I bet if you put the numbers down correctly it would only be 9.5:1 with the 53cc heads. My 1983 305 was about 10.2:1 from the factory. Flat top pistons with 6cc worth of valve reliefs, .025" down the bore, .020" thick steel shim head gasket and the 53cc heads. Pulled a couple of 85-86 305s with the same heads that had dished pistons in them.
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