These Magnum Roller Rocker Arms for Ford V8 289-351W (Rail Type) 1968-89 applications are the ultimate street rocker. With a 3/8 inch stud diameter and 1.6 ratio, they are designed for serious performance enthusiasts. All Magnum Roller Rocker Arms are constructed from 8620 chromemoly 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 valve 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. Don't accept inferior products or cheap knock-offs, look for the COMP Cams logo stamped into the underside of every Magnum Rocker.