Machine Screw Flat Washers are the smallest type of flat washer because their outside diameter (OD) is usually just a little more than twice their inside diameter (ID). If you need a larger OD flat washer, choose SAE or, larger still, USS. Washer size refers to its nominal hole size and is based on screw size. Print Tell a Friend. Code Product Qty. Package Qty. Carton ; WS #2 .