These Magnum roller rocker arms for Ford V8 289-351W (Rail Type) 1968-1989 applications are the ultimate street rocker. With a 3/8 inches stud diameter and 1.7 ratio, they are designed for serious performance enthusiasts. All Magnum roller rocker arms are constructed from 8620 chrome moly steel, which is a superior material that will not flex and assures maximum lift. Their roller tip reduces friction, while stiff design and superior ratio accuracy yield more effective value lift. With a 15 to 30 horsepower increase over other rocker arms, Magnum roller rockers are perfect for performance applications with less than 350 pounds open spring pressure.