First off let me say that Redcat's gear boxes are NOT shimmed correct from the factory. I had a new Backdraft 3.5 and went thru the entire car with thread lock, after setting the gear mesh the way Redcat recommends. Well fast forward to the 1st tank after break in (6th tank) and I stripped 1/3 of the teeth off of the Spur gear. Bought a metal spur and put it on, setting the mesh the same way and lock tite engine mount screws. Ran it for a few mins and pulled it in to look at the spur gear.......it was showing wear already. I stopped running it and took it to my LHS. He took one look at the gear shaft in the center diff which the spur gear rides on and said "wow, that's a TON of play (forward/back). I bought a shim kit, took the center diff out and apart, added a few shims and ZERO problems since. I plan on shimming the front and back diffs in the near future as it will definitely make all of the gears last alot longer.

So, I'm willing to bet you're having a similar issue. Buy a shim kit and a metal spur gear. Shim the center diff and also make sure your clutch bell doesn't have a ton of play in it. If it does then shim it also. I don't place paper between the spur and clutch bell any more. I just do it by feel and make sure there's no binding and just the slightest space between the teeth. 10 tanks since break in and both gears show zero wear on them.