Radiator Caps

[Marv]

Barn members i learned something today. I never knew that Pressure caps of just about any reasonable PSI is easily obtainable, for our old Fords with the large openings like my 39.I took my radiator to a local radiator shop and was going to have a late model radiator cap neck installed and told him why. He told me about the various caps he had that would fit what i had. dHe was a good honest man. I will know the price of repairing my leaks and the cap in a couple weeks. You should be ab le to get one of these caps at any truck radiator repair shop.

[Walt Dupont--Me.]

I wouldn't get more than a 4-7 # cap for a 35 flathead. Walt

[TagMan]

I'm certainly no expert, but I don't believe the pre-war Ford radiators were built to be pressurized, were they ? If so, can you just install a pressure cap and have it seat to the radiator to seal it and make it a pressurized system ? The pressure cap may go on the radiator, but I don't believe it'd make it pressurized. Am I incorrect ?

[gundoca]

I went to the local truck parts store on the recommendation of my radiator guy. The guy came out with a ruler, measured the inside and outside of the neck, went back to his parts book (no computer in the place) and brought a 4# cap from the back. Fit right off the bat. Now I am curious if the pressure is causing the head studs to weep on my fresh 38 build.
.

[Reds34]

Gundoca,

Are you running straight water or anti freeze/water mix? I had the same issue when I ran the engine for my '34 the first time with antifreeze.

Red

[gundoca]

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50/50 mix

[Reds34]

Gundoca,

That is probably why. Exact same thing happened to the engine of my '34. Try water with a rust inhibitor/ water pump lubricant. Just my $0.02

Red

[Walt Dupont--Me.]

On the 48 and older engines with the stud and nut system, It's common to have a few head nuts leaking at first, they usually seal them self in a few days. After torqueing the head about 3 ties after warm ups, the ones that don't want to seal up, remove the nut and smear a little RTV on the threads of the stud and retorque. They will seal up. Walt

[Kube]

Walt is of course correct about the studs leaking. Nearly always this happens and nearly always, they seal themselves up.
I would not even think of applying more than 4# to one of these old radiators. Any more than that and one is surely risking failure. they simply were not built to withstand much more than 4#.

[Marv]

Originally Posted by Kube

Walt is of course correct about the studs leaking. Nearly always this happens and nearly always, they seal themselves up.
I would not even think of applying more than 4# to one of these old radiators. Any more than that and one is surely risking failure. they simply were not built to withstand much more than 4#.

What Kube says is true. However the radiator man told me he thought i could go with 7 because it was in realy good shape. Yes the caps will make it a pressurized system. The problem is the top tanks are not made for more than 7 lbs. and i guess that is a guess but he works with old cars and owns a 37 Ford hisself with this setup. The Amazing part is that you can buy them to fit those radiators.

[G.M.]

Why would you want more than a 4 lb cap?? Your are not trying to make a pressure system. Only reason for a pressure cap on an old Ford is to force the water through the radiator. Without a pressure cap the water backs out the over flow tube and the engine runs hotter from less water in the system. At about 210 degrees you may have about 4 lbs pressure, if it gets over 205 you have a cooling problem. G.M.

[Marv]

Originally Posted by G.M.

Why would you want more than a 4 lb cap?? Your are not trying to make a pressure system. Only reason for a pressure cap on an old Ford is to force the water through the radiator. Without a pressure cap the water backs out the over flow tube and the engine runs hotter from less water in the system. At about 210 degrees you may have about 4 lbs pressure, if it gets over 205 you have a cooling problem. G.M.

GM i am not recommending any pressure. I was amazed that you could buy these caps to fit. I was under the impression you could only buy them from some special company for antique Fords . Not so. I wanted Barners to know they could get what they wanted without getting high jacked with shipping. Some people use other engines than flatheads.

[Bruce Lancaster]

Ford offered a low pressure cap during some of the '46-48 period, from memory I
think 4 pounds. These cars had the overflow above the seal area and so could be pressurized without modification, unlike some years that had overflow where it completely bypassed the cap area.
I ran one of these for a while on my '48, and it was illuminating...the early tanks with large surface area are too flexible for pressure in my opinion. They could be modified with internal bracing, but that would be a serious job. Tanks for later cars that routinely ran pressure were more compact and I would assume heavier gauge metal.
As car warmed up, at about the point at which radiator became too hot for my hand (that would be about 100--110, way below running temp), there would be a sound like "fooomp" and the rear wall would bulge back a bit, sort of hinging where it was soldered to upper plate. As car cooled down, the radiator would pop back.
This was on a good radiator with no leaks in a car that ran right at its thermostat temperature, not at any sort of beyond boiling condition. The radiator handled the load and never popped that seam while I used the pressure cap, but I don't think the long term would have been good.
The caps are still routinely available from regular parts stores and the usual manufacturers...their size is a common truck application still.

[Marv]

Bruce that is true. You do need to add braces internaly for one to hold up.You may be able to brace it on the outside but i would not think it would look very well.

[G.M.]

What you have to be careful of is the length from under the top of the cap to the bottom rubber seat which is in the area of 7/8" (just a rough guess). Most old Fords I run across this length is a little short and the caps don't seal at the bottom. A 1/8" or slightly less rubber washer was required for the cap to seal. I never really got into the various cap length measurements but maybe someone has this info. This radiator was one I added the over flow tube to the neck. The radiator has been on the car for well over 20 years. The clean tubes and plate comes from Skips High Flow pumps and Barr's aluminum granular heavy duty stop leak. I believe the aluminum particals being forced though the tubes is what polished the core. I showed the radiator guy how it looked a few years ago and he couldn't believe it, said he never saw one like it. A new radiator has a dull look from the acid when it was soldered. You don't want the rubber washer to thick as it may increase the pressure it releases at. G.M.