Retro-Fit ProMax High RPM Chevrolet Mark IV, Gen 5/6 Hydraulic Roller Lifters Designed to operate at rpm ranges over 7000 rpm. All new internal valving for increased rpm, torque and horsepower. The lifter body is fully machined from alloy steel bar stock. The nose wheel is machined from 8620 material and case hardened for durability. Note: spring pressure requirements for these lifters are 200-225 lbs. seat pressure, 500-575 lbs. open pressure. Specific lash adjustments are also required. Made in the USA!