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Thread: Radiator caps

  1. #1
    Registered Member chevynut's Avatar
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    Radiator caps

    I have a PRC radiator that came with a 16# radiator cap with a billet cover on it. I have a hose from the radiator overflow connection to the "recovery tank" that I built. The hose is high pressure hose and goes into the bottom of the "recovery tank". The PRC cap doesn't fit under my radiator cover since it has the billet cover on it, and I need just a plain radiator cap there.

    Should the cap on the recovery tank be "vented" to atmosphere, or pressurized? I think some systems have an "open" or low-pressure cap on the radiator and the external tank is called an "expansion tank", which is pressurized. The coolant flows into the tank when hot and gets sucked back into the radiator when cold. I'm thinking of using that PRC cap on the "recovery tank" and getting a plain one for the radiator. If I can do that, what cap should I get for the radiator?

    I read through this and it confused me even more. I don't know what they mean by a "vented" cap. My first thought was that it was "open" and allowed unrestricted flow. But another source said it was pressurized but would vent at some maximum pressure.

    https://blog.championcooling.com/201...xpansion-tank/

    They say an "overflow" or "recovery" tank/reservoir is "vented", and allows coolant to go back into the radiator through a "vented" radiator cap. But they say the "reservoir" is "vented" too.

    Then they go on to say an "expansion" tank is different. They say the tank is vented but the radiator cap is "non-vented". Also they show the tank connected to a radiator hose, which is pressurized. So how does a "vented" tank maintain pressure?

    Not sure how I should set mine up. Would having both 16# caps work?
    Last edited by chevynut; 01-20-2023 at 01:23 PM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


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  2. #2
    Registered Member chevynut's Avatar
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    So i read the Champion website a couple more times and I think I understand it now. Their term "vented" means that it vents if the system goes over the rated pressure.

    The overflow tank is NOT pressurized, and the excess coolant goes onto the ground. The radiator cap is pressurized and "vents" when 16# is reached. On the expansion tank, the radiator cap is open to allow the expansion tank to pressurize with the system, and the cap on the tank is "vented" so it releases pressure above 16#.

    I want my tank to operate as an overflow tank so the radiator has to have a 16# cap, but since I don't have any other vent on the overflow tank except the "nipple" like on a radiator, the cap should be open to vent to atmosphere and not pressurize the overflow tank. But what harm can there be to pressurize it?

    20090228_1242.JPG
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  3. #3
    Registered Member 55 Rescue Dog's Avatar
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    Looks similar to a surge tank setup like used on the C3 Vettes that was like an expansion tank that wasn't vented. On my Camaro the radiator is lower than the thermostat, so I have a non-vented cap there as a high fill point on a Moroso adapter. The cap on the radiator is a vented cap with the vent tube into a non-pressurize reservoir that always maintains the coolant level. I have to drain a gallon of coolant out before I can remove the radiator cap. Can't see the $7 NAPA plastic reservoir with all the car together. Took some long hoses with a 10-inch engine setback.A Radiator Cap Serves Many Purposes - Northern Radiator Knowledge Center (northernfactory.com)

    camera phone 752.jpg
    Last edited by 55 Rescue Dog; 01-20-2023 at 05:00 PM.

  4. #4
    Registered Member 55 Rescue Dog's Avatar
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    Not sure how your tank could possibly work the way it is set up or how you could check and add coolant easily. All you need is a simple assessable reservoir tank that you can check and add coolant. They have been using the same basic setup forever on everything and it works like it should always keeping the radiator completely filled from hot to cold, and nothing leaking on the ground. I have no clue with your setup. Looks like a tank that was designed as a remote fill point that would be mounted above the height of the radiator which they did on the old corvette because the radiator was mounted below the top of the engine. Your tank needs to be open to the atmosphere to work with non-sealed cap and not a radiator cap is all. If you drilled a hole in the cap, it would work fine.
    IMG_3037.jpg
    Last edited by 55 Rescue Dog; 01-21-2023 at 06:48 PM.

  5. #5
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    My GM cars that come with 16psi caps have plastic overflows that are vented. I would vent it if it were mine.

