Gareth, you need a minimum of 120thou bore wall thickness AFTER boring to your desired size. 'Correctly' sonic testing the block before boring to pinpoint the thinnest part of the bore, is the only way to determine where you can bore & how big you can go. Check out this photo of a L28 block that Les milled the top off: (Don't despair - that block had a dirty great big crack running along the deck between the stud holes. It was only ever going to be scrap). It's standard 86mm bore N42 block & if you look closely you can see how much core shift it has in the bore walls. After going through & sonic testing 10's of L28 N42 & F54 blocks for customer stroker builds, Les has found no difference between the two types of block castings in relation to bore thickness's. 'Most' L28 blocks can safely be bored to 88.5mm (some like Gav's can't), a very small number can go to 89mm, and I've only seen two that can tolerate a 90mm bore safely (Owen's and the US L34). It's just all pot luck - you can can't tell until the block is acid bathed & sonic tested.