It is common for the shift shaft seal in GM TH350 and TH350C transmissions to leak. Often this leaking is caused by scoring on the shift shaft itself, preventing the seal from doing its job. Scoring and scratches can be caused by gritty contamination getting in past a worn seal, or from using improper tools to remove an old seal. In these cases the shift shaft must be replaced along with the seal to restore normal operation. For instances where the case itself is badly damaged, installing supplied O-ring into the shaft groove will allow salvage of the case. Stop leaking and contamination for good by installing Sonnax shift shaft and seal kit 35801-02K.
Shaft thread pitch is 7/16"-14 and 3/8"-16.