A/T - THM Steel Clutch Piston Inspection

TSB 88-32 (July)

SUBJECT: GENERAL MOTORS

STEEL CLUTCH PISTONS USED IN THE FOLLOWING GM TRANSMISSIONS:

THM 400 THM 200C
THM 125 THM 200-4R
THM 125C THM 325
THM 200 THM 325-4L




Most steel clutch pistons used in these GM transmissions are not a single piece piston. These steel pistons have one or two lip seal retainers pressed on them to hold the lip seal. (See Figure)

If these lip seal retainers lose their press fit, they can close or open the lip seal groove. If it closes the groove, it may be difficult to install the lip seal on the piston. If it opens the groove, excessive oil pressure may leak around the lip seal and cause an erratic engagement of that clutch, or early clutch failure.

Always check the press fit of the lip seal retainers by gently prying against the inside of the groove with a screwdriver. If the lip seal retainers can easily be moved with a screwdriver, replace the piston.