Comp Cams offers a complete line of valve stem seals, from the stock GM O-ring to the positive stop PTFE seal. These seals are a must to keep unwanted oil from entering the combustion chamber through the clearance in the valve guides. O-ring seals use the standard retainer and oil splash shield. Umbrella seals are normally used when a larger-than-stock diameter spring is used; in the case of a double spring, positive stop PTFE seals are used. There is also a smaller diameter Viton seal available for triple spring applications. O-rings and umbrella seals require no machining, but PTFE seals require the heads to be disassembled so the top guides can be machined with a special .530" diameter cutter. Whenever you're planning an overhaul or cam change, don't forget the valve seals!