A screw fastener, for fastening a workpiece to a fixed member, including a screw having a head having circumferential knurling and a stem having a first stem portion abutting the head and second stem portion abutting the first stem portion, the first stem portion having a smooth surface and the second stem portion having threads; a fixed member having a countersink bore section for accommodating the head and a threaded bore section for engagement with the second stem portion, the countersink bore section having a larger diameter than the threaded section; and a plastic washer having a hole that allows the washer to engage with the threaded section and an outer diameter larger than the diameter of the bore; the second stem portion passing sequentially through the workpiece to be fastened, the plastic washer, the countersink bore section and engage with the threaded bore section of the fixed member.