The Edelbrock #60509 Performer RPM cylinder heads are designed for outstanding performance gains in 1962-79 389-455 c.i.d. Pontiac engines. They are patterned after the 1969-70 Ram Air IV Pontiac cylinder head. The Edelbrock #60509 Performer RPM heads with the 72cc combustion chamber are for higher compression engines. Performer RPM #60509 is semi-machined and must be finished by a professional cylinder head porter. They include valve guides and seats installed with no intake pushrod holes.INSTALLATION NOTE: • Bare heads will have valve guides and seats installed, but will require final sizing and a valve job to match the valves you will be using. • These heads require Ram Air IV (round port) style exhaust manifolds or headers • Edelbrock head bolt kit #8549 • Deck thickness: 5/8" • Rocker stud diameter: 7/16" • Pushrod diameter: 5/16" (width of slot in guide plate).