Generally the diff spur or pinion breaks, althuogh if you get one of the REALLY old savage 21's with the plastic diff cases, they blow internal gears sometimes. However, unless you are running a brushless or a REALLy powerfull mill, like a modded picco or something like that you shouldn't have a porblem granted you keep of the gas/brakes while landing jumps. As said above, hard crashes can take them too. Regardless, you won't be blowing them from haviong too much torque unless you have a really powerful mill or BL setup.