No 4th Gear

No 4th Gear:
 

A common problem with this transmission until GM made an upgrade in later models.  There is a clutch hub inside this trans for 4th gear and has a long hollow shaft with splines on one end, and lugs on the other end for 4th clutches to key into.  The splines on the small end will turn razor sharp over time and eventually wear off.  Constant in and out of 4th gear and lockup in 4th gear puts stress on the splines with constant hammering back and forth from driveline load change puts a lot of stress on the splines which eventually leads to failure and no 4th gear.  Later GM parts as well as the aftermarket both offer a 4th clutch hub with heat treated splines which eliminates the problem.  Must all 2003 and newer vehicles came from the factory with the heat treated version.  Unfortunately the transmission has to be taken half way apart to get to this and is quite a chore to do in the vehicle though it can be done.   Below is a picture of an early non heat treated shaft with stripped splines next to a newer shaft with heat treated splines.  To date I have not seen a heat treated 4th clutch hub/shaft fail