[DaveR]

The real danger with core shift is when the water jacket cores that surround the cylinder walls shift during casting. Then the resulting wall section on that cylinder is thinner than spec. Typically when a builder says that a certain block type like a 351W, or a 460 can be over bored to .030, .060, or .080 safely, it is based on a statistically average wall thickness in the original block casting. Of course, an average is made up of highs and lows, so if the specific block you end up buying has one cylinder with a thinner than average wall, you could have problems, even at the low end of the over bore scale. This wall thickness can be checked with a special process, and when you consider how much money would be lost in a meltdown, it is probably worth the expense up front when doing a first class build.