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Our 348ci to 434ci recipe starts by boring the stock 4.125 bores 0.030 over to 4.155 inches. Though sonic checking is a smart move prior to any cutting, these blocks are known to be plenty thick. Our bore walls measure a super-safe 0.175 average thickness after boring. A second operation enlarges the crescent-shaped combustion chambers atop each bore by 0.060 to a final size of 4.215. Chevy originally specified the oversize region to allow piston ring installation, and it's just as important today.