The Ultra Pro Mechanical Lifters are made from a billet body with a clipped axle and have 8,000+ RPM capability. The .842" & .875" diameter lifters in this series have a .750" diameter roller wheel. The .903" diameter lifters have a .810" diameter roller wheel and the .936" diameter lifters have a .850" diameter wheel. Recommended Spring Pressures are 250-350 lbs Seat Pressure and 600-850 lbs Open Pressure.