Keep breaking 3rd gear

I had a car do this a while back. We were not fixing the trans but just installing other transmmisions. Not 18 spline but not that HP either. And it was always Third gear.

What we found was the bell housing was not indexed to the crank properly. Since then we always index the crank to the bell and found many to be out. Even stock bells when moved to a different block.

If you **** the input it will change the mesh of the gears to tight or loose. Most good bells when purchace have weld on alighnment pin washers for this reason. Buy a magnetic dial indicator and check this first. They make eccentric dowl pins with slots to turn for adjustment. But when using them every time you take the bell off of the motor it has to be reindexed when installed. A large hole on bell and a weld on thick Bushing/washers are the way to go with a steel bell.

Standard bells also flex with that much HP. Blow proof steel with a mid plate is the best. Steel mounts also help. Properly installed and reamed bushing also. Steve