The internal clearance in NSK rolling bearings in operation greatly influences bearing performance in terms of fatigue life, vibration, noise, heat-generation, etc. Consequently, the selection of a proper internal clearance is one of the most important tasks. theoretically, the longest NSK bearing life can be expected when the clearance is glightly negative. However, it is difficult to achieve such an ideal condition, and an excessive negative clearance will greatly shortent the bearing life. Therefore, a clearance of zero or a lightly positive amount, intead of a negative one, should be selected in general. The clearance of NSK water pump bearings is normally selected within the range between 0.010 mm -- 0.040mm, but it may be necessary to change this range depending on the interference, rigidity, material, and temperature of the housing.