Ian, I agree with Ed above. You need to check the thermostat in a pan of boiling water with a thermometer. I was running a 180 degree thermostat in my newly rebuilt 52 GMC and was concerned with it running at or near 200 degrees. After testing it I found started to open at 180 but was not fully open until 200. I think that the number stamped on the thermostat Is actually the temperature that it starts to open rather that the fully open temperature l changed to a 160 degree and now it runs comfortably at 180 degrees.
Mike.
