This metal washer sits between the gearbox housing and the shaft shoulder, providing a precise axial locating surface for the gear train. It maintains the correct end-play of the rotating assembly, which prevents gear clash and bearing preload loss during load changes. A worn or undersized washer allows the shaft to drift, causing misalignment between the meshing gear teeth and accelerated wear on the synchronizer components. The part is manufactured to the original thickness and bore tolerance so that the stack height in the gearbox remains within the specified range without requiring additional shimming. On reinstallation, clean the mating faces and verify that the washer seats flat against the housing shoulder.