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GM uses casting cores sized to the eventual machined finished bore so with some rare exceptions like 1950’s and 60’s 283’s and the late 60’s to early 70’s 307’s which are safe to take to 4 inches, these other engines like the 305 can’t be bored to 4 inches.

A great comment once made perhaps by either Hot Rod or Popular Hot Rodding about the Ford Windsor 221-302 engine was you can’t bore a 221 to a 260 nor a 260 to a 289/302 because Ford casts with “rubber cores”. The intent was to convey that the casting cores are specifically sized to what will be the finished bore not to imply that the cores were actually made from rubber. One needs to appreciate that Ford with the Windsor set the stage for thin wall castings. Prior to this time cross sections were thick because the casting process was quite imprecise dimensionally so the specified cross section thickness was sufficient that the greatest casting error still allowed sufficient thickness to carry the design loads after machining. Back in those days of yesteryear castings you could bore like crazy. It was common to punch anybody’s block an eighth (.125) inch so getting 20 inch gains on a bore job was common and still had enough for another rebuild on the block, yes they were heavy. The dangers were when the OEMs tried to convert old designs to thin wall it didn’t always work. Once again Ford being notable when they tried to recast the FE into a thin wall casting in 65 or 66 they got into a lot of warrantee failures, it cost them a lot of money for fixes and replacements plus the cost to return to the old fat cross sections. Chrysler tried and ran into big problems as well but worked through them without having to redesign their engines. For the most part GM muddled through a semi conversion to thin wall without severe issues. The 350 as an example prior to the change could punch .060 without even thinking about it, point oh-eighty was possible with some study for core shift. After thin wall .030 is perfectly safe, .040 on visually select blocks, .060 with ultrasound testing.

Bogie
 
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