Radiator cap

[aero3113]

I am currently useing a 7 pound radiator cap in my 50 plymouth, is this correct? The overflow tube is at the filler neck, you can kinda see in the picture. Should I keep what I have or use a lower number or a non pressurized cap?

[BobT-47P15]

I think the consensus was that if the tube comes off the neck, the pressure

rating is not an issue. It will overflow anyhow, if it needs to. These systems

actually use a non pressure cap.

Now, we'll see if there are additional answers.

[Norm's Coupe]

The previous owner put a 7 lb. cap on my radiator too. However, as Bob explained, it doesn't do anything because the overflow tube is off the neck. Before I rebuilt the engine, it overflowed several times due to a blown head gasket. So.........even though it doesn't do anything, it also doesn't hurt anything. I still have the same 7 lb. cap on my 48 coupe, but it just works like a regular cap with zero pressure rating.

[aero3113]

but doent the antifreez/pressure have to push on the 7 pound cap in order for it to come out the overflow?

[Norm's Coupe]

The only way any water or antifreeze is going to overflow out the radiator with that or any other cap is: (1) If you overfill the radiator. (2) the car overheats. Otherwise, everything works just like it should. There is no pressure build up in the radiator with that type of overflow. All the pressure goes out the tube.

[Young Ed]

Some of them have the overflow tube away from the cap. This is my p15 radiator with the overflow tube coming from the very left of the picture.

Radiator in my 46 pickup with the tube coming directly from where the cap goes. This one should be able to have a few lb pressure type cap on it. I still run 0 pressure though.

[Norm's Coupe]

Mine is the same as Ed's.

[aero3113]

there still has to be some pressure to open the cap and expel the antifreeze out the tube?

[Norm's Coupe]

there still has to be some pressure to open the cap and expel the antifreeze out the tube?

Nope, no pressure. As mentioned, the only time any pressure may form to make it overflow is if you overfill the radiator, or you overheat. The cap does not seal the tank.

[greg g]

look closely at the filler cap if the overflow tube is connected below the flange agaist which the cap gasket seals, then the system will not build pressure regardless of what cap you have on. If the overflow is above the capt to radiator seal, then they system can be pressurized. You are probably OK with a 7 lb cap. If yu can 3 or 5 you might want to try them.

Are you haveing any problems with your cooing system??? If not what's the difference. If it ain't broke, let sleeping dogs lie.

[steveplym]

there still has to be some pressure to open the cap and expel the antifreeze out the tube?

I'm wondering if you are correct. As my P-15 has the overflow tube in the center of the radiator. So it is an open system and doesn't matter about the cap as the pressure would not build due to that. On your car it appears that the overflow tube in on the neck, so the cap would go down into the neck and cover where your overflow tube goes into the radiator. So wouldn't that mean pressure would have to build up to push the cap up so the coolant could overflow?

[aero3113]

yes,the cap does cover the over flow tube and needs to open to expel the antifreeze.That is why I am questioning what pound cap I should use. Any one else have this same setup,if so what pound cap do you use?

[aero3113]

Thanks for the info guys

[Don Coatney]

I am currently useing a 7 pound radiator cap in my 50 plymouth, is this correct? The overflow tube is at the filler neck, you can kinda see in the picture. Should I keep what I have or use a lower number or a non pressurized cap?

If your overflow tube comes off the filler neck a pressure cap will build pressure. I am not sure but I believe a 4 pound cap was original.

I think the consensus was that if the tube comes off the neck, the pressure

rating is not an issue. It will overflow anyhow, if it needs to. These systems

actually use a non pressure cap.

Now, we'll see if there are additional answers.

The previous owner put a 7 lb. cap on my radiator too. However' date=' as Bob explained, it doesn't do anything because the overflow tube is off the neck. Before I rebuilt the engine, it overflowed several times due to a blown head gasket. So.........even though it doesn't do anything, it also doesn't hurt anything. I still have the same 7 lb. cap on my 48 coupe, but it just works like a regular cap with zero pressure rating.[/quote']

Bob and Norm;

Think about what you said. If the vent comes off the filler neck it is a pressurized system. If the vent comes off the top of the radiator (such as a P-15) then it is not a pressuresized system.

[BobT-47P15]

yep......don.....youre right.

mine

[captden29]

i have a 265 cid. pressurized, uses a 4 # cap. they are hard to find in a local auto parts store. some guy on ebay sells them but they are about $17 plus shipping and i bought one once and it will tarnish fast even though he claims it is chromed. i use a higher pressure cap and put it on half way so it is not sealing tight. leaves me with a non pressure system but it works fine.could matter at altitude but i don't drive in mountains. denis

[aero3113]

Found a 4 pound cap at a local parts store today and installed it. I was once told that antifreeze temp will be higher for every pound you go up on the cap,I forget by how much.

[Young Ed]

Thats true. Its 3 degrees.

[Don Coatney]

yep......don.....youre right.

I thought you would understand Bob. And I also correctly thought that Norm would not admit that he was wrong.

[Merle Coggins]

Found a 4 pound cap at a local parts store today and installed it. I was once told that antifreeze temp will be higher for every pound you go up on the cap,I forget by how much.

The coolant temp doesn't go up... Higher pressure increases the boiling point of a liquid.

Merle

[steveplym]

Glad you found a cap for it. I tended to agree with Norm and Bob at first, but once I started thinking about it and looking at your picture I knew you were right. Just wondering how much you paid for that cap?

[Don Coatney]

The coolant temp doesn't go up... Higher pressure increases the boiling point of a liquid.

Merle

Merle;

You are 100% correct. And the boiling point in a vacuum happens at a lower temperature. That is how a milk condensor works without burning the milk.

[aero3113]

$5.99, not bad

[captden29]

wish i could find one for $5.99. i refuse to pay the guys price on ebay, plus his shipping. dennis

[aero3113]

Im sure you can find a 4lb cap at a local store or they can order one for you. Stant makes the one that I bought.