It's in a saucepan boiling right now, and looks like it has opened about 3/8 inch. Doesn't seem like much, considering there is more room to go -- but that is a pretty big resistance spring.
It did not open until it started to reach a rolling boil -- but then 87 dgrees C is 188.6 degress F -- just 24 degrees shy of boiling anyway.
Also, I heard a rattle -- like a small ball bearing inside -- when I shook it. Is that normal?
Nonetheless, I'll replace the part since it is so easy and cheap, and see what happens.