The TH350 and TH400 are GM's two most iconic 3-speed automatic transmissions. Both were produced for decades and are still widely used in builds and swaps today.
TH350 SPECIFICATIONS: Configuration: 3-speed automatic Overall length: ~21.75" Input shaft: 27-spline Output shaft: 27-spline Weight: ~120 lbs Modulator type: Vacuum modulator (controls shift timing based on engine vacuum) Applications: Chevrolet, Pontiac, Oldsmobile, Buick cars and trucks from 1969–1986
TH400 SPECIFICATIONS: Configuration: 3-speed automatic Overall length: ~24.37" Input shaft: 32-spline Output shaft: 32-spline Weight: ~135 lbs Modulator type: Vacuum modulator or electric kickdown (depending on application) Applications: GM cars and trucks with larger engines, 1964–1991; also used in some Cadillac and Jeep applications
TH350 VS. TH400 — WHICH DO YOU HAVE? • The TH400 is noticeably longer and heavier than the TH350 • The TH400 input shaft has 32 splines vs. 27 on the TH350 — check the spline count at the converter hub • The TH400 was used with larger, higher-torque engines — if your vehicle had a big block, it likely has a TH400 • The TH350 was more common in smaller-engine applications (small-block V8, V6, some inline-6) • The TH400 does not use a TV cable or vacuum modulator in the same way — it uses a detent (kickdown) cable or switch
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