Investigation of the Mechanism of Exhaust Valve Seat Wear in Engines Run on Unleaded Gasoline

Valve seat wear or “sinkage” or “recession” was observed in eight engines run on unleaded fuel. Detailed studies of valve seat wear were undertaken in a 302 cu in. V-8 engine run for 10-1/4 hr at WOT on a dynamometer stand. The seat was oxidized and flaky, and the valve face was spotted with hard (45-55 Rc) oxide nodules. It was concluded that during operation the oxide flakes adhere to the valve face to form nodules which become embedded in the metal (43 Rc). With the embedded nodules, the valve wears the cast iron seat (40 Rc) by abrasion. Loads, rotation, and temperatures are often influencing factors.

The use of leaded fuel in another engine was found to form PbO · PbSO₄ on the hot valve face and Pb (Cl_0.75 Br_0.25)₂ on the cooler seat. These compounds are high-temperature solid film lubricants which, by coating the surface, inhibit cast iron oxidation and also prevent material transfer.