This is the under ported version of Edelbrock's LS1 cylinder head for the professional cylinder head porter or engine builder. It's machined to stock dimensions but with extra thick material in critical areas for greater flexibility in port design. Smaller-than-stock runners with added material around the port allow for hand porting or CNC machining, so any CNC program designed for stock LS1 or LS6 can be run on this casting. Seats, guides, and heli-coils are all installed.