What kj said: get loose bearings and use them rather than a caged bearing. In fact the first thing that anyone buying a bike--especially a box store bike and especially a motorized bike--should do is pull the cages and install loose bearings. Many manufacturers use cheap, low grade, less of them, and sometimes undersized balls to keep their costs down. There isn't enough surface and they aren't hard enough to last. You can feel the difference.
Most rear hubs take 9, quarter inch (.250") per side, (although your hub could take the smaller size --ask your bike shop) and are available at any bike shop for $3 or $4 for a full set. You can buy a bag of 100 on ebay for $8. Pack the races with grease (I like the red lithium grease from Valvoline) and stick the new balls in. The grease will keep them from falling out during reassembly.
bikejohn
