[KTL]

Yes in my opinion you don't need to replace the bottom fitting (pressure bypass). But by all means YES open it up to clean. If you have a t-stat of unknown usage, you never know what it's been subjected to.

When my engine failed a few years back due to an oiling issue, it wasted all the main and rod bearings (along with other engine parts) and I disassembled the t-stat to ensure no debris was in there. I threw away the temperature regulator & installed a new one. Opened the pressure bypass side and found a small amount of debris in there as well.