Reply To: General wisdom on thermostats for oldies…

[Don]

Well, there are a few factors to consider…. Are you just trying to run this engine to evaluate it, have you or are you planning to do a thorough servicing on the engine? Is it a fresh or salt water engine?
Needless to say, if this is an engine you are just going to sell, I would simply run it and evaluate the cooling/thermostat, especially if it is a salt water engine.
But, if this is an engine you are planning to service properly, or use frequently, it might be worth having a closer look. Most of these engines have only three screws retaining the thermostat, but salt water engines might require heat to loosen up the screws.
Like Frank says, thermostats are fairly reliable when used in fresh water. Salt water use plugs up passages, and corrosion hinders proper operation. I guess if I was going through all the trouble to inspect the thermostat, I would probably just replace it and its rubber seal. True, you can test the thermostats in boiling water, but sometimes they will get too hot and get ruined during the inspection/testing.
And again, like Frank says, the failure mode is usually stuck/blown open. It is rare for a thermostat to fail in the closed position, except when jammed up with salt. The one thing that should not be done is just to discard the thermostat and run the engine without it. Leaving the thermostat out creates uncontrolled cooling, and could cause hot spots/partial overheating. An engine that runs too cold will not idle properly, smoke more, and be more likely to foul plugs.