hi. bought my 66 thru Craislist. so. after i got it natch i drove it and the fact temp gauge indicated HOT (almost all the way over yonder a ways!!). so.. had the radiator rodded out. still running hot. so checked Timing (ok). so. removed 160F thermostat... still running hot. so... i had 2 at 180F 'stats which i checked with hot water on my dirtyword stove... both worked... so... installed a 180F thermostat. bought a new Electrical Temp Gauge with freaking numbers on it. they ran 210-212F. so. the radiator never boiled over. began checking with infra-red thermostat, and wow: after driving 6 miles, temp gauge was 210F... stopped and immediately checked the top of the radiator... it was 180F.... the Temp Sending Unit at front of the engine read 167F.... the front of the radiator read from 150-145F. top radiator hose read 170F; bottom was 150F. my own temp checked out at 97F (should have been WAY higher with this dumb overheating MURPHY). checked the infra-red against other temps and okay. so. with the Factory Temp Gauge reading HIGH, and the replacement aftermarket numbers gauge reading HIGH, i think something aint right. oh... the factory sending unit is/was not the same sending unit as was the aftermarket gauge... tried couple of sending units i had stuck away, but they all "pegged" the gauge and would not work. but WHY would the dumb dirtyword snot-sucking finger-licking factory AND aftermarket gauges read practically the same ??? and again, my eng/rad did not boil over at any time. as a precaution i had also changed oil; which i ALWAYS do when an engine has overheated. i need help, physical, not mental (ha!). stay safe. thanks

First, a temp gauge actually reads current flow through the gauge. It's just that the dial is calibrated to show temp, in relation to current flow. The sender unit in the head is the controlling factor. It's a heat responsive resistor that, as it gets warmer, it allows more current, and the gauge responds by raising the needle higher in the displayed range. It's important that there is no thread tape or sealant used on the threads of the sender. It's just as important that the threaded hole doesn't have rusted threads, too.
Now if, by chance, that particular wire between the sender and the gauge has shorted to ground, that will peg the gauge, and the sender will have no control. (Talking about the factory stuff, here.)

(Side note to Dan; is that gauge a direct acting, or inverse acting gauge?)

I can't for sure say why your aftermarket gauge is showing the same, (or close to), what your factory gauge is showing. It might be that you bought a rather low cost, and low quality gauge, and those are notorious for have sending units that aren't calibrated well to the gauge.

Now.......do any of your other factory gauges act funny? I ask because there's a gauge voltage regulator mounted on the backside of your instrument cluster that drops the 12vdc to about 5vdc for, (I think), the temp and gas gauges. If it's bad, that could very well be the root problem.

Just a couple things to consider.....Roger

the only other issue i have is my Gas gauge is not working. i did check it at the tank. stay safe. thanks

i did not use tape or sealant any kind on either of the 2 sending units. stay safe. thanks

Just an idea: can you check the aftermarket gauge with boiling water? (I guess is easier to uninstall and test than the factory one). Temperature should be 212 F (if you are at or near sea level, at 2000 feet above sea level is 208, but the difference is small), compare with the gauge. If temperature is right the problem is somewhere else. If not, check how to calibrate, or return the aftermarket one.

Then with a mercury thermometer you can check more temperatures.. if you have one handy.

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Check the resistance of your sending unit to ground to check gauge accuracy. If you measure from the sender to battery negative, you should be about 73 ohms at the coldest reading, 23 ohm's at midpoint and 10 ohms or less at full hot. If you're gauge doesn't match then you have wiring issues or a bad instrument panel voltage regulator. It should put out 5 volts at all times.

There used to be gauge tester boxes that you could use to check all your gauges. They had different resistor circuits in them. Most older cars used a 0-90 ohm gauge. You can make your own tester with some simple resistors. The fuel gauge and oil pressure gauge (if you have one) work the same as the temp gauge. Just put the resistor in place of the sending unit and connect the other end to ground. You should be able to make your gauge show low, middle and high.