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, but you may not have access to this information if the transmission is still in the car.
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
Year 1963-1965 Type: M20 Rings: None Wide Ratio: 2.56 / 1.91 / 1.48 1.00 / 3.16 Main Drive Gear - 24 Teeth Counter Gear 17-19-22-29
Year 1966-1974 Type: M20 Rings: Two Wide Ratio: 2.52 / 1.88 / 1.46 1.00 / 3.11 Main Drive Gear – 21 Teeth Counter Gear 17-19-22-25
Year 1963-1974 Type: M21 Rings: One Close Ratio: 2.20 / 1.64 / 1.28 1.00 / 2.27 Main Drive Gear – 26 Teeth Counter Gear 17-19-22-27
Year 1967-1974 Type: M22 Rings: None Ratio: 2.20 / 1.64 / 1.28 1.00 / 2.27 Main Drive Gear – 26 Teeth Counter Gear 17-19-22-27
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 1970-1974 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 M-22 gearbox has a 20 degree helix angle on the gearset as opposed to a 45 degree angle. Also M-22 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 "rockcrusher". 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 M-22 boxes were created. But all 3925661 castings had drain plugs.
Serial numbers for Muncie 4-speeds always begin with the letter "P". P stands for Muncie. The serial number is a date code showing that 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.
January Letter: A May Letter: E September Letter: P
February Letter: B June Letter: H October Letter: R
March Letter: C July Letter: K November Letter: S
April Letter: D August Letter: M December Letter: T
Letter: A M20 Ratio
Letter: B M21 Ratio
Letter: C M22 Ratio
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 66.)
The most popular question has always been: Do I have, or is this one of the famous "rockcrusher" M-22 4-speeds? This transmission had the same gear ratios as the M-21 but with a heavy duty gearset. You might be wondering WHY? It wasn't designed so that you could go drag racing on Saturday night or impress girls in front of Mel’s Drive-In by doing burn-outs!!!! It was primarily designed as a road racing transmission. The straighter angle of the gearset produced less heat and less end-loading of the gear train. Combined with high impact alloy gears this 4-speed really pushed the limits of it's aluminum case in drag race applications. Although the gears are not spur gears ( completely straight ), they still produced a fair amount of gear noise, thus the "Rockcrusher" name. These 4-speeds sound like a blower drive.
The 2.20 first gear ratio which is the same in the M-21 is not that good for off the line acceleration. You would need at least a 4.56 rear end gear to compensate for the dead first gear. That was OK when gas was cheap in 1967 and you did not mind pulling 3000 RPM at 50 MPH while going to work. Times and requirements have changed, making both the M-21 and M-22 the closest ratio production 4-speeds ever produced. They are not practical for today's driving habits unless road racing is your thing.
For today’s cars, looking for better gas mileage, an M-20 wide ratio, 2:52 1st gear or 2:56 1st gear would be better for acceleration. A good example of the new car gear set-ups for today are Mustang GT's with a 3.35 1st gear and 3.08 rear. New Camaros with a 2.68 1st gear, 3.73 rear gear and .59 overdrive. ZR1's with a 2.68 1st gear, 3.54 rear gear and a .59 overdrive.
Cast Iron Case w/23/64” Bore, 3rd gear 1.37
No thrust flange for 2nd gear, Mainshaft 3rd gear has bushed hole, Reverse idler halves lock with 4 lugs
Cast Iron Case w/ 1.5” Bore
Mainshaft 3rd gear has a ground hole
Reverse Idler halves are splined (27)
Everything else is the same as above
Except 1st gear bushing on Mainshaft was eliminated
Aluminum Case w/smaller bore than before
Mainshaft 1st & 2nd gears are given wider driving surfaces
Main drive gear re-designed
Snap ring clip was changed to threaded nut
Aluminum Cases from GM & Cast Iron-After-Market
Offered TWO new gear ratios: 2.64:1 & 2.43:1
Side-Loaded w/side-lever shift linkage
Counter-shaft was enlarged to 1.00 inch
Mainshaft resdesigned redesigned for larger 1st gear Journal
Same as above with MORE variations of a minor significance
Main Shaft is 32-spline
Gear Ratios are 2.43, 2.64, 2.88 or 3.42