The serrated washer is important as it prevents the special conical oil pump nut from loosening off the oil pump mounting stud.
Applicable for 1961-1974 350 & 500 models, 1963-1972 650 models, and 1973-1979 750 models.
The serrated washer is intended for one-time use, as it’s soft composition disintegrates when removed from the oil pump mounting stud. It secures the conical nut under a light torque specification.
The washer fits into the concave mounting hole on the brass body of the oil pump body. The conical nut is locked into place when the washer deforms. The washer is not reusable once deformed.