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The aim of this work was to investigate the wear mechanisms occurring in passenger car diesel engine inlet valves and seat inserts using a bench test-rig designed to simulate the loading environment and contact conditions to which the valve and seat are subjected. The investigation has shown that the valve and seat insert wear problem involves two distinct mechanisms: impact of the valve on the seat insert on valve closing and sliding of the valve on the seat insert under the action of the combustion pressure. The prevailing wear mechanisms have been shown to be related to critical operating conditions such as valve closing velocity, combustion load and valve misalignment relative to the seat insert and valve and seat insert material choice.