Factory Block Oil Passage Drilling

Yep, we were doing the Guitar Jones oiling mods; the holes to the mains increase to 5/16"; that mod is a fun one. Getting the drill to start from the web end where multiple passages meet up is a trick. We started with a vary narrow angle, small reamer to open up the oil supply hole a bit, then followed up with a very small grinder to open it up to 5/16' deep enough to get past the edge of the cam oiling passage. Then, you could start the drill without it grabbing the edge of cam oiling passage.

But the GJ mods don't discuss the oil galleries per se for complete step drilling end-to-end; so that I why I am suggesting an inspection of them. If you find any unusually small lifter oiling holes, that is a distinct clue to that potential issue.