Valve Stem Oil Seals, Metal Body Viton Seal 520-16 CO520-16 Suit 5/16" Valve, .530" Guide, For Use With Triple Valve Springs VALVE STEM OIL SEALS, 5/16" COMP Cams offers a complete line of valve stem oil 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. The o-ring seals use the standard retainer and oil splash shield. The umbrella seal is normally used when a largerthan-stock diameter spring is used, and in the case of a double spring, the positive-stop PTFE seal is used. Smaller diameter Viton seals are also available for triple spring applications. Both the o-ring and the umbrella seals require no machining, but when using the PTFE seal it is necessary to disassemble the heads and machine the top of the guide with a special cutter, which can also be found on this page.