The M4 x 18W8 JIS B1188 Phil Pan Split Washer Flat Washer Sems M/S Zinc is a type of fastener that is used to secure two or more pieces of material together. It consists of a pan head screw, a split washer, and a flat washer. The pan head screw has a round head with a slotted drive. The split washer is a thin, springy washer that is placed under the screw head. The flat washer is a larger, flat washer that is placed over the split washer. The screw, split washer, and flat washer are all made of zinc-plated steel.