When the valve is then opened a small particle of the iron seat is pulled away. This particle is converted into iron oxide in the hot exhaust gas. As the valve then closes again these particles are imbedded in the valve face. With these hard lumps of iron oxide imbedded in it, the exhaust valve becomes, in effect, a grinding wheel. Thus any rotation of the valve will cause it to grind away the seat.
Valves tend to rotate under the action of the valve springs and by engine vibration.
Alternatives Advice on the precise fuel requirements of a particular vehicle should be obtained from its manufacturers, the appropriate one-make club or other authoritative source. This is the approach used on modern engines designed to run on unleaded petrol and is totally effective. However it can be expensive and is not possible on all engines. TEL was first added to petrol during the 1920's as an anti-knock additive and its function in forming deposits of lead compounds which acted as high temperature solid film lubricants on exhaust valves and seats was not understood until later. It was quickly realised that although there were many lead substitutes available for which great claims were made there was no standard test against which these claims could be validated.