Bearing Buddy replaces the dust cap in the axle hub. The hub is filled with grease via a fitting in the piston of the Bearing Buddy. This spring loaded piston then holds a slight, constant pressure inside the hub. When wheels are submerged, this higher pressure, together with the hub being full of grease, keeps out water. The piston rides on an O-ring seal. Any attempt to overfill the hub forces the piston out beyond the O-ring, and excess grease escapes around the edge of the piston. This automatic pressure control feature limits pressure in the hub to 3 psi to protect the rear seals from rupture. Since the hub is full of grease, the bearings are constantly lubricated and the rear seals last longer as they ride on a lubricated surface. To convert a model number to the hub bore in inches (or dust cap diameter), simply place a decimal point after the first digit in the model number (1.980 for example).