New engine running HOT

Figured it out. The next day after work I made sure the radiator was full of antifreeze (I was able to add almost a whole gallon!) and I left the radiator cap off and started it up. Checked to make sure my timing hadn't slipped (it was spot on). Waited a little while and I could feel the hoses warming up, but I couldn't see any real movement of the fluid in the radiator ...

At that point I walked over to the cab to check temperature and--yikes--it was already 205-210 and climbing. So I shut it down, removed the thermostat, grabbed a second thermostat I had, and went into the kitchen and filled a pot with water. They were both 195 deg items, and as the water heated up and approached boiling, the "off" thermostat opened right up, but the one I just removed from the motor stayed shut, even after the water boiled and had boiled for 5-10 minutes!! BAD THERMOSTAT ! ! !

I won't tell you the brand, because I don't think that's too rare a thing to happen. Oh, what the hell, it was a Mr Gasket, high performance high flow unit. i guess it can happen to any mfr. I wound up feeling that, with a brand new engine, why didn't I check it to begin with ??!?

Thanks for everyone's thoughts and help.