i've melted a bearing in a 5300 on 12 cells (75k rpm). but I ran that setup in my tc3 for about two three months before I had any problems. the rotor came apart on one of my Hacker 8S' on 16 cells (75k rpm). that was when i blipped the throttle unloaded. loaded, I never had any problems w/ my other 8s on 16 cells.. I'd stay away from 75k, as that's the only time i've had motor problems.. 50k, I don't worry though.