The Edelbrock #60587 Performer RPM NHRA Stock/Super Stock cylinder heads are designed for outstanding performance gains in 1962-79 389-455 c.i.d. Pontiac engines. They feature 87cc combustion chambers designed to maximize performance while using less expensive, low-octane fuel with a streetable 9.5:1 compression ratio. They are patterned after the 1969-70 Ram Air IV Pontiac cylinder head. Performer RPM #60587 is not port matched or bowl blended and is NHRA accepted for Stock & Super Stock. All Edelbrock Cylinder Heads are designed from scratch using the latest in computer technology by our own highly skilled engineer's. Using highly efficient ports and state-of-the-art combustion chamber shapes, Edelbrock cylinder heads offer improved performance throughout the rpm range for great throttle response and top-end horsepower. Top-quality features include threaded inserts in rocker stud and exhaust bolt holes for superior strength and durability, and manganese-bronze valve guides with ductile iron valve seats. These are features that some other companies don't include! INSTALLATION NOTE: • 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).