Ql What is valve seat wear? Valve seat wear, or more accurately, exhaust valve seat recession (VSR), occurs when an exhaust valve impacts upon a soft cast iron valve seat at engine operating temperatures. The mechanism consists of; i) localised welding of valve to seat resulting in transfer of material from seat to valve when the valve reopens, followed by oxidation of this material to iron oxide. ii) accumulation of these iron oxide nodules on the valve, which stand slightly proud of the valve face and are harder than the iron valve seat. iii) abrasion of the valve seat by the nodules on the valve (acting as a grinding wheel) which is exacerbated by valve rotation. These situations are observed when vehicles with soft valve seats are forced to operate on unleaded petrol. Refractory lead salts such as oxide, sulphate and halides act as solid lubricants preventing contact between valve and seat. VSR protection additives act to lay down material in a similar way but are generally less effective than lead. See also the answer to Question 7.