One of the most common repairs on centrifugal pumps is replacing worn or damaged wear rings. For pumps with closed style impellers (impellers with a front shroud as described below), there will be a casing wear ring and possibly an impeller wear ring which is fitted to the outside diameter (OD) of the impeller suction eye. Impellers may also have rear wear rings which are important for controlling axial thrust. Pumps with open style impellers generally do not have suction eye wear ring clearance concerns, but they often have rear wear rings. The clearances between the stationary casing wear ring and the rotating impeller wear ring are critical to proper pump operation. Although many pump manufacturers will provide proper clearances and dimensions, some do not; there are plenty of old pumps around from now defunct manufacturers where dimension data is simply not available.