How to Identify a Turbo 350 or Turbo 400 Transmission

by Robert Bayly
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The General Motors TH 350, introduced in 1969, was developed by Chevrolet and Buick. It was used in GM cars and trucks through 1984 with small-block engines. The TH400 transmission was first used in 1964 in Cadillac and Buick automobiles. The other GM brands got the TH400 in 1965. The TH400 was used in passenger cars through 1982 and trucks through the early 1990s. Both transmissions are noted for strength and durability but the TH400 is the stronger of the two. There are numerous aftermarket parts and complete units available from a host of suppliers.

Step 1

Park the vehicle on a paved level surface and set the parking brake. Raise the front of the vehicle and support the front with jack stands.

Step 2

Measure the transmission case with a tape measure. The Turbo 350 case measures 21 3/4-inches long from the front of the bellhousing to the rear of the case. Do not include the tailshaft housing which bolts to the rear of the case. The Turbo 400 measures 24 3/8-inches long from the belllhousing to the rear of the case.

Step 3

Look at the driver's side of the transmission. The Turbo 350 has a "kickdown" cable attached to the right side attached to a manual linkage on the transmission. The Turbo 400 has an electrical switch.

Step 4

Look at the transmission pan. The Turbo 350 has a basically square pan with one corner cut off. It actually has five sides. It measures 13 1/2-inches long and 13-inches wide. The Turbo 400 pan is longer than it is wide. The part of the pan at the rear of the transmission looks like the lower part of the state of Texas (really). It measures 16 5/8-inches long and 13-inches wide.

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