Coil Pack Failure?

Yeah, it's the thermal expansion of a physical substance. It is going to happen. If you do set up some way to immovably block the extension of the rod, you're creating a "what happens when an immovable object meets an irresistible force?" question. Something else will bend. Or break. Or burst.

There was a document from Stanton (maker of many wax thermostats) saying there were not many ways of failing, but an easy way for a thermostat to be killed is by severe overheating (happening for some other reason at first). The wax expands beyond the ability of the thermostat to open, and bursts the part of the thermostat that contains it, and the thermostat's no good anymore after that. Won't open when it should, so then the overheating recurs even after fixing whatever caused it the first time.

I'm pretty sure the EHRS section of the manual describes a test where you measure the extension of the rod at such-and-such temperature and compare it to the distance in the manual.