Radiator Cap

[Joe/Ct]

I have some leaking from the radiator cap on my '39 Ford coupe. I appears that I need to replace it, as the seal on the inside is broken. Can I replact the seal or should I just buy a new cap? What is the pressure rating that I need?

[David W Juliano]

Joe: If you have a stock 39 radiator the system is not under pressure. Original and original style caps are readily available or if your cap is good otherwise, just put in a new seal. Seal material is available at any good parts store such as NAPA.

[BIG MIKE]

Just make sure cap dosent say "made in Mexico" lol

[Joe/Ct]

Originally Posted by David W Juliano

Joe: If you have a stock 39 radiator the system is not under pressure. Original and original style caps are readily available or if your cap is good otherwise, just put in a new seal. Seal material is available at any good parts store such as NAPA.

The cap that I have on the radiator, which I think is an original '39, has a spring, which makes me think that it is under pressure (either 4 or 7 lbs). Also, when I went to the local parts dealers (NAPA, AutoZone, etc), none of them carried seal material.

[Ken/Alabama]

Like was posted earlier if you have a stock 39 radiator it wont build pressure. If it has the overflow tube coming straight out of the center of the top tank then you know it wont build pressure because its just open to the atmosphere ,so it wont matter if you have a pressure cap or not. I took my radiator to the shop and had them plug the overflow pipe with solder where it comes out of the top tank and then had them solder in a tee where the overflow pipe runs beside the radiator neck and drill a hole in the radiator neck and soldered the tee there. It still looks stock other than than that small tee at the neck plus I can now run a 7# pressure cap . Ive been in some really hot weather stuck in traffic and it didnt loose any water.

[G.M.]

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Joe the original 39 radiators were setup with a seat down in the neck for a pressure cap but as far as I know pressure caps were not supplied on 39s. The original over flow tubes came out of a fitting on the top of the tank. The tube can be cut off and a plug soldered in as one poster mentioned. A 1/4" hole needs to be drilled in the filler neck between the bottom seat and the top then reach in from the inside of the neck with a large center punch and and tap the punch so the hole is pushed out making a counter sunk seat. Take a piece of copper tubing and put a SMALL flare on the end, clean the tube and inside the neck, insert the tube through the hole from inside of the neck and solder the tube in the neck. Use a 4 lb cap and make sure it seats down on the bottom seat. At lot of these caps don't hit down on the bottom and need a thin washer type gasket. With 4 lb cap you can fill the radiator up into the neck. Filled with more water the engine runs cooler and if there is no compression leak the water will stay filled for months. The radiator tubes in the old Ford radiators add up to 3 1/2 square inches and the hoses filling the raditor tank are 4 1/2 square inches. The greater amount of water comming in can not flow through the tubes by gravity and must be forced through the tubes. This action of being forced causes the water to back up in the tank and runs out the over flow tube. The water level in the radiator drops more then a gallon which causes higher temperatures. If there is a compression leak which a lot of old Fords have the compression pressure opens the pressure cap and water escapes. Usually a large bottle of Barrs heavy duty sealer will stop these leaks before they get to bad. G.M.

[Joe/Ct]

GM & Ken/Alabama,

I just checked the radiator in my '39 coupe and found the following:
1. The upper tank has a raised code on the motor side/passenger side. It is "11A/G", that is an "11G" over a DASH over the letter "G". I don't know if this makes it a '39/'40 radiator or not.
2. I looked for the outlet on the upper tank as you described. There was none, nor could I find a soldered hole. However, it may have been obscured the the other soldering that had taken place.
3. There is an outlet/tube in the fill tube/neck, between the lower seat and the upper end of the neck. The hose from this tube goes to the bottom of what I believe to be a overflow tank (long, thin vertical cylinder). This looks exactly like what you guys are talking about above.
4. The distance between the lower seat and the top of the neck is 3/4".

I need to replace/repair my existing pressure (has spring) cap because the rubber on the bottom is cracked and the metal bushing on the top is spilt. How can I tell what pressure it is (no markings)? What is the difference between using the 4# and 7# cap? Where can I get one, still retaining the period look?

Thanks for alll your help.....Joe

[Ken/Alabama]

The "correct" radiator for 39-40 will be stamped 91A and the filler cap will be located on the top drivers side corner of the radiator. It sounds like your radiator is set up to run a pressure cap. A 4 or 7# cap will work fine , should be able to get one from Napa or Carquest. I think the last one I bought was from Carquest . The 11A is a designation for 41 .

[G.M.]

I sounds like someone put a pressure tank in the over flow. I never used or saw need for these. I believe there is a pressure cap on the top of this tank. When this type is used the cap on the radiator is a plain cap, not a pressure cap. The tank could also be just an over flow tank. The over flow tube go's into the bottom of this type tank or bottle and an over flow tube comes off the top of this tank to release pressure or excessive coolant. If the engine gets hot or compression pushes some water into the tank it stores it and as the temperature drops through 155 degrees when cooling down the jiggle valve on the pressure caps opens from a vacume in the system and sucks the water from the tank back into the radiator. The purpose of the pressure cap is to hold the water in the system as coolant is forced through the radiator tubes plus the pressure cap is also a safty valve. The pressure rateing of the cap determines the safty factor. A 4lb cap is all that is required on a good working cooling system on these cars. The temperature should never get much over 200 degrees, at 200 there is less then 1 1/2 lbs pressure. Driveing the radiator is cooled by the air forced through the radiator from forward motion and cooling is not a problem. What has to be avoided is idleing or heavy stop and go traffic on very hot days. The fans in these cars don't provide enough air for these conditions. G.M.