Fender washers, also called flat penny washers, are distinguished by their wide outer diameter compared to their inner diameter. The outer diameter consists of a solid material ring encircling a fastener, while the inner diameter forms an empty central hole without any material.

These washers' distinctive construction allows them to function beneath nuts or bolt heads to effectively spread force during tightening. Their expansive outer diameter provides superior protection against wear compared to conventional washers. They effectively distribute the fastener's pressure over an extended surface area. Additionally, fender washers prove useful when securing fasteners in components that have experienced rust or corrosion damage.