Valve stem seals are small relative to other gaskets and seals in an engine, but play an important role in lubrication. What makes valve stem seals different than almost every other type of seal? The answer is simple they are designed to leak. Seals designed to leak may sound counter-intuitive, but the amount and way in which they leak is precisely controlled to achieve a specific goal.
Valve stem seals provide a controlled leak of oil to allow the valve stem to be lubricated as it slides in the valve guide. The amount of oil that passes by the valve stem seal must be precisely controlled, as too little oil causes stem and guide wear. Too much oil causes carbon buildup leading to valve seat damage, decrease in volumetric efficiency, increased emissions and excessive oil consumption.
There are two basic valve stem seal designs:
- Deflector seals, also called umbrella seals, deflect oil away from the valve stem. They are secured to the valve stem and move with the valve to shield the valve guide from excess oil. Umbrella type seals were commonly used prior to the development of positive type seals.
- Positive seals, attach to the valve guide boss and function as squeegees, wiping and metering oil on the stem as they pass through the seals.