Is your crankshaft clean where the bearings sit? You may need to clean off those little rust areas where the bearings sit with some fine sandpaper. If there is enough rust on there the bearings wont last very long. Another thing is if your gear mesh between the cluth bell and spur gear is to tight then when the chassis flexes it puts way to much stress on the bearings so they wont last very long that way either, and the chassis will flex every time you land a jump or hit a rut or whatever, so that can be a problem.