This washer sits between the engine block and the mounting face of a driven accessory, such as a pump or idler pulley. It provides a controlled thickness to maintain the correct axial position of the attached component relative to the belt or drive line. A worn or missing washer allows the accessory to shift, causing belt misalignment and uneven wear on the pulley face. The original thickness matches the stack-up height of the original assembly, so no shimming is required during reinstallation.