well something i have come to learn first off is that the pistons are not the first thing to go. you'll burn the exhaust valves first. as far as what temp is safe the rule of thumb has always been 1200. now ive run trucks to 1600+ and held them there for several minutes pulling loads i have yet to have problem from too much heat. however i dont recomend it. ive done it, but dont like to, its just plain too hot. i like under 1200 but 1350 is ok.