four 4 Speed Transmissions

To be able to identify the correct Muncie, several things must come into consideration.

  • First, the main case casting number

  • Second, is a count of the input shaft and output shaft splines and if any rings or grooves exist around the input shaft splines.

  • Third, are the date codes and VIN numbers that help confirm that the above case and gears belong together. Tooth counts on the input shaft will help confirm a certain gear ratio, but you may not have access to this information if the transmission is still in the car or an unwilling vendor at a swap meet won't remove the cover.

Case casting numbers, relating production years and ratios available

Casting:

Years

Ratios

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

Ratios of the M20 , M21 , M22

Year

Type

Rings

Ratio

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
 

Input Shaft Tooth and Spline Count Related to Year

Year

Type

Rings

Spline

Tooth Count

1963-1965

M20

None

10

24

1966-1970

M20

Two

10

21

1970-1974

M20

Two

26

21

1963-1970

M21

One

10

26

1970-1974

M21

One

26

26

1967-1970

M22

None

10

26

1969-1974

M22

None

26

26

There are 7 different Muncie input shafts that you should take note of. All 26 spline inputs came with 32 spline output shafts and all 10 spline inputs came with 27 spline output shafts. Another misconception is that all “fine spline” 26 spline input shafts are M22 heavy duty types, which is not true, because M22 gearbox has a 20 degree helix angle on the gear set, which is different to a 45 degree angle. Another thing that makes M22 different from a 26 spline is, M22 gear sets were of a higher nickel alloy that allows more impact of the gears. It was nicknamed "rockcrusher" because of how it was made and has become, the straighter angle was designed to produce less end loading of the gear train and less heat but created more noise. Also another thing that people makes mistake of is the thought that if you have a drain plug you have an M22 which is only true during the creation of the first M22. However all 3925661 castings had drain plugs.


MUNCIE Four 4 Speed TRANSMISSION Service Repair Manual is available now.

This is the same Muncie Transmission used on all the GM cars of the 60's & 70's era including Corvette, Camaro & Firebird very informative and illustrated with easy to follow directions. All the information you need to show you how to completely rebuild this Muncie transmission by yourself.

For the downloadable PDF e-manual version

click here

Automotive Detailing Manual


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