this can be quite different for different motors i've found..
one thing i try and do is find a ratio that keeps the motor in the 60-70 deg C range and call it the safe ratio (cruisy but competitive etc)
probably no more than around 80-90 deg C if you're pushing the motors, but it really depends on what motor
an example of one that likes to run hot..
my SP 3.5 likes to run over 100 deg C when geared at 10.5 FDR on an outdoor asphalt track with 55 deg track temps and 30-35 deg ambient/air temps
the same wind from LRP geared at 9 FDR pushes 70-80 deg C
Thanks again for the info Mok! I just been to my LHS and my MS has just arrived at the Fed Ex office however I can only pick it up tomorrow. I ll post again during assembly if I have any issue building it.