Originally Posted by ThePhoenix

Because you have to pry out the black o-ring first. I was trying to avoid that in case the t-stat was still good, not wanting to damage it. Dug it out with a pick and the t-stat came right out.

By the way, the boil in water test showed no movement whatsoever. I noticed that there are loose rattly bits inside the t-stat. Is that a sure sign of a bad one?