Washers listed are extra thick. Thru hardened SAE flat washers are used under bolt heads or nuts to increase the bearing surface area, or to offer protection so that the nut or bolt can be rotated without distortion of the bearing surface. Flat washers are also used to give tightness to a joint, to prevent leakage, and especially to distribute pressure under the head of a bolt or nut. USS (United States Standard) flat washers have a larger outer diameter than SAE (Society of Automotive Engineers) flat washers, and can be utilized in nearly any application. These fasteners are available in yellow zinc plated finish. Zinc plating sacrifices itself, corroding before the base material, offering added corrosion resistance.