GM Transmission Identification & Decoding

 

General Transmission Identification - GM Vehicles

Transmissions are marked and identified by:

Casting numbers on the case, extention housing and side cover (on manual trans).
Date Casting Codes
Assembly Date Code Stamping - can be stamped anywhere...
Chassis Vin Number stamping - beginning in 1962

Transmissions are also expressed via the RPO number, usually beginning with "M".

Automatic Transmission Identification & Description

Automatic transmission types are generally identified by their pan shape.

Automatic transmission types are generally identified by their pan shape.

The THM200 and the THM250 have pans shaped like a THM350, HOWEVER, the THM200 will have the words HYDRAMATIC DIV. and METRIC stamped into the pan.

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

THM250 - Used in small 4 cylinder cars (Vega, Sunbird, Monza) from 1973 to 1981 and some Nova/Camaro/Chevelle 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 Olds and 301 pontiac from 1981 to 1989. This is a 4 speed, overdrive unit.

THM350 - Medium duty 3 speed trans used from 1969 to 1989, Used in Buick, Olds, Pontiac and Chevrolet cars and Chevy & GMC light trucks until 1987. Generally used in straight and V6 and small block engines. Generally able to handle up to 400 foot pounds of torque.

THM375B - This trans is identical to the THM350, but is considered a heavier duty transmission.

THM375 - This transmission is a derivative of the THM400 sharing the external appearance of the 400. It'll be marked 375-THM on the bottom of the tailshaft housing. It's a light duty version of the THM400, has ABSOLUTELY NO RELATION to the THM375B, and was found in 1972 to 1976 full-size cars. The Turbo 375 was modified to accept the Turbo 350 driveshaft yoke.

THM400 - The THM400 was GM's Heavy-duty 3 speed transmission used from 1964 to 1990. Used on large displacement, high torque engines and engines with towing packages. Generally found in Chevy & GM trucks, cadilacs and large displacement engined, full size GM rear wheel drive cars.

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

4L60 - Early 1990's saw the 700R4 renamed to the 4L60, and "electronic" models were named 4L60E. 4L60 means 4 forward speeds, L = longitudily mounted (rear wheel drive). 60 is the strength rating (less than the 4L80).

3L80 - The 3L80 is a renamed THM400, used in the early 1990's. The 3L80HD would be the THM475 (heavy duty unit).

4L80 - An overdrive model of the THM400. The 4L80E would be the "electronic" model.

Transmissions with a suffix of "C" on the end of the type (THM200C) indicate the transmission has a lock up torque converter.

 

Automatic Transmission Identification & Decoding

Specific methods of identification are as follows (and yes, this section needs to be rewritten).... Some transmissions have a code stamped onto them. Others have a tag rivited to the case.

Most transmissions will have a "source serial number" (chassis vin) usually found close to the transmission code. This number will contain division ID #, model year, assembly plant and production sequence (last 6 digits) of the VIN stamped onto the transmission. Example: 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 drivers 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 drivers 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.


Prior to 1967, transmission ID numbers contained the plant prefix code, month and date of production (expressed numerically) and a shift code (D = Day, N = Night). From 1967 on, the ID number contained the transmission type or plant prefix, Date (coded below) and a shift code. The constants in decoding the trans ID number are the date the transmission was produced.

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: (NOTICE this is NOT in alphabetic order)
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

Manual Transmission Eyeballing

Muncie (left) Saginaw (center) Borg Warner (right)

Muncie (left) Saginaw (center) Borg Warner (right)

To quickly finger what you have, notice in the above picture:
The muncie reverse lever is in the extention 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.
Muncies are the hot tip, since they usually were used with high performance engines. Again on the muncie, and looking at the cover, 3rd/4th gear lever is on the left. 1st/2nd gear lever is on the right.

Further Muncie identification
3 Muncie 4 speeds were offered. The M-20 wide ratio, the M-21 close ratio, and the "Extra heavy duty close ratio" M-22, also known as the "Rock Crusher". The M-22 was generally installed behind severely powerful (high torque) big-block engines. The M-22 is distinguished from an M-21 by the angle of the gear teeth. The M-22 has "straight cut" gears. Due to this, the box tends to be noiser (whines or howls almost like a blower, which is why it's called a "rock crusher") than other boxes. The M-21 has a higher pitch angle on its gear teeth.

The close ratio gearboxs generally came with 3.73+ gear ratio'ed cars. 3.55 geared and higher (numerically lower) cars used the wide ratio gearbox.

Due to the wide combination of id's, stampings, numbers, etc... varied and even GM couldn't keep track of what was what, it's best to ID your gearbox visually, taking what is stamped on the box FWIW. The best way to ID the transmission is to count teeth.

Muncie 4 speed spline, groove and tooth count characteristics.

Type/Year

Input Spline

Input Groove

Input Gear
Tooth Count

Cluster Gear
Tooth Count

Output Spline

Gear Ratio

M20: 1963 - 65 10 none 24 29-22-19-17 27 Note 1
M20: 1966 - 70 10 2 21 25-22-19-17 27 Note 2
M20: 1971 - 74 26 2 21 25-22-19-17 32 Note 2
M21: 1963 - 70 10 1 26 27-22-19-17 27 Note 3
M21: 1971 - 74 26 1 26 27-22-19-17 32 Note 3
M22: 1965 - 70 10 none 26 27-22-19-17 27 Note 3
M22: 1971 - 72 26 none 26 27-22-19-17 32 Note 3


Note 1: M-20 gear ratio: 1st 2.56:1, 2nd 1.91:1, 3rd 1.48:1, 4th direct
Note 2: M-20 gear ratio: 1st 2.52:1, 2nd 1.88:1, 3rd 1.46:1, 4th direct, Rev 2.59:1.
Note 3: M-21/22 gear ratio: 1st 2.20:1, 2nd 1.64:1, 3rd 1.28:1, 4th direct, Rev 2.27:1

Effective October 21, 1968, an additional letter was added to the plant prefix number to help identify the gear ratios in Muncie transmissions. The additional letter codes as follows:

Muncie 3 speed Manual Muncie 4 speed manual transmissions
Suffix 1st gear ratio
A 3.03:1
B 2.42:1
Suffix 1st gear ratio
A 2.52:1 wide range
B 2.20:1 close range
C 2.20:1 Rock Crusher

Of course like engines, transmissions come and go over time, so below is the (as complete as I can make it) transmission plant prefix list:

Plant Type Prefix   Plant Type Prefix
Cleveland Manual Powerglide A   Saginaw overdrive O
Cleveland Turbo Hydramatic B   Warner Gear 3 & 4 speed P
Cleveland Powerglide C   Muncie 4 speed P
Hydramatic Turbo Hydramatic CA   Muncie 4 speed R
Saginaw Overdrive D   Saginaw 4 speed R
McKinnon Powerglide E   Muncie 3 speed S
Muncie 3 speed H   Saginaw 3 speed S
McKinnon 3 speed K   Toledo Powerglide T
GM of Canada Turbo Hydramatic L   Cleveland Turbo Hydramatic X
Muncie 3 speed and Overdrive M   Toledo Turbo Hydramatic Y
Muncie 4 speed N        

 

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