Based on what I can read and have observed the cooling system for a '49 3100 was not pressurized from the factory. However, in the operator's manual there is a note that the design of the radiator neck will allow the owner to install a pressure cap. I agree that the design of the radiator neck and the location of the vent tube will work with a pressure cap. As a side note I did see that the 5100 series used a 3 1/2 to 4 1/2 psi cap.

I am interested in your experiences and thoughts about this. Do you get any better cooling performance with a slight pressure in the system? If a pressure cap is used I expect it should be very low pressure (maybe 3 or 4 psi). My understanding is that the primary reason to pressurize a system is to increase the boiling point of the coolant.

Right now our '49 3100 tends to run a little hot (about 190 or so) with a 180 thermostat. It goes to this temperature when you idle right after driving for 10 or 15 minutes. When you start driving again the temperature drops back to about 180. Our ambient temperatures are only between 60 and 70 right now. I expect things will get worse as we get into 80 and 90 degree days this summer.

My thinking is that we have some gunk/crud in the block and head that hopefully we can flush out (this weekend's project). The engine was rebuilt at least 15 years ago so it sat exposed to the atmosphere for quite a while. We did have the radiator cleaned when we assembled the truck. I do not think that adding a pressure cap will address our concern.

Thanks for the ideas.

Rusty