Redmond49

Hi, 
 
Here's the follow-up, it's been an interesting learning experience. The repair manual says thermostats are supposed to open and close within a few degrees of the stamped temperature.
 
I bought this car (46 Plymouth) going on 4 years ago now. It ran but overheated right off the bat. To start off, I had the radiator re-cored, installed a new water pump, new hoses, pulled the distribution tube, and flushed the block. Then I installed a new 160 degree NOS thermostat. The engine overheated anyway. I hadn't tested the new NOS thermostat before I put it in (oops), so when I pulled the cooling system apart again, I did. The NOS thermostat didn't open at all, so I threw it away and tested the original one, which seemed to open and close nicely at 160 degrees. 
 
In the end, it turned out the engine needed a complete rebuild, so fast forward to this spring. When I assembled the cooling system on the newly rebuilt motor, I kept the original 160 thermostat. While the engine no longer overheated, it did run around 175 degrees based on the gauge and thermometer readings, and the head had surface temps quite a bit warmer than my other flathead Plymouth (a 49 with a 51/52 Dodge truck block in it). The temp gauge was somewhat active between 160 and 175 but mostly on the high side. Since the thermostat was the only thing I hadn't changed out in the cooling system, I decided to try a new one.
 
I bought the "Premium Quality" Murray Plus thermostat that O'Reilly's website indicates is right for these cars. Even though it's in a Murray box, it is stamped MotoRad, so I assume it's sold under several brands. I tested it in the stove pot, and even though it's stamped and sold as a 160 degree thermostat, it opened around 175 degrees and closed around 160. I installed it anyway just to see how it would behave in the system. The car heated up to 175, then it opened and the temp gauge dropped to 160 for a moment then it started to climb back up. With this Murray thermostat, once past the initial opening of the unit, the car seemed to run at a steady and consistent 175 degrees.
 
A buddy mentioned that he had a lot of issues like this with thermostats for his GTO, and he pointed me to the Holley/Mr Gasket thermostats. I found this one with the right diameter and profile - it's not listed for flathead sixes, but I thought I'd try it out:
 
Mr. Gasket 4366 Mr. Gasket High Performance / High Flow Thermostat - 160 Degree (holley.com)
 
In the test pot, it opened promptly at 160 and stayed open to about 150. When I put it in the car, it took care of the running-warm problem and now the temps and patterns I see on the gauge are exactly what I see on my other car. If it's cool outside, the car runs consistently at 160, on warmer days a bit above that but not up to the 170-175 mark. 
 
So it only took 4 thermostats and some persistence to get to the one that works.