50/50 change the Tstat housing will crack when attempting to get the cap off. Mine was on real good, couldn't get it off.
bought a replacement cap ... but by the time my degas bottle started to leak I had already ordered in the Aluminum Jag Stat.
Did end up getting the "very" stuck OEM cap off without cracking or breaking the housing. The
formed Allen key was useless, ended up using a rag and wise grips while engine temp was warm. 10+yr old PVC plastics that's been exposed to excessive heat cycles ... proned to break.
PS: if you are down in coolant level in the degas bottle, do not get much heat through the heater core and/or have troubles getting a solid stream from the bleed valve tube during bleeding, there is then potentially a lack of pressure in the system due to leaks. Degas bottle may be in your near future and do not purchase the Dorman brand.