The compression in a 215 CID engine was changed with the size of the head cavity. The “volume” of a head is measured in cubic centimeters of liquid, usually water.The 10:75/1 automatic trans 1963 4 bbl engine has a 38cc head volume. The 4 bbl standard transmission and all Jetfire heads are 43cc or 10:25/1. The 2 bbl engines were all 51cc or 8.75/1. The exception is the export-low compression models. They were 8.25/1 for the 2 bbl and 8.5/1 for the 4bbl engines. Attached is the 1963 head numbers from my parts book and a chart for Piston/Rod/Compression ratio swaps for a 215 CID engine.
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