Judd,
I don't know where you got your information on 250F is the minimum temperature to damage a thermostat. I was told ths information by American Radiator which had been in business for 50 years in St Louis. When you walked in the place had the smell of antifreeze soaked into the concrete. You know, one of those places that the neighborhood has changed, they have fences and lockable gates with barb wire but does a land office business because they know their stuff.
I never mentioned this before but the guy told me to change my thermostat after my overheat and did not even try to sell me a new thermostat. So him telling me tht information cause him to make a sale.
Before I had talked to him I was thinking like you that a thermostat should be able to 250F for a short period of time with no problem. He insisted that seeing 225F one time, even if it is a short period of time, can damage the thermostat. Why would I doubt the guy, cooling systems "is his business".
After my overheat due to a broken fan belt I was getting wider swings of the temp gauge and I was running at 210 vs the historical 200-205F I had been running. I was in the radiator shop to find out what a new radiator would cost, that when he told me how little it takes to mess up a thermostat.
I just want guys on here to know to consider the thermostat when they have cooling system problems, it is a part that gets overlooked.
Martin