Unlike the plastic gears in the later GM transmissions, where you could get away with just changing the driven gear, the drive and driven gears in an SM420 have to be changed as a set. There would have been a set for a 4:10 one for the 4:57, and a set for the 5:14. If you change only the driven gear you will chew up the drive gear and good luck finding another.
Denny Graham
Sandwich, IL
