TP Pro-Series® Forged Roller Rocker Arms TP Engineering's Roller Rocker Arms are forged from 4140 steel and CNC machined for a perfect fit. Each rocker arm includes solid bronze bushings for extended life, and roller tips that reduce operating friction so well they can give extra horsepower over your stock rockers. Pro-Series® Roller Rocker Arms fit all Evo-style, Big Twin, Sportster, and Twin Cam engines. Sold in sets of four.