VCCA Home Forums TECHNICAL - GENERAL Electrical - 12 volt 1965 Nova Temperature sending unit

I have a Edelbrock manifold and need a 3/8 NPT temperature sending unit that will work with the stock gauges. I know that the original is 1/2 NPT sending unit but it cannot be used in the available hole. Does anyone know of a 3/8 NPT sending unit that will work properly with my gauges.

Paul it is a simple fix! You need a NPT reducer from 1/2 inch to 3'8 inch. Here is the McMaster link: https://www.mcmaster.com/1162T54
Rory
1934 Chevrolet Standard Holden Roadster under new rebuild
1933 Ford 3 Window Coupe
1934 Ford 4 Door Sedan rolling under construction

Maybe I missed something in the original post. I think the Edelbrock manifold is drilled and tapped for 3/8" NPT. The stock/original sending unit is 1/2" NPT so it is too big for the manifold.

The reducing bushing will not help for this situation.

He could possibly use an adapter fitting that has 3/8" external threads and 1/2" internal threads.

3/8" Male Pipe to 1/2" Female Pipe Adapter

I am not sure that will work very well in this application. The sending unit has to reach into the coolant flow.

Is it possible to drill and tap the opening for 1/2" NPT?

If you can find a part# for the original sender, you could google your fingers off cross referencing to find something compatible in the smaller size. Probably more common than you think, just annoying to compare. Sometimes just walking into Napa with it in your hand can work if you get a guy who can search their system. Might get lucky with a universal unit from Napa with the correct thread, but may have to try a few.

I have also seen hose mounts for sending units in lots of sizes. Meant to fit heater hose close to the head or in upper rad hose close to the thermostat housing. Not as accurate as right in the manifold but may do what you need if you could hide it on the underside of the hose for appearance.

Yes the hole in the manifold in 3/8 NFT and I cannot fit in a 1/2 NFT. That is why I need a temperature sender that is 3/8 NPT that has the same ohm resistance as the OEM 1/2 NPT.

One simple question. Does you car have a temperature gauge or an idiot light?

There are lots of 3/8" NPT temperature sending units for 3 different applications: on/off for overheat, analog gauge, digital gauge. Just check the listing at Summit racing to drive yourself crazy.

Hi Rusty. My car has a temperature gauge. I have located many 3/8 NPT on various website. The problem is that they are all calibrated differently for various gauges and various cars. I have tried to locate a site that has specifications that indicated the ohms reading at various temperatures and then I can match them up with my gauges requirements. I know that the specs for my gauges are around 368 ohms at 100 F and 80 ohms at 220 F. The temperature sending units on the website don't indicate the ohms specs for their units.

Thanks for the details. I wonder if Gene on this chat site has access to the engineering specs for the sensor that might help us. Plus I cannot believe you are the first person that has had to solve this dilemma.

If Gene does not catch this I will PM him to have him look at this thread.

Still looking for someone who may have this information or knows where to look to get the information. OEM was 1/2" temp sending sensor and the manifold that is on my car (not stock) requires a 3/8" temp sending sensor. I have gauges and not lights on my car.

You might try Dolphin gauges, I bought a set from them and I believe the temp sending unit is 3/8 threads., maybe they can help.

Thanks for the information Air Force but as with SW their temp sending units are calibrated to their own gauges.

Have you talked to Edlebrock? For some reason they are saying this is the correct manifold for your car but they did not make the hole the correct size.

I still think your best bet is to drill and tap to 1/2" NPT. They can tell you if there is enough material to do that.

Hi Rusty:
I have not done so, but that is an excellent idea to check with them to make sure that there is enough material to drill and tap the hole so that I can fit the proper sending unit. I looked at the manifold and it looks like that would not be a problem. So, I will contact them and see what they say. I read through the instructions as to installing the manifold and they are very picky about the procedure for tightening the fittings into the manifold. They appear to be concerned about over-tightening will crack the manifold. I am guessing that they have had a number of issues with that situation. So I am not sure if they will respond in a positive way. But nothing ventured nothing gained. It doesn't hurt to ask. Also, this is not a manifold that I purchased, it was already on the car.

Hi Paul,

Yes, it is easy to crack a casting (aluminum or iron) when you overtighten a tapered pipe fitting. People underestimate the force a taper can create.

Have you posted on any of the rod and custom forums? While many of us who use this forum also work on other types of vehicles, I greatly value the expertise of this group for helping me understand how things were originally.

My guess is that if you find the right forum you will learn that you are not the first person to encounter this situation.

Keep us posted.

Ok, the response is that there is not enough material there to expand the hole to accommodate a 1/2" temperature sending unit. The likelihood of cracking the manifold is drastically increased and most likely to occur. Finding a 3/8" sending unit with the appropriate resistance is a challenge as most do not publish what resistance they are working with. At least I have not been able to find that information yet. There are some unofficial readings that I have found, and I tried a few, but found that the information is not accurate and the response on the gauge did work as hoped. If anyone knows of a website or other source for the ohm reading on temperature sending units, I welcome the information.

My solution that I am experimenting with is to put in a resistor in parallel on the sending unit. I have had some success doing this. So far I have tried a 100 ohm resistor but that appears to be too low and the gauge is reading too high. So, I will try a 150 ohm resistor and see if that sets the needle half way when I am at about 180 to 190 degrees.

Paul

You have the right idea with the resistor - that was my next suggestion if the gauge works but is just not accurate. Sounds like you are very close.
Do you have a meter to measure OHMs so you can measure the resistance of the current sending unit? The resistor needed can be calculated roughly once you know the resistance of the sender and the current position of the needle...

A method for determining what resistance you need is to temporarily place a variable resistor in place, when you have a correct reading measure the variable resistor and put the fixed in place.
Tony

Ok I measured the ohm resistance at 180 degrees to be 275 ohms for the thermostat. I then put on the variable ohm resistor and measured the total ohm resistance at 180 degrees to be 156 ohms. The formula for total resistance is (calculation is 1/total resistance = 1/thermostat resistance + 1/variable resistor). When I calculate the value for the variable resistor I get 365 ohms resistance. That puts the needle just shy of exact middle. Now my thinking is that I will leave the variable resistor in place as it is giving me the reading I need. I could replace it with 365 ohm static resistor, but why bother.

Using a variable ohm resistor was an excellent way to achieve the desired results.

Thanks Everyone

Paul

Nice! Small blob of silicone can hold it still if needed.

Where did you buy your resistors? I have a lifetime stash of bits and pieces but it's getting harder to find this stuff locally in a brick/mortar store so wondering who is still selling this stuff in Canada...

Sayal Electronics with locations in Toronto, Markham, Barrie, Vaughan, Mississauga, Burlington and Cambridge. They sell almost any kind of electronics that you would need to repair or build just about any project. They had thousand of resistors to select from.

Paul

Used to shop at Sayal in Markham when I worked back there...