I know things... A lot of things.
Join Date: Feb 2015
Motorcycle: GSXR 750 K4
If it did not open at all (as opposed to you just somehow missing it) in boiling water then it's way out of spec and probably entirely jammed. Note that, as described in the service manual, page 7-09, the thermostat should just start to open at 82°C, but it should have opened by at least 8mm at 95°C. If the thermostat stays completely closed, the cooling system of the engine will be effectively disabled and the engine should overheat, so if it did start to open at 120°C, your engine would run at 125°C, or so, instead of 85°C.
Finally, note that you should be careful when testing the thermostat, in order to make sure you're heating it at the temperature you think you are. You can't just throw it and the thermometer in a pot with water, as it will touch the bottom, which is at quite higher temperature than the water in the pot. So you need to make sure both the thermostat and the thermometer are suspended (and stirring the water once in a while, would also help).