Ultimate Duty 3/8 .531 PC Valve Stem Seals The Best Seal Made, Period! Howards Premium PC type seals are made of a space age material which is tolerant to pump gas, alcohol, and nitromethane. Temperature tolerant to 350?. They are self-lubricating and offer controlled radial tension during changes in operating temperatures. Color Coded. Controls oil while lubricating the valve guide.