Muncie 4 Speeds


Muncie 4 speed transmissions were used in General Motors cars from 1963 to early 1975. These gear boxes were used when power levels exceeded the limits of the early Borg Warner T10 4 speed. It was actually based on the T-10 design and uses the same basic gear layout. The T10 made it's appearance in GM cars in 1957. From 1957 to 1963 many changes took place inside the gearbox to meet the increasing horsepower the new engines were putting out. It was replaced by the Muncie in late 1963. The Muncie 4 speed was used in production vehicles until power levels dropped in 1975. 1975 was the first year catalytic converters , smog pumps, and low horsepower were introduced. These new requirements led to the demise of the Muncie and the reintroduction of the improved T10 called the Super T10.

While General Motors was switching gearboxes around from 1963 to 1975 other companies such as Ford and AMC kept the T10 in production. So we got to see T10, and First Design Super T10 transmissions end up in these cars. It was also an opportunity for Borg Warner to design replacement Super T10's for the Muncie. These 4 speeds offered 9310 nickel gears, had a wide variety of gear ratios, and were available from any auto parts store carrying the Borg Warner parts line. A variation of this Super T10 ended up in GM cars from 1975 to 1983. Also as the 4 speed section of the 4+3 Corvette Overdrive in 1984 to 1988 Corvettes.

Well enough of this little 4 speed history. I figured I pass these facts on to you so that one day you can tell your children and just maybe all won't be lost. Lets face it today's muscle car is the Honda Civic. A majority of kids today will never feel the awesome torque of the big blocks as well as the feeling of ripping thru a few sets of gears.


        Identifying Muncie 4 Speeds


Many people are always calling me or asking me at shows on how to identify Muncie 4 speeds. Not to many people actually know what type they have and most publications have done a good job of giving out wrong information. This includes performance parts books from GM as well as a multitude of poorly written "How To" articles in just about every car magazine around. One important point I'd like you to know is that most of the so called "Muncie Specialists" are in business because of me. Now with the use of my rebuild kits and knowledge you too can become a Muncie Specialist, and if your lucky a "Muncie King".

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 or an unwilling vendor at a swap meet won't remove the cover.

Case casting numbers, relating production years and ratios available

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 gearset as opposed to a 45 degree angle. 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 "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 M22 boxes were created. But all 3925661 castings had drain plugs.

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 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.

Month Code Chart

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

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.

Muncie 4 Speeds  the “Rock Crusher”


The most popular question has to be that everyone thinks they have the famous "rockcrusher" M22 4 speed. This had the same gear ratios as the M21 but with a heavy duty gearset. The reason you may ask? Well it wasn't designed so that you could go drag racing on Saturday night or impress girls in front of Burger King by doing massive hole shots. It was primarily designed as a road race 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. For the real gear head the best documented movie that sports a real M22 in a car is the famous movie " Two Lane Black Top ". You can really hear this gear box howl in the background as James Taylor and Dennis Wilson talk shop in a 55 Chevy. Later on we get to see the same car with new paint in the classic film "American Graffiti" with Harrison Ford at the helm. Since they used a dummy car for inside dialog shots you miss the flavor of the M22 in that flick.

The dead 2.20 first gear ratio which is the same in the M21 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 M21 and M22 the closest ratio production 4 speeds ever produced but also not practical for today's driving habits unless road racing is your thing. Today's examples of the new car gear set ups 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. Get the picture? If you want to learn about this stuff DOWNLOAD our RPM Calculator software for windows its free.

Below is an M20 gear set. Notice the 45 degree helix angle.

m20 gears


Below is an M22 gear set. Notice the angle of these gearsis about half the angle of the M20 gears.

m22 gears

                           m22 gears



Muncie 4 Speed  Casting #’s


Main Case


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


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


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


1964 –1965 7/8" Bore Patent Number


1965 7/8" Bore Patent Number


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


1966 -1967 1" Bore Patent Number


1968 – 1970 1" Bore Patent Number


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




1963 "Thin Fin Tail" Driver speedo 27 spline


1963 "Thin Fin Tail" Driver speedo 27 spline


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


1964 – 1965 Pontiac Catalina Long Tail Driver Speedo 27 Spline


1966 – 1970 Passenger side speedo, 27 spline


1970 – 1974 passenger speedo, 32 spline output


Side Cover


1963-1965 Early side cover stud type shift shafts


Cover issued with "584" tail stud type shift shafts


Short boss with bolt on type shift shafts - no switches


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


Short boss bolt on type w/ TCS switch on 3-4


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.

 Information From:


Medatronics Corporation
208 North US Highway 1
Unit #1
Tequesta, FL 33469
Phone: 561-743-5600
Fax: 561-741-7921

Paul Cangialosi