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In this investigation, worn valve seats and inserts have been examined to obtain a fundamental understanding of the primary wear mechanisms. Seats and inserts from a wide variety of engines were studied using optical and electron microscopy. The recessed region on seats and inserts displayed a common feature which can be described as a series of ridges and valleys formed circumferentially around the seat axis. These ridges and valleys were found to be due to either a relatively severe gouging process or to a milder deformation and wearing process. These processes were related to assembly and product geometry factors which caused misalignment between the seats and inserts.