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A valve stem seal is designed to reduce oil flow from the valve train area of the cylinder head into the intake and exhaust port of the engine. However, these seals have to allow adequate lubrication of the valve stem / valve guide contact area and therefore need to provide a minimal amount of oil flow. Even such small quantities of oil passing the seals into the ports and the combustion chamber will effect engine emissions. Exhaust emission measurements on a modern 16 valve gasoline engine illustrate the seals contribution on total engine emissions and allow prediction of the effect of adjustments of the oil metering rate to actual engine requirements.