I have zero knowledge on the max bore... but when I was building my .060 481988 400 stroker I asked Butler Sr on a phone call if .060 was a concern on these early 70's 400 blocks, he chuckled and said 99% of the time its not a concern, he didnt get into core shift or sonic testing, just said usually not a problem at this power level of ~500hp.
He then went on to say how back in the day before strokers were available he, Arnie B & others used to bore mid 60s 389 blocks to .090 & .100 with no real issues, and that was for hard racing use... no hard block or sonic testing, just pushing the limits of what those blocks could do.
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