On my Nitro Shockwave, I have changed a many spur gears and usually find the new ones to be slightly out of round. I also bought a new entire transmission which came with a new spur gear. I found that it was also slightly out of round (the shaft is good, it's the gears that are not round). So, the result is that there is never a perfect adjustment for the spur gear. The thing is, it doesn't appear to affect the gear if the car isn't jumped or slammed hard (at least on my car). I have trashed two spur gears in just a few hours when I run the course at a local hobby shop's dirt track which has several large jumps. Then, I have gone many weeks with the same gear just driving around the street or yard with minimal jumping. My car has the steel clutch bell and it is round (I did a runout check on it). I think that my Redcat would benefit from a slipper clutch arrangement on the spur gear to help reduce the loads on it during landings. My HPI gas truck has this type of set up and the plastic spur gear has survived several years of serious punishment at the track.
Just my thoughts....