Not too sure without seeing the pinion gear, but I drill & tap out most of my setscrew holes and install the next size up. It is my experience that the little tiny 3mm setscrews are just to small and weak to holdup, and usually end up stripping either the head or the hole. I believe that all of mine are now 4mm hard steel setscrews with blue loctite(
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXLR68), and they hold up much better for me. In some cases I even place some shrinkwrap over the gear end to keep the setscrew from comming out even if it does come loose depending on the location or access. Good luck with your setup and we look forward to seeing your updates and photos.