The GM 4L60, 4L60-E, 4L65-E and 4L70-E input drum assembly is known to crack where the input shaft is pressed into it (where the overrun clutch rides).
Sonnax offers a hardened steel sleeve that reinforces the input drum to prevent this failure. OE overrun pistons must be machined when using Sonnax input drum reinforcement kit 77733-02K.
*Will not repair a cracked drum. Requires 96-earlier aluminum forward piston or Sonnax billet forward piston 77764-01, cannot be used with stamped steel forward piston.