Been years since I've rebuilt an old 621 frame up so my memory may be a bit clouded. There is adjustment settings that need to be done to the transmission after install, the exact procedures would have to come from CAT dealer or SIS if you have access. If memory serves me correct the adjustments are done with long allen wrenches for the gears. After putting the machine back together I had several testing, adjustments to make and twice had to do the transmission once initially and then after traveling the machine and shifting through the gears once up to operational temperature. I would get hold of the information through SIS / CAT