As a rule block temp (or head temp) depending on what engine you have is more critical to monitor than the radiator temp.
When the thermostat is closed very little fluid will flow through the rad and be cooled....like when the mercury drops to -20C. The coolant in the rad will be warm but not as warm as the fluid in the engine which is what the thermostat is designed to maintain. In fact they are designed to open or close depending on the temp of the coolant in the engine which is in direct contact with the wax plug inside the body of the 'stat'.
You can sometimes find coolant passages in the head or the block that are merely plugged with a threaded cap/bolt. This would allow a person to install secondary tempurature system. Auto-Meter's website has a wide array of gauges and thermo-probes that you can look at in detail on line.
Yeah I joined the Dark Side because the medical plan is top shelf!
