Catalina 30 TRBS #2889 "Northern Light"

Replacement for Universal 5411 Thermostat

The rest of the modifications.


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:

  1. They provide some water into the exhaust flow even when the thermostat is closed. The stock plumbing for this engine on the C30 has the exhaust water from the engine fed into the muffler. A closed thermostat during warm-up means no water to cool the muffler.
  2. They prevent totally blocking pumped water flow (while the engine is cold) which would result in back pressure at the water pump. Back pressure in the water pump tends to push water down the pump shaft, through the oil seal, and out the weep hole in the pump. That's too close to getting water into the engine oil for my comfort.

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.