Dust Cap
A metal or plastic cap pressed onto a wheel hub to protect the outer wheel bearings from dust, water, and debris.
A dust cap (or grease cap) is a small, dome-shaped metal or plastic cup pressed into the center of a wheel hub or brake drum. Its primary function is to protect the outer wheel bearings and grease cavity from road dust, water, mud, and road salt, which can contaminate the lubricant and cause bearing failure.
Inside the hub, the cap also acts as a reservoir to retain the bearing grease, preventing it from throwing out onto the wheels or brake assemblies under centrifugal force. It seals the threaded end of the wheel spindle, the spindle nut, and the cotter pin assembly.
During wheel bearing service, the dust cap must be carefully pried off to access the spindle nut. If a cap is missing, dented, or loose, water will enter the hub, emulsifying the grease and leading to bearing rust, noise, and overheating. Replacing damaged dust caps is a cheap way to ensure wheel bearing longevity.