Bad sender or bad temp gauge?

Good news is the gauge and wiring is OK. The other news is your testing by grounding was only a few seconds as advised by "drolston". The better news is you avoided a fun trip under the dash.
Your car should have a 81A-6049 head on the right side and a 81A-6050 head on the left side as they have different combustion chamber volumes to match the different valve angles. However this has nothing to do with temp sensing as your car and all 1940 and '41 Fords had only one temp sensor. Even a deluxe or a super-duper deluxe would have only one sensor and it was mounted onto the left head. The outlet for the right head would be plugged if no heater was fitted.
The right side head is not original to your car.
Original King -Seely OEM sensors will be marked with a "KS" stamped on the hex head and the body of the sensor will not be cylindrical but will have a couple of grooves in it as shown in reply #8.
I have redrawn the crude temperature chart for the gauge and it is shown below. I have rerun the test with another K-S sender and the results are identical, showing how precise these instruments are.
The short hash marks on the gauge are at 150 F on the left side and 200 F on the right side. Note that 200F is still within the standard operating range according to Ford. They go on to say that going above that is OK for severe conditions and overheating does not occur until there is actual boiling. The gauge will show "boiling" when it hits the right side marker line labelled "H"
I have also included the pages from the 1940 owners manual with the text about the "Heat indicator".
Keep us posted and let us know if the faulty sender unit is a real K-S or not.
 

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STATUS REPORT:


Replaced the existing sensor, it's a King Sealy, with NOS early 1950's Lincoln King Sealy sensor.


Better but not great. Rather than reading hot when it's cold, it now reads 3/8 of the way from cold to hot when it's cold.


Only just started it up, didn't have time to go for a ride and bring it up to operating temp. I'll do that later this week.
 
Maybe we are making progress here. Remember that for the temp gauge that backwards thinking is required. I am an expert at that. No current flow through the dash gauge equals pegged to full hot-right hand side. Current flow through the dash gauge "pulls" it back toward cold, depending on the amount of current.
So, if we assume the new sensor is good and correct, and the dash gauge is good and correct, then we have a poor connection in the wiring between the dash gauge and the sensor, thus limiting the current flow, preventing the needle from being pulled all the way back to "cold" when the engine is in deed cold. A trip under the dash is required and also measure the voltage to the instruments. I have seen turkey wiring where the instruments were fed from the coil resistor. That will make you crazy. Check, clean, measure etc
Report back
 
Actually did get a chance to take a short cruise around the 'hood. After a couple of minutes the sender became acclimated to it's first permanent home in over 70 years of life and the temp dropped to 'C'. Cruising, it's steady at about 165 water temp on 3/8 according to my $25 infrared sensor.



(No, I really didn't sit under the hood with the infrared gun, steer through the window, and drive down the road looking over my shoulder. I left a few parts out of the story.)



The brass head of the sensor is consistently about 10 degrees warmer than the radiator or radiator cap. That seems consistent with combustion going on inside the head. Could be different emissivity of the two materials but the reading seems close. At idle it seems to heat up about ten deg to 175 at about 7/16 of full scale.


I"ll be playing around with the temperature gun a lot to understand what the car is really doing and comparing it to the gauge.



Anyway, I now have an 81 year old temperature gauge that's giving me useful information and so far the car doesn't appear to be overheating.


I could make it easier on myself and add a modern analog temp gauge by tapping into one of the radiator hoses but why would do that?
 
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Real Ford parts in Ford boxes made by King-Seeley properly treated, installed correctly and maintained are way mor reliable and accurate than any combination of cobbled up Pep boys add on crap even if it says "SW". (SW was a instrument company back in the day ,1905 to the fifties), then selling out to too many buyers to keep track of. )
Good news, you are up and running!!! However, under the strict rules of old car caretaking, no trips under the dash may be considered cheating.
The sensor is placed in the cooling circuit where the temp is maximum, leaving the cylinder head and on its way via the hose , through the thermostat and on into the radiator for cooling.
The emissivity of the brass could actually be lower, but only in a polished state. Sounds like you are a fan of Stephan-Boltzmann equation.
Go drive your car!!!
 
Back in the 60's Smiths had a small 2" combination mechanical gauge that showed oil pressure and water temp together for English sporty cars.

I'd love a small mechanical gauge that showed left and right cylinder head temps together at a glance...
 
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