#373820-S Copper washer for attaching the 8 inch or 9 inch third members to the axle housings. Steel washers will leak, made from copper which is a soft metal and seals under the attaching nuts. Copper washers like the OEM Ford installed washers. Takes 10 washers total for both 8 and 9 inch rear ends.
*Tip - It is a good idea to inspect or replace your old washers, many times they have tool marks from the housing or nuts that cause small troublesome leaks.