I bought a simple
chamfer tool at harbor freight to cure the problem I had with the seals squishing out from under the steel washers- just be sure to block the through-bolt passage from the other side before you chamfer the hole, and be sure to clean/blow out all the little metal bits that result from the chamfering, and be sure to take it slow and easy to make sure the chamfer is nice and smooth... it seems that one side of the case is always worse than the other, so you can use a good chamfered hole as a reference for how far to go.
Curt