This washer sits between the knife drum and the chopper housing on a forage harvester. It provides a wear surface that prevents the rotating assembly from contacting the stationary housing directly. Over time, the contact face wears, increasing clearance and allowing vibration that can damage the drum bearings. The manufacturer machines the washer to the same thickness as the original so that shimming is not required during replacement. On reinstall, ensure the mating surfaces are clean and free of debris.