sbc was running crappy after 97,000 mlles. Would not get up to OT. stayed around 120*.
Pulled the ThermoStat and boiled it. Opened right up.
Dropped the new 190* in the boiling H2o and it opened a little but remained about 1/2 closed.
Installed the new one and now she runs (perfect) @ 210* on the factory gauge.
Never compared before so I figured if it opened it was OK.
The spring must have weakened after so many cycles ( metal fatigue ? )
Anyway, just my experience that might help somebody.