Radiator cap

[oldmopar]

It looks like they switched to a 7 lb cap here is some information from a Jan 1952 plymouth service bulletin

[55 Fargo]

I have run this type of rad with both 4 or 7 lb caps. 1950 or 1951 PDDC went to a pressurized cooling systems, with a 4 lb pressure cap.

Think of the bright side, with a 7lb cap, you can now raise the boiling point by 20 degrees, combined with 50/50 antifreeze water, ou add another X amount of boiling point. So the end result is, you can run your engine at 180 to 200 degrees, and still have a good margin of error before boiling over.....Fred ps go to NAPA and get a 4lb cap if your concerned

[aero3113]

my car runs at about 190 ar a little higher. Do you think I should stick with the 7lb cap or try out the 4lb I bought?

[Plymouthy Adams]

at 190 degree..the use of pressure is moot...either will suffice..

[De Soto Frank]

One more exception, brought to you by MoPar...

My '48 New Yorker (straight-eight) has an original, factory, pressurized (?) cooling system...

Not sure what the pressure rating is/was...

From the outside, the cap looked like the non-pressurized caps, except for a rivet in the center. Remove the cap and turn it over, and there was a collar about 1 inch in diameter and about 1-1/2 inches tall attached to the cap. In the center of this collar was a plug with a hole at the center, and a spring-loaded ball behind it. There was a relief hole in the side of the collar.

So, when the cap was installed, the bottom of the collar sealed against the flange at the bottom of the radiator filler. When the pressure on the cooling system exceeding the rating of the cap, it forced past that spring-loaded ball, up through the collar, out the relief port, and through the overflow tube in the side of filler neck. Pretty much like how a "normal" Stant pressure cap works...

In my foolish youth, I thought that OEM cap looked pretty tired and I replaced it with a new Stant cap ( 7 pound ?).

All of a sudden, I started finding puddles of coolant under the car... looking like they were coming out of the over flow tube.

After some head-scratching and trying other caps, I discovered by accident that the original cap was almost 1/2-inch taller than the newer caps...

I measured the filler neck on the radiator, and from top flange to the inner flange at the bottom, the filler neck was taller too...

SO, I found that the sealing disc on the new style caps was never reaching the sealing flange of the radiator filler... and I had the equivalent of a non-pressurized cap on my system.

I could not find any Stant (or other pressure caps) with an "extended reach", so I just put the original cap back on and the puddles went away...

I don't know if there were any of this sort of irregularities with the six-cylinder cars; the eight-cylinder New Yorker had some unique features, including a 26-quart cooling system with a fin & tube radiator, as opposed to the cellular ("honeycomb") type used on the six-cylinder cars...

De Soto Frank

[Don Jordan]

Okay - while we're on the subject - yet a little off topic - ready?

I blew out a freeze plug the other day. Is it possible that too much pressure (along with it being 60 years old) could have caused that? It is obvious I don't completely understand the pressure issue. Since I was a kid when the car began to over heat my father would turn the radiator cap to the first click. It's probably a wives' tale. I can't think of a reason the freeze plug would just blow out.

[Don Coatney]

Okay - while we're on the subject - yet a little off topic - ready?

I blew out a freeze plug the other day. Is it possible that too much pressure (along with it being 60 years old) could have caused that? It is obvious I don't completely understand the pressure issue. Since I was a kid when the car began to over heat my father would turn the radiator cap to the first click. It's probably a wives' tale. I can't think of a reason the freeze plug would just blow out.

In my opinion excess internal pressure could contribute to freeze plug failure.

[dirty dan]

Okay - while we're on the subject - yet a little off topic - ready?

I blew out a freeze plug the other day. Is it possible that too much pressure (along with it being 60 years old) could have caused that? It is obvious I don't completely understand the pressure issue. Since I was a kid when the car began to over heat my father would turn the radiator cap to the first click. It's probably a wives' tale. I can't think of a reason the freeze plug would just blow out.

Overpressure of a coolant system can blow out a freeze plug.

I think that on engine coolant systems as old as these, I wouldn't risk installing a higher pressure cap. At 190 engine temp it won't matter, once you get past 212 it starts to become a concern. If the Mopar engineers designed this system for a 4 pound cap, it certainly wasn't designed to hold 7 pounds of pressure 60 years down the road.