valve stem oil seal

Oil seals are fitted to the valve stems or the guides on both intake and exhaust valves. They prevent too much oil passing down into the combustion chamber.

The valve stem oil seal must produce a controlled flow (regulated) leak. It is much more difficult to achieve controlled flow leakage because the margin for error is so small since it is so important for a thin film of oil to remain between the valve stem and guide. However, the amount of oil used to form this film must be strictly controlled. For this reason, do not use the Original Equipment O-ring type valve stem seals. The design of this type of seal only permits it to prevent oil trapped in the valve spring retainer cup from draining down the valve stem and puddling atop the valve, a condition that induces smoking upon startup and leads to heavy, flow-restricting carbon deposits atop the heads of the valves, as well as carbon accumulation on the roofs of the combustion chambers and the crowns of the pistons.