Reading Serial and VIN Numbers

Serial numbers for Muncie 4 speeds always begin with the letter "P". P stands for Muncie Plant.  Not for passenger car as some self proclaimed specialists may think. The letter M was used to ID the Muncie plant for the Muncie 3 speed, so P was the next logical letter. The letter O would add confusion since it resembles zero. The serial number is a date code the transmission was built for a particular year. Serial numbers from
1963 to 1966 included only the month and day. P0101 would indicate January 1st. From 1967 to 1968 the serial number got a year designator and a letter designator for the month such as P8A01, meaning January1st 1968. One important point is that if you have a Muncie dated with a December build date it was actually built the prior year. An example would be the date code P8T13. This is for a 1968 production car. The T stands for December and 13 is the day. To confirm this simply look at the VIN number. It will usually begin with a 18S101350 or a 28N12950. This means the Muncie was assembled December 13, 1967 for the 1968 model year. The VIN number will usually be a low number.  1969 to 1974 Muncies got a ratio designator at the end of the serial number. An example would be P4D23B. This equates to April 23, 1974, M21 ratio.

Month Code Chart 

January: A
February: B
March: C
April: D
May: E
June: H
July: K
August: M
September: P
October: R
November: S
December: T


Ratio Code Chart

Letter: A    M20 Ratio
Letter: B    M21 Ratio
Letter: C    M22 Ratio

Important Notes

Some input shafts produced by the aftermarket and General Motors have no identifying rings on them. The rings originally corresponded with rings or grooves on the counter gear so that the assembler matched a one or two ring input with a one or two ring cluster. When manufacturing was stopped, GM stopped making inputs with these marks, probably to save machining operations. Also 3831704 and 3851325 castings come with a 7/8" counter shaft all others come with a 1 inch counter shaft. There are some odd ball castings out there produced in 1963 to 1966 years. I've left them out since they really are not common.



Main Case

3831704       1963 Only small 6207NR Front Bearing Pat. Pending, 7/8" Bore

3839606       1963 - 1964 Regular Bearing Pat. Pending, 7/8" Bore

3851325       1964 -1965 7/8" Bore Patent Pending Mostly 1964

3851325       1964 –1965 7/8" Bore Patent Number

3864848       1965 7/8" Bore Patent Number

3864_____     1965 Milled off last 3 digits 7/8" Bore Patent Number

3885010       1966 -1967 1" Bore Patent Number

3925660       1968 – 1970 1" Bore Patent Number

3925661       1970 – 1974 (some early 1975 cars) 1" Bore Patent Number

 

Tailhousing

3831731       1963 "Thin Fin Tail" Driver speedo 27 spline

3846429       1963 "Thin Fin Tail" Driver speedo 27 spline

3846429       1963-1965 Regular thick web, driver speedo, 27 spline

9779246       1964 – 1965 Pontiac Catalina Long Tail Driver Speedo 27 Spline

3857584       1966 – 1970 Passenger side speedo, 27 spline

3978764       1970 – 1974 passenger speedo, 32 spline output Side Cover

3831707       1963-1965 Early side cover stud type shift shafts

3884685       Cover issued with "584" tail stud type shift shafts

3950306       Short boss with bolt on type shift shafts - no switches

3952642       Long boss bolt on type w/ TCS switch on 3-4

3952648       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.

Casting: 3831704  Years: 1963 Only    Ratios: M20 2.56 1st , M21 2.20 1st 

Casting: 3851325  Years: 1964 - 1965  Ratios: M20 2.56 1st , M21 2.20 1st 

Casting: 3885010  Years: 1965 - 1967  Ratios: M20 2.52 1st , M21 2.20 1st, M22 2.20 1st

Casting: 3925660  Years: 1968 - 1970  Ratios: M20 2.52 1st , M21 2.20 1st, M22 2.20 1st

Casting: 3925661  Years: 1970 - 1974  Ratios: M20 2.52 1st , M21 2.20 1st, M22 2.20 1st

Ratios of the M20 , M21 , M22

Year 1963-1965    Type: M20   Rings: None   Ratio: 2.56 / 1.91 / 1.48 / 1.00 / 3.16

Year 1966-1974    Type: M20   Rings: Two    Ratio: 2.52 / 1.88 / 1.46 / 1.00 / 3.11

Year 1963-1974    Type: M21   Rings: One    Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27

Year 1967-1974    Type: M22   Rings: None   Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27


Input Shaft Tooth and Spline Count Related to Year

Year 1963-1965    Type: M20   Rings: None  Spline: 10  Tooth Count: 24

Year 1966-1970    Type: M20   Rings: Two   Spline: 10  Tooth Count: 21

Year 1970-1974    Type: M20   Rings: Two   Spline: 26  Tooth Count: 21

Year 1963-1970    Type: M21   Rings: One   Spline: 10  Tooth Count: 26

Year 1970-1974    Type: M21   Rings: One   Spline: 26  Tooth Count: 26

Year 1967-1970    Type: M22   Rings: None  Spline: 10  Tooth Count: 26

Year 1969-1974    Type: M22   Rings: None  Spline: 26  Tooth Count: 26

Please note that there are 7 different Muncie input shafts. All 26 spline inputs came with 32 spline output shafts and all 10 spline inputs came with 27 spline output shafts. A common mistake is thinking that all "fine spline" 26 spline input shafts are M22 heavy duty types. This is not true. An M22 gearbox has a 20 degree helix angle on the gear set as opposed to a 30 degree angle. It's roughly a 10 degree difference. From time to time I get people disputing what exact angles are on the gears. Personally, I think the angles vary from gear to gear. 1st gear on an M22 may be 19 degrees, 3rd may be 20 degrees. Also M22 gear sets were of 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". The higher nickel alloy allowed for more impact of the gears. Another misconception is if you have a drain plug you have an M22. Again this was only true when the first M22 boxes were created. But all 3925661 castings had drain plugs.

I found this information on http://www.5speeds.com/

There are several things are needed to identify a Muncie correctly. The first is a main case casting number.   Second account of the input shaft and output shaft splines and if any rings or grooves exist around the input shaft splines.  Third are 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