I have sonic checked only a small handful of non-Mexican blocks that would be safe for a .060" overbore race engine. Of the many dozens of Mexican blocks i've sonic checked, there were still a small handful of those that wouldn't make a safe .060" overbore either. Of the ones that were truly thicker in the cylinder walls, they were only thicker by an amount enough to allow them a safe minimum margin to be bored to .060", provided core shift didn't bump it out of contention for consideration. Of the Mexican blocks that were thick enough to bore to .060" over, they were still required to be filled.
Of the many Mexican blocks that I have sonic checked that were thicker and had little core shift, I only saw maybe 2 or 3 that were thick enough to take a chance to bore to 4", and you still would have had to fill it. None of the non-Mexican blocks I ever sonic checked were ever thick enough to bore to 4" with or without filling. I know a lot of racers have run 4" bore engines before, maybe our standards were just higher than theirs or they were just lucky! But if your going to try a 4" bore, luck better be on your side. Headrick, like everyone else tried some 4" bore engines and they never lasted for one reason or another, mostly poor ring sealing because of too much cylinder distortion.
So even if you don't split a cylinder from boring one to 4", there are other issues that will arise that will make a .060" overbore a better alternative, especially if you plan to use it for competition.
