Head gaskets are always part of the compression equation. The 252cc 1-inch-down cylinder volume, 86cc postmilling chamber volume, and 0.039-inch-thick Fel-Pro composite gasket (bottom) yield a calculated 9.75:1 compression ratio. If we substitute the 0.015-inch-thick P4286754 MP steel shim head gasket (top), compression rises to a calculated 10.4:1. We'd love the added cylinder pressure but refuse to be slaves to race gas or additives in 91-octane California. So we'll go with the thick gaskets on this all-iron mill for pump-gas compatibility.