Jims Powerglide™ tappets will give you a superior hydraulic function when compared with OEM style equipment, a billet steel body with hand-honed hydraulic unitcavity and an oil fed axle. To get full power, you must have zero valve lash and eliminate collapsing of the hydraulic unit as is found in the OEM unit, also called ‘the shock absorber syndrome’. Failing to keep the valve open the full amount results in power loss. The Jims Powerglide tappets come as close to a solid tappet as possible, but are still able to compensate for heat expansion. They will even work well with solid style camshafts. Fits as a improved oversized diameter replacement for OEM 18538-99C. Note: Standard is .8420″ O.D.