Does anyone know the range of the stock water temp gauge in a 66? I have the electric fan set to hold the red gauge needle straight up, but what temp is that?
This is a question that's impossible to answer, given that your temp gauge is 40 years old. It's a sure bet, it's not reading accurately. Invest in a set of mechanical guages and there'll be no guess work.
Fair enough, but since the car puked out a cup of coolant from the overflow hose when the needle hit barely on the "H" when the fan didn't come on the other day, I figured the gauge was pretty close to whatever was intended years ago. My WAG is that H is supposed to be 250 degrees F and the halfway point is about 225.
You're going to bet your engine on that assumption? Puking coolant only tells you that the pressure rating for the radiator cap was exceeded, not that any certain temperature was reached.
I just put new Autometer gauges in my 65'. Before the new gauges, the 40 year old temp gauge ran right around half way between L and H. I am running a 160 deg. thermostat and the new temp gauge reads between 150 and 160 when it's warmed up. I know the old gauge with L and H is a crap shoot in determining what the heck "L" and "H" actaully means but hope this helps.
I stuck a a dial thermometer in the radiator fins right next to the water inlet and get a reading around 160-180 range when the needle is around the halfway point. When the engine gets too hot it will start to ping, thats when the needle is getting close to the H. I used to have a mechanical gauge but realized it was way off compared to the thermometer I stuck in the radiator. The gauge was was reading 225 or higher when the thermometer would read 160.
Not sure if this will help but my stock temp gauge "normal range" changed when I put in a new sending unit in the manifold. Was reading 1/4 at normal, now is half. I use a non-contact laser thermo on the manifold and it's reading around 180 and the lower hose is at 135 degrees.
I recommend you use one of those candy thermometers and measuring the coolant temp with cap removed with car running, verify temp when thermostat opens and then note your gauge reading. That will at least give you an idea and a place to start.