Here is some info on my first mod, It's a robinson racing 72t spur. You have to file the corner's of the top shaft very little to get the bearing on. The gear is heavier so it carries more inertia and gives the car quicker throttle response at high speed.

[link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXBJB6&P=7]spur gear[/link]