- The most-common kind of round washer, Flat Washers feature 2 flat surfaces for distributing the force/stress of a nut or bolt. - They also help to reduce friction and heat during the installation of a threaded fastener. Flat Washer Type / Flat Washer Washer Basic Material / Stainless Steel System of Measurement / Inch Washer Material Grade / 18-8 Fastener Finish / Plain Fits Bolt Sizes / #8 Washer Inside Dia. / 3/16 in Washer Outside Dia. / 7/16 in Thickness / 0.036 in to 0.065 in Rockwell Hardness / B70 For Use With / Screws and Studs Washer Standards / ASME B18.22.1 Package Weight / 0.4 Package Quantity / 50