The washers are bright zinc plated for increased corrosion resistance, and have been given the name 'Penny Washer' due to their similarity in size to an old penny. Washers provide a smooth surface for the nut or bolt to bear on, making it less likely to loosen as a result of an uneven fastening surface. The main purpose of a washer is to distribute the load under a bolt head or the nut face. OD - Outer Diameter