A water pump bearing is a device used to support the end of the water pump impeller shaft. Fitting securely around the impeller shaft and tightly into the water pump housing, the water pump bearing is designed to withstand the stress and load placed upon the water pump shaft by the engine's accessory-belt drive system. In many cases, the water pump bearing is charged with performing double duty as a bearing for the rotating impeller shaft as well as a seal to prevent the coolant from leaking out of the water pump housing.
The sign of a damaged water pump bearing can come in one of several ways. A squealing sound is a common sound of a failing water pump bearing, however, many bearings have been known to go bad without making a sound. Often, a bad water pump bearing will be noticed by leaking coolant coming out of the bottom or the backside of the water pump. In this event, it is time for a new water pump or a rebuild for the existing pump. In some circumstances, a rebuild kit is not available for the water pump, so a new water pump must be installed when the bearing goes bad in the existing pump.
The style of bearing used in the typical water pump is a sealed bearing. The sealed bearing allows the component to hold the coolant inside of the engine without seeping out around or through the bearing. The bearing is commonly pressed onto the water pump impeller shaft since the fit is extremely tight. The outer surface of the bearing is known as the race. This is commonly machined to a slight angle on a water pump bearing to allow the bearing to better resist leaking.
Manufactured to exacting tolerances, the water pump bearing is designed to operate at extremely high speeds for long periods of time without failing. The working environment is very harsh for the coolant pump bearing. Operating in heated liquid, under severe load and at higher speeds than the typical bearing is exposed to, the water pump bearing is subjected to many abject conditions that would destroy a lesser bearing in a very short time. It is because of this extremely harsh operating condition that most water pumps are typically not able to be rebuilt. Many manufacturers are not able to warranty the inner components of the water pump against do-it-yourself rebuild and installation.