Over the years I've had a few thermostats that lasted less than two seasons. The price tag (around $50 at the time) was quite high. In May 2019 the Catalina Direct price is $108.60!
A few years ago I decided to find a less expensive exact replacement. If that failed I was going to machine a new thermostat housing to accommodate a reasonably priced automotive thermostat. (Yes, I know "automotive" is not marine-rated, but the only thing that is marine-rated is a fish. On second through, every thermostat is marine-rated...they all sit in water.)
After some searching I landed on the iBoats.com web site. I found some thermostats that looked similar enough to be hopeful. I called them and the sales rep was quite helpful. Since iBoats doesn't have detailed specs for thermostats he offered to put me in touch with the manufacturer, Sierra. A call to Sierra to request a spec sheet came up empty handed because they "simply go by application." Doesn't someone, somewhere, have design documents on these thermostats??? However, I was able to get some key sizing information from Sierra so I went back to iBoats and ordered one that looked right.
The result? It's an exact mechanical fit (1.390" flange diameter) and exact temperature rating (143°F), and is used in a number of Johnson, Chrysler, and Mercruiser applications. In other words, it's a marine thermostat.
The old thermostat had two holes drilled in the flange that allow a small flow of engine cooling water to get to through the thermostat at all times. The total hole area amounted to two 0.125" diameter holes so I simply drilled two 1/8" holes in the new thermostat. I believe these bypass holes accomplish two things:
Observation showed that the two 1/8" holes may be too large because at idle the engine never really heated up. I closed one of the holes. Subsequent observations with an IR thermometer show that both thermostat housing and exhaust manifold settle at something like 144°F regardless of throttle setting, so the thermostat is clearly regulating to its temperature.
It's part number 18-3557, $30.49 (May 2019), at iBoats.com, 800-914-1123.(The image of that thermostat on their website appears to be a generic photo.)
By the way, it seems that sediment accumulates in the lower section of the thermostat housing. I remove the bottom plug and drain the housing annually.