S&S adjustable pushrods allow the engine builder to set identical pre-load for each tappet to prevent tappet noise and tappet "pump-up". S&S adjustable pushrods are made of aircraft quality steel tubing. For Evolution Big Twins & Twin Cam models there also are special "Lightweight" kits, made from aircraft quality aluminum. Both aluminum and steel versions are stronger than the original equipment push rod. Pushrod "flex" can initiate valve train harmonics that reduce power and cause damage to valve springs and tappets. Each set of adjustable push rods contains two intake and two exhaust length pushrods for the most accurate geometry and the best strength characteristics.