This flat washer sits between the cutterbar knife drive and the header frame, distributing clamp load over a larger bearing surface than a standard fastener provides. It prevents the bolted joint from loosening under the cyclic shock loads transmitted during cutting. The washer's inner and outer diameters match the clearance hole and the housing boss, so the clamping force is applied evenly across the mating face. A worn or deformed washer allows the joint to relax, which leads to play in the knife-drive linkage and accelerated wear of the connecting pins. On reassembly, ensure the washer is seated flat against the machined surface before torquing the fastener.