Radiator Cap...what pound rating or part number?

[Dan Babb]

The cap on my truck isn't a pressure cap...looks like the spring and seal rotted off a long time ago.

What pound rating do you guys use for the radiator cap?

[geopcanuk]

Very little pressure...maybe 6-10 lbs.The cap is more of a cover than a pressure seal.

[Jim Gaspard]

I may be off on this, but in our Pilothouse truck cooling system, its the temp rating of the radiator camp that's important, not the pressure rating. In fact, I don't think our radiators are meant to be under pressure at all. In modern car/truck radiators, pressure will raise the boiling point of radiator fluids, but that concept was not necessary in our era trucks.

[greg g]

What year and where is the vent tube located??? If the vent tube is located remote from the radiator cap, it is a no pressure system. If the vent tube is under where the cap gasket seals its a no pressure system. If the vent tube is located in the neck of the filler between the gasket and the and the top of the cap it is a presurized system radiator and 7 lbs would likely be max pressure.

In the first two scenarios the cap pressure rating will have no bearig on the system as the overflow tube is below where it seals and will vent before any presure can build up.

So you can use any cap that fits on a non presurized system.

Edited October 8, 2009 by greg g

[grey beard]

That Greg guy is right again . . .

Our Pilothouse trucks had no pressurized cooling system until 1952 or thereabouts, so it depends on your year of manufacture, PLUS the fact that in the past sixty some odd years the radiator may have been swapped out.

I just bought a cap from NAPA that fit my large-mouth radiator opening, and peeled the bottom rubber gasket off so no pressure can possibly build there, as a favor to my old honeeycomb-style radiator core.

My Pilothouse has almost two thousand miles on it - all this past summer. I'm amazed how cool the engine stays. In all the wamest weather, idling or pulling hills, I have never seen the dash gauge go above the 180 mark - this with a 180 degree stat in there. Prolly should have tried to get a 190 or 200 degree unit - keeps the sludge from building up if the block gets hot enough to evaporate the condensation buildup, dontchaknoo. JMHO:D

[Dan Babb]

My truck is a 52.

The vent is located in the neck of the radiator filler between the gasket and the and the top of the cap, so it's a pressurized system.

So, true or false:

With a pressurized system, you need a pressure rated cap for the engine to stay cool. Otherwise, the fluid can boil and won't keep the engine cool.

I assume the above to be true because my motor runs a bit hot now, but I don't have a pressure rated cap.

[Merle Coggins]

Adding pressure to the system only raises the boiling point of the coolant. I don't remember the exact spec, but 2 degrees per pound (psi) of pressure seems to stick in my head. So, if you had pure water in your cooling system, the water would begin to boil at 212 degrees (f). If you added a 7 psi cap you could get the water up to 226 degrees before it begins to boil.

So, adding a pressurized cap does NOT make the engine run cooler, but it will allow it to get hotter before the coolant boils.

If your engine runs hot I would recommend flushing your radiator and engine block. These engines tend to get a good deal of sludge deposits in the block. You will need to pull the casting plugs out of the side of the block and flush thoroughly.

Merle