The Edelbrock #60609 Performer RPM CNC Bare Cylinder Heads have the same specifications as our popular Performer RPM cylinder heads for 1962-79 389-455 c.i.d. Pontiac engines, but feature a CNC machined combustion chamber. The CNC machined chamber results in a consistent and precise chamber for improved flow performance. Performer RPM CNC cylinder heads are available in two chamber sizes. The 87cc chamber version is ideal for 9.5:1 engines and will deliver excellent street performance on low octane fuel. The Edelbrock #60609 Performer RPM CNC Cylinder Heads are patterned after the same port flange configuration as the 1969-1970 Ram-Air IV Pontiac Cylinder Head.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 • Match with Edelbrock complete head gasket set #7382. • Deck thickness: 5/8" • Pushrod diameter: 5/16" (width of slot in guide plate).