Re: Where's the thermostat?
Let there be no doubt about it, your Merc 500 4-cyl does not have a thermostat.
When Mr. Carl Kiekhaefer in his Infinite Wisdom built one of the best fishing motors of all time, he did not include a thermostat in the design.
Therefore, it obviously was not necessary!!!!
So, best thing you can do for your Classic Four is to keep the cooling system properly maintained by renewing the impeller periodically and making sure you replace any worn pump parts at the same time.
HTH & Happy Boating.............ed