Make sure you run the heavy-duty gears and when shimming you need a little play. All so make sure you are running the cast metal spider gears and not the pot metal ones. All so make sure you have your slipper clutch set right.

Here are some links

http://www2.towerhobbies.com/cgi-bin...7&I=LXJYM2&P=K

http://www2.towerhobbies.com/cgi-bin...?&I=LXBTA4&P=Z