Performer Pontiac heads utilize a D-Port exhaust port configuration for use with standard D-port exhaust systems. They are designed for 1962-79 389, 400, 421, 455 cid V8 Pontiac engines with a 4. 060in. or larger bore. They feature optimized oil return passages for increased oil flow. These heads are available in three heart shaped combustion chamber sizes, including 65cc, 72cc and 87cc. The 65cc combustion chambers are as-cast while the 72cc and 87cc chambers are fully CNCd to provide a 9. 5: 1 compression ratios when used with flat-top pistons. The spark plug has been relocated closer to the exhaust valve for improved performance. These heads are cast with extra thick port walls and a compact port size, making them ideal for CNC and hand parting. Complete heads include guide plats, rocker studs and high quality valve train for direct bolt-on application. Head crossover machined for 50-state emissions legal performance.