Gathering information from your other post regarding overly tight connecting rod bearings on a recent rebuild (216 bored out to 235), I would suspect that your machinist either is unfamiliar with these engines because he is too young, or he is old enough to be familiar with them, but too old to see what he is doing anymore.
He didn't get the connecting rod clearance right, or he didn't check for bent connecting rods, or bent them while tightening the wrist pin bolts, which will cause the appearance of them not having enough clearance.
If he did not know how to do those tasks properly, then it would stand to reason that he was not familiar with the main bearing shimming procedure. That would cause low oil pressure if too many shims were used.
My apologies for abruptness, if said machinist was you.
Carl
