I've been using the kyosho 46t gear from mike's store. I believe the cause is the motor not staying put.. it seems to slide away from the diff just a tiny bit.. I'm not sure, but it's not sheering the teeth off completely, it's just kind of flattening them 1/2 way down. I'm trying some new screws tommorow (hex head, instead of phillips), and I'll use loctite, and see if it holds.