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Does any one know how wide the opening should be on a 180 degree thermostat when its fully opened ?

Fully opened, it should be almost 90 degrees. The edge will be almost even with the bar across the center.

Hi Sal,
I ran my M most of last sumer, thinking the temp guage was broken. I decided to invistigate and found that there was no thermostat. I bought a thermo from the local parts store on Rockaway Beach. It was a two piece affair. an adaptor and then the thermostat. Now it runs at 180 no matter what the o/s air temp is. I know this does not answer your question, just another piece of useless info.
Bob

Thanks Lifer and Bob. I replace the NOS one I had in there with the same type as you have Bob. which happens to be the Napa # 155. After I took it out, I put the NOS one and in hot water and it opened approximately 3/16 of an inch. no matter how hot I got the water. Is that good? Or is it stuck partly closed ?
I noticed the truck started to run a little hotter then it normally does and I have no faith in NOS parts...so I replaced it. And waiting on a new radiator cap so I can try it out...

Sal wrote: I put the NOS one and in hot water and it opened approximately 3/16 of an inch. no matter how hot I got the water. Is that good? Or is it stuck partly closed ?

No, 3/16" is not good. That's just begging the engine to overheat. With the water at a boil, it should be wide open.