temp gage/thermoswitch (Frank Help)

I have a 1971 55HP Chrysler. I want to put on a water temperature gage. I went looking at some today. On the top of the head to my engine is the thermoswitch. It looks to be about 1/4 inch or 3/8 inch hole that it is screwed in to. The gages that I saw all had a thermoswitch that was much larger in diameter. Can I buy and put a temp gage on and use my old thermoswitch? It also looks like its the original. Looks like it is actually a part of the head. Seems like it will be a pain to get out or possible break off. Are there any temp gages with the same diameter thermoswitch or is mine so old that it is no longer made in that diameter? Any advise on this mater would be greatly appreciated, also can you tell me who makes one that would fit in the same hole if I do need to put a new thermoswitch in.
thank you

Re: temp gage/thermoswitch (Frank Help)

If it is indeed a temperature sensor / sender, they all work the same way. resistance decreases with heat so more voltage goes through the gauge and it reads higher. You should be able to use it with a new gauge, BUT: With a non matched sender, you may need to recalibrate the gauge.

The ones I have seen on Chrysler engines are brass and 1/8 NPT which looks to be about 3/8 diameter. Pipes threads are designated by the pipe inside diameter so 1/8 NPT is 1/8 inside diameter plus the pipe wall thickness on each side. Comes out close to 3/8 inch.

The overheat sensor was usually a 1/4 inch bolt with a 5/8 head and a bimetal switch glued on top. This usually replaced one of the bolts next to a spark plug holding the water jacket cover if it were that type head.

Later engines had a hole machined into the side of the head. A bimetal sensor plugged into the hole and was capped with a rubber plug to hold it. The plug was retained with an internal snap ring.

Re: temp gage/thermoswitch (Frank Help)

Hi Frank
My sensor or whatever it is called is located on top. I might be confusing but I will try. I do not mean on top of head as the head is on the side of engine next to plugs. I mean on top as pointing straight up. Mine points straight up, right beside flywheel. Can I find one that has the same diameter hole and use that to replace mine? If so, how can I remove mine. Looks painted over and l looks like it will be a byatch to get off. Do you recomend any type of temp gage?
Thank You