The information in your post is correct except for the "quality factor" in the current thermostats. I assume you know how they are constructed. The thermostat "Bulb" is the pellet that rests in the coolant. It contains a wax with a specific melting point. As it heats, it exerts increasing pressure on the pin sticking out that is trapped in the tower of the thermostat body. Much like a single piston brake caliper, as force is applied, an equal and opposite force is applied to the sealing disc of the thermostat, compressing the spring and opening the disc, allowing small initial flow through and more as it continues to heat. The spring rate, effects the closing speed, and to a small extent the closing temperature.
Now enter the Chinese and their thermostats. Low quality, Chinese made thermostats, may not have thermowax that melts at the exact correct temperature, Also they don't always get a full fill when the wax is installed, In addition, the wax often leaks out at the pin due to a poor seal, so they work for a few weeks or months, then fail. Finally, the closing spring may have the wrong spring rate and not close fully, or close too fast and cause "bounce-back" you can watch on your gauge as the vehicle warms. You can see the needle make pretty wide sweeps until the engine finally warms up all the way and stabilizes. My advice is to avoid ALL cheap thermostats, and if possible buy a USA made unit. Stant Super Stat, and Robert Shaw are the only two that seem to be consistently accurate. The cheap Stant thermostats are marginal as are Gates, and many others. A good thermostat will keep operating temps typically 15-20 degrees above the rating as you stated. Good luck.
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