Cylinder Head Specifications: , Combustion chamber volume: 87cc, Intake runner volume: 215cc, Exhaust runner volume: 175cc, Intake valve diameter: 2. 11in, Exhaust valve diameter: 1. 66in, Valve stem diameter: 11/32in, Valve guides: Manganese bronze, Deck thickness: 5/8in, Valve spring diameter: 1. 46in, Valve spring maximum lift: . 575in, Rocker stud: 7/16in, Guide plate: Hard ended steel, Pushrod diameter: 5/16in, Valve angle: 14, Exhaust port location: Stock Ram Air IV, Spark plug fitment: 14mm x 3/4 reach, gasket seat. These 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. Performer heads No. 60579/No. 60575 are 50-state emissions legal for 1965-79 vehicles. They feature 87cc combustion chambers designed to maximize performance while using less expensive, low-octane fuel with a streetable 9. 5: 1 compression ratio. Our Performer RPM heads with the 72cc combustion chamber are for higher compression engines. Performer RPM No. 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. Performer RPM No. 60587 is not port matched or bowl blended and is NHRA accepted for Stock and Super Stock. INSTALLATION NOTE: These heads require Ram Air IV (round port) style exhaust manifolds or headers and Edelbrock head bolt kit No. 8549. Deck thickness: 5/8in, rocker stud diameter: 7/16in, pushrod diameter: 5/16in (width of slot in guide plate).