E-Street cylinder heads are designed for entry level street performance engines operating in the idle to 5,500 rpm range. Manufactured in the USA, these heads feature the quality and details of higher priced cylinder heads, starting with A356 aluminum castings that are heat treated to T-6 spec for superior casting strength and quality. Their modern combustion chamber designs improve the combustion process and larger than stock intake and exhaust ports move more air through an engine; these features improve efficiency, resulting in more horsepower, broad torque and improved mileage potential. Every E-Street is assembled with manganese bronze valve guides, powder metal interlocking valve seats, stainless steel one-piece valves with hardened tips, 1.25" or 1.55" diameter valve springs, 3/8" or 7/16" screw-in studs and hardened guide plates. As added value, E-Street heads include hardened valve springs cups to prevent casting wear and Heli-coil™ threaded inserts in the rocker stud bosses for proper torque; these are the quality details that most low-cost competitors leave out of their heads. All E-Street cylinder heads are Made in the USA.