Fender Washer A fender washer is a flat washer with a larger outer diameter in proportion to its center hole. They are made from a thinner gauge metal than most flat washers and are designed to spread the load on thin sheet metal. Interestingly, they are named after their use on automobile fenders. Often used in sheet metal, plumbing, and electrical work. Zinc plated.

To increase surface area (more than flat washer) & avoide crushing due to tightening of fastener.A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a bolt or nut.