Valve Stem Oil Seals, Positive Stop Tefflon 512-16 CO512-16 Suit 3/8" Valve, .500" Guide, .600" O.D VALVE STEM OIL SEALS, 3/8" Comp Cams offers a complete line of valve stem seals, 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 larger than stock diameter spring is used, and in the case of a double spring, the positive top Teflon 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 with the Teflon seal it is necessary to disassemble the heads and machine the top of the guide with a special cutter.