Keeps water and dirt out of wheel bearings and keeps them properly lubricated. Makes hubs safely submersible. Easy to install. Automatic pressure control feature limits pressure in hub to 3 psi to protect seals from rupture. Allows no air to be forced in, eliminating corrosion. Lubricant level can be checked by pressing on edge of piston. Stainless steel internal parts and triple chrome plated barrel. To convert model number to hub bore, place a decimal point after the first digit. Outer bearings given.