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In this investigation, more than one hundred valves from forty seven fired engines have been examined. The worn surfaces of valve and insert seats from a wide variety of engines were studied using optical microscopy, SEM and EDX. The purpose of this investigation is to obtain a fundamental understanding of the primary wear mechanisms on the valve/insert seat of the engines. The wear mechanism which dominates the valve/insert seat wear depends on the engine operating conditions, seat contact configurations, valve/insert seat materials, and dynamics etc. Besides adhesion, corrosion and abrasion wear mechanisms, this study also found that shear strain is a major factor contributing to engine valve/insert seat wear. When shear strain at the seat surface exceeds the limit of the material, delamination wear is produced.