Replacing the Tsat, cap, and installing a secondary temp monitor so I can verify what’s going on.
I do have an alternate theory:
Lower radiator hose... originally, they had an external spring to maintain shape. Over time, somewhere along the line, Triumph stopped putting that on. Mine hasn’t got one, and when I look at my lower hose it does, indeed, appear to be slightly flattened. I’m suspicious that it may be completely collapsing at high RPM when the pumps making serious flow demands on a 3/4 ID hose. Luckily I have a brand new OEM part on hand to also swap out, so I’ll switch all three and see what happens.
@warp9.9 has pictures posted showing the spring, don’t think I’ve seen many R3s with the spring.
If it turns out to be the lower hose I’ll be a bit annoyed with SAMCO, and will engineer a fix for their hose to prevent the collapse.
For once, and for all, please allow me to dispel three myths:
1: Running without a T-Stat will never, in any case, directly cause your engine to overheat. It will, 100% of the time, be something else in the system.
2: Running a lower T-Stat won’t hurt the motor, and will provide an engine with a greater margin between combustion and detonation provided the radiator has the capacity to expel enough heat to run it at a lower temp. It “may” hurt MPG and emissions.
3: There is no such thing as “too much or too fast flow”. More flow = more cooling, period. All the fud about “moving to quickly through the radiator” is an ancient shade tree mechanic myth.