Identifying Muncie 4-Speeds

Muncie ID

Several things are needed to identify a Muncie correctly. The first is a main case casting number. Second a count 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.

Main Case Casting Numbers

Casting Number Year Notes
3831704 1963 Small 6207NR Front Bearing , Pat. Pending, 7/8" Bore
3839606 1963 - 1964 Regular Bearing, Pat. Pending, 7/8" Bore
3851325 1964 - 1965 Patent Pending, 7/8" Bore (Mostly 1964)
3851325 1964 - 1965 Patent Number, 7/8" Bore
3864848 1965 Patent Number, 7/8" Bore
3864___ 1965 Patent Number, 7/8" Bore, (Milled off last 3 digits)
3885010 1965 - 1967 Patent Number, 1" Bore
3925660 1968 - 1970 Patent Number, 1" Bore
3925661 1970 - 1974 Patent Number, 1" Bore

Tail Housing Casting Numbers

Casting Number Year Notes
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 Speedo, 27 spline
3978764 1970 - 1974 Passenger Speedo, 32 spline

Side Cover Casting Numbers

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.

Casting Number Notes
3831707 Early side cover stud type shift shafts (1963 - 1965)
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/ Transmission Controlled Spark switch on 3-4
3952648 Short boss bolt on type w/ Transmission Controlled Spark switch on 3-4
335308 Long boss bolt on type w/ Neutral Safety Switch.

Ratios of the M20, M21, M22

Muncie 4-speeds were produced in two different ratios wide-ratio (M-20) and close-ratio (M-21). An extra heavy duty close-ratio version (M-22) was also offered on many of the big-block high-performance models.The choice of transmission was dictated by the engine size and rear-axle ratio. Axle ratios of 3.73 and lower (numerically higher) came with close-ratio transmissions, while axle ratios of 3.55 and higher (numerically lower) used wide-ratio transmissions.

Year Type Rings 1st 2nd 3rd 4th Rev
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 Count and Splines

      Input Shaft Output Shaft
Year Type Rings Splines Tooth Count Splines
1963 - 1965 M20 None 10 24 27
1966 - 1970 M20 Two 10 21 27
1970 - 1974 M20 Two 26 21 32
1963 - 1970 M21 One 10 26 27
1970 - 1974 M21 One 26 26 32
1967 - 1970 M22 None 10 26 27
1969 - 1974 M22 None 26 26 32

A M22 gearbox has a 20 degree helix angle on the gear set as opposed to a 45 degree angle and 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 "rockcrusher". The higher nickel alloy allowed for more impact of the gears.

Reading Serial and VIN Numbers

Serial numbers for Muncie 4 speeds always begin with the letter "P". P stands for Muncie Plant. 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 January 1st 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.

Code Month
A January
B February
C March
D April
E May
H June
K July
M August
P September
R October
S November
T December
Code Ratio
A M20
B M21
C M22

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.