USS Flat Washers United States Standard thread (USS thread), also known as Sellers Standard thread, Franklin Institute thread and American Standard thread is a standard for inch based threaded fasteners and washers.
Flat washers are used under bolt heads or nuts to increase the bearing area or offer greater hardness, 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 nut or bolt.
USS flat washers have a larger outer diameter than SAE flat washers. Unless the buyer has a specific USS or SAE requirement the buyer should consider the measurements, based on need and not whether the flat washer is USS or SAE.