This washer sits between the concave assembly and the frame of the threshing mechanism. It maintains the correct spacing and alignment of the concave relative to the threshing rotor. A worn or missing washer allows the concave to shift, which can reduce threshing efficiency and increase the risk of mechanical contact between the rotor and the concave. The integrated nose on this washer provides a positive locating feature that prevents rotation during operation. On reinstall, ensure the washer is seated fully against the locating face before tightening the retaining hardware.