GM Turbo 400 Automatic Transmission Overview

The TH400 transmission was GM's first design of automatic transmission in the Turbo-Hydramatic product line.  It was first introduced in select 1964 models then rolled out completly the following year in 1965.  Early 1964-1967 model Turbo 400 transmissions were equipped with a switch-pitch torque converters with variable-pitch stator.  These model TH400s can be identified outside of the transmission, when the torque converter is off, by looking for a narrow front pump spline.  Switch-pitch Turbo 400s also had two electrical connectors where the non-switch-pitch Turbo 400s only have one electrical connector.

GM had adpated the TH400 to be able to work with a large number of vehicles between its almost 30-year lifespan.  The majority of internal components are interchangeable.

• THM375 is a Turbo 400 transmission with a long output shaft that mates to the smaller Turbo 350 drive shaft yoke.  On the tailhousing "375-THM" is typically cast into it.  The other major difference between TH400's and TH375s is the TH375 had fewer frictions then the TH400.
• GM's 3L80 transmission, also known as the Turbo 475, has a stright cut planet gear set.  There is no visual way to determine if your TH400 is a 3L80 from the outside of the transmission housing.
• TH425 transmissions are front-wheel drive automatic transmissions and share almost all internal parts with the TH400.