I had a few thermostats that each lasted less than two seasons. Although I had paid something around $50 for the thermostats at one of the major marine engine distributors, a look in 2016 revealed that this part was difficult to find. Catalina Direct had the thermostat for $107.92 (with apologies) but I decided to make some changes before paying that price.
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 happy to help. 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" when called to recommend something. Doesn't someone, somewhere, have design documents on these thermostats??? However, I was able to get some key sizing information 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:
Observations seem to show that the two 1/8" holes may be large enough that, at idle, the engine never really heats up. I may close 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, $23.37, at iBoats.com, 800-914-1123.
By the way, it seems that crud accumulates in the lower section of the thermostat housing. I remove the bottom plug and drain the housing annually.