Identifying Muncie 4-Speeds
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.
| |
|
|
Ratios |
| 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.
