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Brian told us it's not only possible to bore out a 318 to 4 inches, but he's done it many times. He thoroughly sonic-tests the block to make sure there are no weak areas in the cylinder barrels before doing this, though. He likes to leave at least 0.120 inch of material on the thrust side of the bore and 0.100 inch on the nonthrust side and won't bore out a block that will measure less than 0.090 wall thickness anywhere in the cylinder. That's a 4-inch head gasket held up to a 318 block, demonstrating the difference in diameter.