The Pro X Steel Exhaust Valve Spring Kits are manufactured so close to OEM specs, you can reuse your stock retainers, bases, and seals without any modifications. It all begins with a 21-4N stainless steel valve that is black nitrided for an unprecedented combination of core strength and surface friction reduction. Then comes the chrome vanadium, heavy duty springs built specifically to handle the extra weight of the steel valves. Even at high rpms these springs hold a tight seal, so your valves won't flutter or float even after seasons of you being heavy on the throttle. Everything comes together to create a valve kit that restores compression so your machine runs at peak performance.