radiator water temp vs. head temp

51f1truck

51f1truck

Junior Member
I have a freshly rebuilt 239 ci V8 flathead that I have been running for several months. The dash gauge has always indicated that is runs just a tick below "H" (hot).

I thought for a while that the engine would break in, but now I am questioning the accuracy of the gauge reading.

Today I ran it until it got up near the "H" again. I carefully took of the radiator cap and stuck in a candy thermometer to measure the radiator water temp. I then ran the engine at a good rpm to circulate water for about 5 minutes. The thermometer measured the radiator water temp right at about 160 to 170 deg.

Can this be an indication of the head temp? I am beginning to question my temperature sender or the dash gauge...

Any thoughts? Have any of you tried this before?

Fred
 
51f1truck said:
I have a freshly rebuilt 239 ci V8 flathead that I have been running for several months. The dash gauge has always indicated that is runs just a tick below "H" (hot).

I thought for a while that the engine would break in, but now I am questioning the accuracy of the gauge reading.

Today I ran it until it got up near the "H" again. I carefully took of the radiator cap and stuck in a candy thermometer to measure the radiator water temp. I then ran the engine at a good rpm to circulate water for about 5 minutes. The thermometer measured the radiator water temp right at about 160 to 170 deg.

Can this be an indication of the head temp? I am beginning to question my temperature sender or the dash gauge...

Any thoughts? Have any of you tried this before?

Fred
Click to expand...

Get at cheap laser temp gun at Harbor Freight and you will get accurate readings to correlate with your gauge.
 
There used to be several part numbers for the single terminal temp bulb that is installed in the passenger side head. After more than 50-years they get mixed and matched plus the aftermarket types get slipped in there. It is the one that actually is calibrated to give the gauge a temp reading. The twin terminal bulb in the driver side head is just a thermal on-off switch so if it gets hot enough to trip the bi-metalic switch, it just goes straight to "Hot". For piece of mind, it wouldn't be a bad idea to rig yourself up a test set up with a hot plate and a thermometer that can be trusted.

Kerby
 
Mike in AZ.

Yes you can. You will be able to see the temp on each head with that set up.

51f1truck,
The temp at the top of the radiator will be very close to the head temp.
You could add 2 guages like above or do a little trouble shooting. If you find the single post sensor is working properly you can bypass the 2 post switch. Then install only one new gauge to read the other head temp. Another way would be to install another single post sensor and wire both sensors though a switch to the gauge. The only problem I see doing that is response time before you see a true reading.
This is a way to check the temp gauge system.
1. Disconnect the wire from the gauge at the two post switch. Ground it to the block. Turn on the ignition. The gauge should go all the way to cold and maybe beyond. If it does the gauge is good and you have an issue with one of the engine units. Turn off the ignition. If the gauge doesn't go all the way to cold it needs to be adjusted or replaced.
2. Disconnect the other wire at the two post switch. Connect it to the wire from the gauge bypassing the switch. Turn on the ignition and see if the gauge goes to cold. If it does, start the engine and let it warm up. You should see the temp climb on the gauge without going to hot. If this reads good then the switch unit is bad. If it goes to hot and you are sure it's at 180° then the single post unit is bad.
 
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I would check first at the temp probes and then the hose neck on the heads and then all over the head to see how even it is.
 
The best place to take the temperature on the 32 to 36 engines is on the buldge on the head right behind the water pumps. On the 37 to 48s on the cast iron neck just below the radiator hose and on the goose necks of the 49 to 53s. A laser pointed thermometer as described in 41s post above is the best. Both sides of the engine should be within about 5 degrees and the laser gun must be held the same distance and the same angle from the SAME SPOT being tested every time. The radiator temperature is a mixture of both sides of the block so you could have a problem on one side and not know it until it really got bad. After a few testings when driving and comparing the gun readings with the dash gauge you can be confident of the dash readings. The picture is a friend of mine checking his temperature. G.M.
 

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Thanks for all of the advice. Much appreciated. This morning I went and bought/installed a mechanical temp gauge into the head. It was reading within about 5 degrees of the radiator water temperature measured with a $4 candy thermometer.

The mechanical gauge and candy thermometer read about 170 when the dash gauge is on the "H". This was very comforting to see!

My next step will be to replace the single terminal temp sender and see if that makes a difference.

Best to you all, Fred
 
Before you swap out the single terminal sensor. Did you disconnect the 2 terminal unit and tie those leads together? If you get a reasonable reading on the Ford gauge that way the single terminal unit is good. Then the 2 terminal unit is the one to replace.
 
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