Whats the average head temp on, say a DT360. Trail Tech TTO
Avg outside temp 80deg, max pulling hills or on flats in hi high gear.

Maximum is what you saw just before the piston melted and the engine quit!

Seriously, 400 deg F should be safe... at 425 I'd back off on a big bore.

One thing to keep in mind is the head heats slowly... if you have a condition like lean or timing off damage could occur before the head temp registers too high. If you see the gauge rising rapidly under heavy load, that can be a danger sign.

Only on a 175 but I try to pull over and let it idle at 395 just to be safe, as Mark said the gauge can keep rising, have not seen over 410 on my gauge but my piston showed signs of a near seizure at some point last time I had the cylinder off.

78' Dt400 100°f day flat ground 50-55 MPH 325-350°f CHT. Mixture and timing are set correctly. I won't let it get to 400° unless it's for short periods of acceleration.

On my RT 360s they run cooler than my DT400s. The 360s don't get above 330 and the 400 gets to maybe 375. I really have only seen that one time and I don't like it that high...but I'm overly cautious...these are my babies! I tend to have them tuned slightly rich. Like RT325 said once...rich is just a little annoying but lean can break things (paraphrase!). And yes, Trailtech TTO CHT.

I think i need to fatten it up. Im at sea level, but usually ride at cal Dreamin' 3-5000 ft. On flat today 75 ambient temp, 360 deg.
Actually the bike is sold, Ill relay the info to the new buyer, his sign-on is Frogman.