Cylinder Head Specifications:; Combustion chamber volume: 87cc; Intake runner volume: 215cc; Exhaust runner volume: 175cc; Intake valve diameter: 2.11"; Exhaust valve diameter: 1.66"; Valve stem diameter: 11/32"; Valve guides: Manganese bronze; Deck thickness: 5/8"; Valve spring diameter: 1.46"; Valve spring maximum lift: .575"; Rocker stud: 7/16"; Guideplate: Hardended steel; Pushrod diameter: 5/16"; 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 #60579/#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 #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 #60587 is not port matched or bowl blended and is NHRA accepted for Stock & Super Stock. INSTALLATION NOTE: These heads require Ram Air IV (round port) style exhaust manifolds or headers and Edelbrock head bolt kit #8549. Deck thickness: 5/8", rocker stud diameter: 7/16", pushrod diameter: 5/16" (width of slot in guide plate).