"Almost all aftermarket cylinder heads measure 4.100 inches across the chamber," Naegele says. "Most gasket bores measure 4.100, and a standard 302 bore is 4.00. A decent rebuild is 4.030. You can see in the illustration that you end up with a lip at the top of the cylinder that really causes havoc with flow. The shrouding effect of the cylinder and disruption of smooth flow into the cylinder is detrimental to power. If you want to maximize your investment in high-flow cylinder heads, put them on a big bore or use a Level 20 block with the elliptical chamfer."