The 5/16 x .063 NAS1149 Military Flat Washer is a thin, circular metal ring with a hole in the center. It is made of 18-8 stainless steel, which is resistant to corrosion and rust. The washer has a diameter of 5/16 inches and a thickness of .063 inches. It is designed to be used with a 5/16-inch bolt or screw. The washer helps to distribute the load of the bolt or screw and prevent it from damaging the surface of the material it is being fastened to. It also helps to prevent the bolt or screw from loosening over time.