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If you would like to add some information about transmissions, please let me know, and I will make sure to incorporate it into the article.

Transmission Identification & Decoding

General Transmission Identification

Transmissions can be identified in the following manner:

Casting numbers :    Located on the case, extension housing                                        and side cover (manual).
Date Casting Codes
Assembly Date Code Stamping:     Located almost                                                                  anywhere.
Chassis Vin Number stamping:     Beginning in 1962

Transmissions are also expressed via the RPO number, which usually begins with an "M".


Automatic Transmission Identification and Description

Their pan shape generally identifies automatic transmission types.


The THM200 and the THM250 have pans shaped like a THM350.  The THM200 will have the words Hydramatic div. and metric stamped into the pan, however.


Automatic Transmission Description & Usage
THM200 - Light duty, "metric" 3 speed transmission.


THM250 - Used in small 4 cylinder cars from 1973 to 1981 and some 6 cylinders in 1974 & 75. The 4 cylinder models have 4 large cooling holes near the torque converter, while on the "bigger cars" there are no cooling holes.


THM200-R4 - Used in GM rear wheel drive cars with the 3.8 V6, 305 & 350 from 1981 to 1989. This is a 4 speed, overdrive unit.


THM350 - Medium duty 3-speed transmission used from 1969 to 1989.  It was generally used in straight and V6 and small block engines, and generally able to handle up to 400 foot pounds of torque.


THM400 - The THM400 was GM's Heavy-duty 3-speed transmission used from 1964 to 1990. It was used on large displacement, high torque engines and  with towing packages.


THM700-R4 - 4 Speed transmission used in GM cars and light trucks with the 2.8, 3.1, 4.3, V6, and Chevrolet built 305, 350 and 454 engines from 1982 to 1992.


Automatic Transmission Identification and Decoding

Some transmissions have a code stamped onto them while others have a tag riveted to the case.

For all you "numbers matching" types out there, most transmissions will have a "source serial number" (chassis VIN) usually found close to the transmission code. This number will include the division ID #, model year, and the assembly plant and production sequence (last 6 digits) of the VIN stamped onto the transmission. An example would be 19N500001. The source serial number is from the car the transmission originally went into, not the assembly plant.

Location of source serial number:

On the THM350, the VIN will be stamped on either the driver’s side housing near the shifter, on the right side of the housing just above the pan or on a boss behind the bellhousing flange on the passenger side of the transmission.

On the THM400 the VIN is stamped on a machined surface just above the pan on the drivers side.

On the THM200-R4 the VIN is stamped onto the housing towards the rear of the pan, on the driver’s side.

On the THM700-R4 the VIN is either stamped onto a boss behind the bellhousing flange on the passenger side, just above the pan on the drivers side, or just behind the governor cover, stamped vertically where the case meets the tailshaft housing.



Before 1967, transmission ID numbers contained the plant prefix code, month and date of production (expressed numerically) and a shift code (D = Day, N = Night). After 1967, the ID number contained the transmission type or plant prefix, Date (coded below) and a shift code.


Pre-67 Example: C213N - (C = Cleveland Powerglide, February 13, Night Shift)
Post-67 Example: P9E03 - (P = TYPE, 9 = year (1969), E = Month, 03 = Day of Month)

Month code:
A = Jan, B = Feb, C = Mar, D = Apr, E = May, H = Jun, K = Jul, M = Aug, P = Sep, R = Oct, S = Nov, T = Dec




Manual Transmission Identification & Description

External differences:

The Muncie reverse lever is in the extension housing, and the cover has 7 bolts.
The Saginaw reverse shift lever is on the cover, and the cover has 7 bolts.
The Borg-Warner has a 9 bolt cover.
Muncie Identification
Both Muncie three and four speeds were offered during the sixties. The M-20 wide ratio, the M-21 close ratio, and the extra heavy duty close ratio M-22, which was also known as the "Rock Crusher." The M-22 was generally installed behind extremely powerful big-block engines. The close ratio trannys usually came with 3.73+ gear ratio cars. Cars 3.55 geared and higher (numerically lower) used the wide ratio gearbox.

The M-20 Cluster gear teeth are 25-22-19-17. The Input shaft/gear teeth: 21.
M-20 gearing: 1st 2.52:1, 2nd 1.88:1, 3rd 1.46:1, 4th direct, Rev 2.59:1.
The M-21 & M-22 Cluster gear teeth are 27-22-19-17. Input shaft/gear teeth: 26
M-21/22 gearing: 1st 2.20:1, 2nd 1.64:1, 3rd 1.28:1, 4th direct, Rev 2.27:1
The M-22 is further identified against an M-21 by the angle of the gear teeth. The M-22 has "straight cut" gears. Due to this, the box tends to whine more than other boxes. The M-21 has a higher pitch angle on its gear teeth.
To summarize, the M-20 differs from the M-21 by the input shaft and the cluster gear. The M-21 differs from the M-22 by the angle of its internal gears.

Note, remember that transmission pans can be changed easily, so you should avoid relying upon them directly for a code if possible.

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Last Updated:

10/12/99 04:46 PM