How to Identify a 350 Transmission

by Floyd Drake III
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The Chevrolet Turbohydramatic 350 automatic transmission, or THM350, was available on all Chevrolet and most GM models staring in 1969. As the successor to the Powerglide, it was produced until 1989. This three-speed medium-duty automatic was used primarily with six-cylinders and small-block V-8s. The 350 transmission is easily identified visually, by counting the pan bolts and by locating transmission number. This number contains the year of manufacture, but does not identify the transmission as a THM350.

Step 1

Count the number of bolts on the transmission pan. Located underneath the transmission, the pan has 13 bolts. The pan is square with one corner removed, creating a fifth side. According to Roadkill Customs, only the THM350 has this pan shape and 13 bolts.

Step 2

Locate the transmission code. According to's GM turbo 350 transmission page, the code for 1969 to 1972 transmissions is on the passenger side accumulator cover. The accumulator cover is a small, round dome-shaped cap, located in the middle of the transmission. For 1973 and later transmissions, the code is located on the governor cover, located at the rear of the transmission surrounding the output shaft

Step 3

Decode the serial number. The top number on the accumulator cover gives the last two digits of the year of manufacture. The code below the year of manufacture is specific to the original engine and the transmission's calibration code.

Step 4

Compare the transmission to a visual chart similar to the one found on the Roadkill Customs website (see Resources).

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