The Edelbrock Sure Seat Valve Spring is made of the highest quality spring wire and is precision-wound to precise specifications. They are designed for use on Original Equipment Manufacturer (OEM) heads when used in conjunction with their Performer-Plus, Performer RPM and Torker-Plus camshafts. It closes the valve while also ensuring that the valve train stays in contact with the cam lobe. They provide adequate seat pressure to allow a tight fit between the valve face and the valve seat to seal the combustion chamber. This valve spring seat provides protection for aluminum heads and locates the valve spring to prevent it from moving around at higher engine speeds. It requires checking the clearance between the rocker arm and the retainer before starting the engine. The inside diameter of the inner and outer spring is 0. 880 and the outside diameter of the outer spring is 1. 265.