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 larger than-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. PartNumber : cca500-12 Description : Positive Stop PTFE Seal; 12 Pieces Title : 500-12 Valve Seals, 5/16" PTFE California Proposition 65 : WARNING: This product contains a chemical known to the State of California to cause cancer, birth defects or other reproductive harm. Seal Outer Diameter : 0.63 Valve Guide Outer Diameter : 0.53 Valve Seal Type : Positive Stop PTFE Valve Stem Outer Diameter : 5/16 Guide Size : .530" Valve Size : 5/16" / .626" Engine Components::Valve Seals