Here is a shot of a motor showing the spacers/shims that are used to center the rotor. Below that is a blown out bearing, where the cage failed. Another thing you need to watch is when installing bearings always try to push them in by there outer race. Some people just slip them over the rotor shaft and hammer them in, what this does is it puts pressure on the inner race, which then transfers that to the balls. This can leave microscopic dents, which will cause premature wear. I made a simple aluminum tool out of some scrap, that has a small cone shape piece in the middle that the center hole of the bearing rides in (this centers the bearing), then it is dished out slightly and only the outer part contacts the bearing (the outer race). This way I can push in the bearings with pressure only on the outer race.
