Boat trailer hubs get warm while traveling to the ramp, and when they're backed into cool water during launching, this contracts any air inside and creates a vacuum. The vacuum pulls water and road grit inside the hub, contaminating the grease and causing corrosion and rapid wear. These chrome-plated Smith bearing protectors prevent this problem by filling the entire hub with grease. Most air that is trapped inside will be expelled by the bearing protector's pressure release valve. Once the hub is full, the spring-loaded piston in the bearing protector exerts a slight pressure to the grease inside the hub, which prevents water and grit from being drawn inside. Automatic pressure relief feature prevents over-filling the hub which would damage seals. Fits hubs with inner diameter of 2.328".