The poppet valve in an internal combustion engine is subjected to some very harsh operating conditions, with rapidly changing stress and thermal cycles. There are generally four other components that are in contact with the valve - the lash cap, stem seal, valve guide and valve seat. Some engines run without stem seals though, and lash caps were dealt with in another RET-Monitor article recently. Although they have a very similar cycle of operation, the inlet and exhaust valves are...