The Stovebolt Site Forums The Shop Area Engine Shop Correct "plumbing" for 261 water pump

What does the correct thermostat housing, thermostat, and water pump for a 261 look like and how is it plumbed so that it doesn't overheat on initial startup when the typical thermostat is completely closed? I understand it is critical to always have some coolant flowing and have heard of a dual function thermostat. I would like to see the correct parts and learn how the entire system works. Mine has the typical hoses to the radiator and the loop to the heater, but there is another plugged hole under the top hose to the heater.

The "correct part" stopped being manufactured a few decades ago. It's a thermostat with a flapper valve built into it that plugs off a large diameter bypass hose after the engine warms up. With a conventional thermostat, the bypass runs all the time, so there's no possibility of overheating during warmup, or at any other time. The big hose bypass system was only used on heavy duty vehicles like school buses or dump trucks. Unless the heater hoses are disconnected or there's a shutoff valve that completely stops flow through the heater core, there's a large volume circulating through the cooling system as soon as the engine is started. You're concerned about a problem that doesn't exist.
Jerry

Thank you Jerry. OK, but since I'm a newbie here let me ask for clarification. If a hose to the heater has a shutoff valve, which I assume is needed or you'd have hot coolant (oxymoron) and heat in the cab always, and it's shut off/closed, does this prevent circulation through the block when cold starting? Would a "bridge" connection between the heater hoses ahead of the heater core, creating a "primary" loop for circulation that's always open, then the heater valve after that used to allow circulation in the "secondary" loop (heater core) be appropriate? Sounds like the heater valve being closed and stopping the flow could be problematic.
Rob

I doubt that closing off the heater hose would cause a problem. That's what the thermostat is for. It opens when the coolant gets warm to allow circulation thru the radiator. My daily driver has no AC, and in hot weather, the cab overheats during short shutdowns, so I installed a shutoff valve. No issues with overheating on cold starts with that arrangement. You could try it and see what happens. Monitor the back of the head with an infrared thermometer on a cold start and watch as it warms up.

You are looking for a problem that doesn't exist. A good waterpump flowing through a good radiator with clear engine passages doesn't care if the truck has a heater or not. Most 261s were used in big trucks running full loads against a governor allowing 3500/4000 rpm's all day long. Many didn't even have a heater, which was a dealer-installed option throughout the 235/261 production years. They came from the factory with pipe plugs where the heater hoses go.

I went through this issue as I have the 261 thermostat. Yes, closing the heater valve may be an problem if it shuts off the bypass. I have a tee plumbed into the heater hose so the bypass works in any case. There has been a lot of discussion about this in the past. e.g.

https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1445405/all/261-bypass-thermostat.html