These Magnum Roller Rocker Arms for Ford V8 289-351W (Rail Type) 1968-89 Applications are the ultimate street Rocker. With a 3/8" 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 lbs. 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.