All this is exactly the reason I say put the seal in the block FIRST, with the ends at 12 and 6 oclock. Then you do whatever you have to do to make the retainer slide over that seal cleanly and freely, with NO influence either direction. With it all dry and no bolts, drop the retainer on and see how it feels and look in the holes to see if it is where it NEEDS to be, not where the bolts, or those idiotic side seals put it. One thing to understand.......the SEAL is what seals the crank. The retainer only holds the seal where it needs to be. With the seal in the block around the crank, the seal IS where it needs to be. Now make the retainer cleanly hold it. This is a piss poor design and always has been, but not unfixable. You need to drill the retainer holes out, drill them, makes no difference. This is NOT a critical piece. It ONLY holds the seal, nothing else. It's just not this hard

Other motors with two piece seals don't have this issue. The reason, is the seal is retained by the rear cap and is always consistent and goes on where it should every time. BBM doesn't have that luxury but is not hard to MAKE it work in a similar way.

If you have never had a BBM rear seal leak.........you simply haven't built enough of them. But since I started doing the above procedure....30 years ago or more........no more leaks

Many also want to "face" the retainer on the bottom, to make sure it sits flush. It MAY need it, but usually not. This generally just makes the seal groove not round any more and changes the crush. If you do that and put the seal where the ends are flush with block and retainer, it will NEVER seal. Its an easy measurement to determine if the retainer NEEDS a shave. Don't do it "just because"