This pressure-adjustment washer sets the clamping force on the header's knife-drive or belt-tensioning mechanism. It sits between the adjustment screw and the spring pack, providing a controlled shim thickness that determines the preload on the assembly. An incorrect or worn washer shifts the clamping force outside the design range, leading to belt slip or excessive bearing load on the drive components. The manufacturer machines the washer to the same thickness as the original part so that the adjustment range of the screw remains centered. On reinstallation, clean the contact faces and verify that the washer seats flat against the spring retainer.