This washer sits between the concave adjustment mechanism and the threshing drum frame. It provides a controlled wear surface and maintains the correct axial clearance as the concave position is changed during operation. A worn or deformed washer allows the adjustment linkage to develop play, which leads to uneven concave gap and reduced threshing efficiency. The part is manufactured to match the original profile and thickness, so it restores the factory clearance without requiring shimming. On reinstall, ensure the washer sits flat against the locating face and that the retaining hardware is torqued to specification.