There is a ton of Muncie information on this page. It will be reorganized ASAP, but for now I just wanted to get it out there.

The Muncie consists of two basic sections. They are the transmission case (towards the front of the car) and the rear case extension (towards the rear of the car). The forward section contains the four forward gear assemblies and their synchronized mechanisms. The rear section contains the reverse gear assembly.

Incorporated throughout the transmission are gears specifically designed to provide high torque capacity without additional weigh, and gear teeth symmetrically positioned to operate at high speeds with neither excessive heat creation nor excessive friction losses. Muncie four speeds were produced in two different ratios. There was the wide ration M-20 and a close ration M-21. There was also a heavy duty close ration version which was of course the M-22. The M-22 was offered on many of the big block muscle cars of the 60's and 70's. The choice of transmission was dictated by the engine size and the rear axle ratio. Axle ratios of 3.73 and lower (numbers getting higher) came with close ratio transmissions, while axle ratios of 3.55 and higher (numbers getting lower) used wide ratio transmissions. Many GM high performance engines came with the M-22 "Rock Crusher" in the years 1970 through 1973. Earlier versions of this four speed were available as early as in 1965 in the Corvette Stingray.

	M-20 Wide Ratio 1963-1965	M-20 Wide Ratio 1966-1974	M-21 Close Ratio 1963-1974	M-22 Close Ratio 1967-1974
1st	2.56 to 1	2.52 to 1	2.20 to 1	2.20 to 1
2nd	1.91 to 1	1.88 to 1	1.64 to 1	1.64 to 1
3rd	1.48 to 1	1.46 to 1	1.28 to 1	1.28 to 1
4th	Direct	Direct	Direct	Direct
Reverse	3.16 to 1	3.11 to 1	2.27 to 1	2.27 to 1
Rings on Input Shaft	0	2	1	0

Gear Ratio Code Chart
Letter	Ratio
A	M20
B	M21
C	M22

Input Shaft Tooth and Spline Count By Year
Year	Type	Rings	Spline	Tooth Count
1963-1965	M20	0	10	24
1966-1970	M20	2	10	21
1970-1974	M20	2	26	21
1963-1970	M21	1	10	26
1970-1974	M21	1	26	26
1967-1970	M22	0	10	26
1969-1974	M22	0	26	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 oposed 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.

The Muncie 4-speed transmission was used with many different GM models and engine combinations from 1963-1974. The basic design and operation theory of the gearbox changed little during that time, though several design upgrades and modifications were made during the total production run.
The Muncie was used with most performance engines and is not to be confused with the Saginaw or Borg-Warner 4-speeds. The Muncie is distinguished from the Saginaw in that the reverse lever on the Muncie is mounted in the extension housing while the Saginaw reverse lever is mounted in the side cover. The main difference between the Muncie and the Borg-Warner is that the Muncie has a 7 bolt side cover and the Borg-Warner has a 9 bolt side cover.

Muncie Date Coding

Many Muncie 4-speed maincases, extension housings, and sidecovers have a casting date code. On maincases the code is usually found on the passenger side of the case below the casting number. The extension housing code is usually within a few inches of the casting number also on the passenger side. Muncie casting date codes are very different from other transmission manufacturers. The code consists of two circles, both 1/2 inch in diameter and divided in half. One circle is called the date marker, the other the status marker.
The date marker has a month designator for when the part was cast or manufactured. This is represented by the numbers 1 through 12 (Jan-Dec) in the years 1963-1966. They were marked with letters as shown in the table below from then on. The opposite side of the date marker has between 1 and 5 dots representing the week of the month. The easiest way to discern the proper date mark circle is to look for the week designator spots. Always check to see that the date marked precedes the final assembly stamp code. The status marker has the latest blueprint change number on one side of the circle and a deviation notice letter on the other side if the part change letter is active. This infomation was only useful to the Muncie plant by allowing the tracking of problem parts. This circle is usually blank.
The way that the serial numbers and date codes were "coded" changed somewhat over the years. 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 date coded 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 on 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	April = D	July = K	October = R
February = B	May = E	August = M	November = S
March = C	June = H	September = P	December = T

Differences in Muncie Models
Muncie model/year	Main Drive gear (teeth)	Muncie model/year	Countergear teeth
M20 1963-65 wide ratio	24	M20 1963-65 wide ratio	17 19 22 29
M20 1966-70 wide ratio	21	M20 1966-74 wide ratio	17 19 22 29
M21 1963-70 close ratio	26	M21 1963-74 close ratio	17 19 22 27
M22 1965-70 close ratio	26	M22 1965-72 close ratio	17 19 22 27

Muncie 4-speed Aluminum Component Casting Numbers
Year	Maincase Casting #	Extension Housing Casting #	Sidecover Casting #	Front Bearing Retainer Casting #
1965	3851325	3846429* (up till 2/65) 3857584 (after 2/65)	3831707	3851326
1966- 1967	3885010	3857584	3884685	3851326
1968- 1969	3825660**	3857584	3884685 (1968) 3950306 (1969)	3915020

*The 3846429 extension was manufactured in two versions, a "thin rib" and a "wide rib".
**Muncie data shows 1968 and 1969 M22 received maincase #3925661. All documented M22 transmissions have the #3925660 maincase. It's unclear as to why this discrepancy exists.

How can you tell if the transmission in your car is the one that the car was originally built with? All original Muncie transmissions have ring grooves on the input shaft and counter gear. These grooves were put on for the Muncie assembly line workers to readily identify which input shaft to install and therefore shouldn't be used for positive identification of transmissions since many of these shafts have been replaced. Also several Muncie parts -primarily the maincase- were found to have documented "oddball" casting numbers on production vehicles. None of these maincases are shown on the origianl build records from the Muncie plant and should also not be used for identification purposes.
Many transmissions over the years have been rebuilt or replaced, upgraded from a 3-speed to a 4-speed etc. which can make identification somewhat difficult at times.
Below here are some diagrams that will describe where the serial number is stamped on all transmission cases. The 9-digit code will tell you which division, year, model, assembly plant, and car the transmission was pulled from. The first digit tells what division the transmission is built for (Oldsmobile's number is 3). The second digit is the last digit of the model year (7=1967, 9=1969, etc.). The third digit is the code for the assembly plant the car was built in. The last six digits should match the last six digits of your VIN (generally found on the door hinge of 1964-1967 models and on the left top side dash of 1968-1972 models). If the numbers don't match, your transmission has been changed, though remember that even an original transmission may have had parts swapped for repair work at some time through it's life.

I found a great exploded diagram of a Muncie while surfing once. It is quite large so it is displayed on a separate page. Click here to go to that page. It is in printable form for ya!