Well, the last thermostat lasted less than two seasons. I think I paid something around $50 for it at one of the major marine engine distributors. I looked around a bit and noticed that Catalina Direct now sells the thermostat for $107.92 (with apologies). I decided to make some changes:
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, Evan (ext 198), 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. Someone, somewhere, must have design documents on these thermostats, don't they??? 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.
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.