  6. #6
    Registered Member chevynut's Avatar
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    Quote Originally Posted by 55 Rescue Dog View Post
    Not sure how your tank could possibly work the way it is set up or how you could check and add coolant easily. All you need is a simple assessable reservoir tank that you can check and add coolant. They have been using the same basic setup forever on everything and it works like it should always keeping the radiator completely filled from hot to cold, and nothing leaking on the ground. I have no clue with your setup. Looks like a tank that was designed as a remote fill point that would be mounted above the height of the radiator which they did on the old corvette because the radiator was mounted below the top of the engine. Your tank needs to be open to the atmosphere to work with non-sealed cap and not a radiator cap is all. If you drilled a hole in the cap, it would work fine.
    What do you mean by "vented" in your prior reply? The Champion article says "vented" is pressurized, and when it exceeds the rated cap pressure it "vents". To me vented means it doesn't hold pressure, but different sites use the term differently. This statement from Champion seems wrong to me, depending on the definitions of "vented" and "sealed"....

    "A system with an overflow will have a vented radiator cap, and usually a sealed reservoir cap. The reservoir is never under steam pressure, which allows the coolant to return via atmospheric pressure."

    The term "sealed" must mean that the cap doesn't vent to atmosphere, but the tank does through a different vent tube. They use the term "vented" to mean it's pressurized, and "vents" when the rated pressure is exceeded. That's what's confused me about their article.

    My system is quite simple, if you understand how a recovery tank works. The top of my overflow reservoir cap is almost at the same height as the radiator cap by design. I don't think it matters if it is or it isn't, because the radiator cap is "vented" or pressurized. The reservoir just holds the coolant until it's sucked back into the radiator, so it could be mounted below the radiator. If the radiator cap wasn't pressurized, as in the case of an expansion tank, it would matter. A pressurized expansion tank has to be above the engine and radiator because the radiator cap doesn't restrict flow in or out of the radiator.

    If the recovery tank was significantly above the radiator, and you removed the radiator cap, the coolant in the tank would flow into the radiator, overflow it, and dump onto the ground. Apparently that's how you designed yours.

    I can add coolant in two ways. First I can take the air intake/radiator cover off and remove the radiator cap and add coolant. Or I can remove the panel behind the front tire, and fill the recovery tank. I plan to fill the radiator before I put the radiator cover on, and put some coolant into the recovery tank. Once the engine warms up everything should stabilize. If it pukes some more coolant into the recovery tank, no big deal, as it will suck it back into the radiator when it cools. It's quite simple. And I don't expect to have to add coolant very often in this car so access is secondary to having a nice, clean, uncluttered engine compartment.

    I'll probably just cut part of the lower reservoir cap seal away so the reservoir is vented to the atmosphere. It should work fine that way.
    Last edited by chevynut; 01-23-2023 at 10:47 AM.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

  7. #7
    Registered Member 55 Rescue Dog's Avatar
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    That explains it much better than the original post and I understand how your setup will work. Pretty much the same thing I have on my 55 other than the $7 reservoir that never nothing dumps on the ground and radiator is always full.
    IMG_3047.jpg

  8. #8
    Registered Member chevynut's Avatar
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    I got a simple, cheap "vented" 16# NAPA radiator cap and topped off the radiator with coolant, then I took the bottom seal off of the cap that WAS on the radiator, and put it onto the recovery tank. That should make it into just a "sealed" cap that vents out of the tank overflow tube. I filled the recovery tank up part way so it might overflow but it should stabilize.

    I was surprised to see so many kinds of radiator caps for sale, not just different pressures, but different sizes and depths. I finally called PRC and asked what they used on their radiators, and the guy there told me "they're all the same". I told him to go to O'reilly's or Autozone's website and look for a radiator cap. I finally found out that there's an SAE standard "type A" cap, and that's what I used. It's 3/4" deep. O'reilly's doesn't even list a 3/4" depth 16# cap,and Autozone doesn't specify depth.
    56 Nomad, Ramjet 502, Viper 6-speed T56, C4 Corvette front and rear suspension


    Other vehicles:

    56 Chevy 2-door BelAir sedan
    56 Chevy 210 4-door sedan
    57 Chevy 210 4-door sedan
    1962 327/340HP Corvette
    1961 Willys CJ3B Jeep
    2001 Porsche Boxster S
    2003 Chevy Silverado 2500 HD Duramax
    2019 GMC Sierra Denali Duramax

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