Azca
If there is a harder way - I will find it...
I had a problem with this o-ring as well. The new one I installed leaked and had to get another. Not sure if it was the incorrect diameter or there was a nick in it. These get hard and should not be reused as C-Dan is saying.I assume you meant T-stat, not T-case.
Did you replace the round rubber gasket that sits on top of the thermostat?