Stovebolt.com Forums The Shop Area The Engine Shop radiator cap

I'm curious what you guys use and recommend in regards to what pressure of radiator cap you use on the original radiators in the AD trucks. My truck is a 53 half ton with the original radiator, and a 235 engine.

Original radiators and heaters are only built to use lower pressures, if modern higher pressures are used to raise the building point, older units will leak!

Ed

My 38 is a non pressure system.

If there is a housing that resembles a tuna can on the upper radiator tank with a vent tube attached to it, you have a "zero pressure" system that does not use a pressure cap. Later systems with the vent tube attached to the radiator fill neck can handle a little pressure, like maybe 4 pounds, or possibly 7 on vehicles from the late 1950's, but be aware that the heater core is probably the weak point. Too much pressure will cause the radiator core and/or the heater to spring a leak. Modern replacement radiators can handle up to 15 pounds or so, but the heater must be upgraded also. Each pound of pressure on a cooling system raises the boiling point of the coolant 3 degrees F. at sea level so a 7# cap will let the coolant reach 233 degrees before it boils.
Jerry

My truck came to me with a 4 pound cap. I installed a coolant reservoir and it works perfectly filling up when hot and lowering to the cold line as it cools off. Radiator is filled to the very top.

I’m running a 4 lb. cap on my ‘59 235 in my ‘52 with the original radiator.

Hmmm. I have a 7 lb. cap. I have a new aluminum radiator, but the heater core is original. No leaks so far, but maybe I should buy a 4 lb. cap.

The only purpose of any pressure cap is to allow the coolant temp is to rise above the boiling point and stay liquid- - - -the same principle a pressure cooker uses. With a good coolant recovery system, anything that escapes past the pressure cap gets recycled back into the system when the engine cools down. That's probably a better idea than trying to keep all the liquid in the radiator all the time, and a lower pressure cap can be used to protect the heater core. Very few stovebolts with a clean, proper capacity radiator run 200+ degrees for very long if the water jacket is clean and the right temp thermostat is used. A 180 stat is probably the best choice to get the engine up to normal temp and still have a little cushion before the boiling point is reached. Engines that run cold all the time try to beat themselves to death with wide piston skirt and bearing clearances.
Jerry

My last Stovebolt, a 1950 3600 with 216 had what may have been the original zero pressure cap. The shop manual calls for zero pressure for 1950, but not sure about others.

The 48-51 Service manual says that all trucks except COE's use a no pressure radiator. It says the COE uses a 3.5-4.5 psi cap.