I read in the current "RC Car" mag, that you shouldn't tune to temp.. Says to tune to performance and the temp will take care of it's self??  As was said before, as long as there's a nice trail of blue smoke when ya throttle it up, you should be good to go!  But getting back to your motor siezing up.. I dont think 250 is near hot enough to cause it to seize.. Dont hold me to this, but Im pretty sure it'd take well above 300* to get these cast parts to begin to deform?? Is it possible that ya just had a bad part in the motor that let go and locked it up?