Unfortunately, I have been dealing with a top gasket leak on a Billet pressure tank. What is the pressure these caps are designed to hold or give at? This tank is definately defective from day one, brought it to an aircraft machine and tool shop to build up the top sealing surface and get a good polished gasket mating surface, but they could not do anything with the bottom of the neck where the tabs of the cap ride (actually gets looser as you tighten the cap). Got the cap to hold to about 22 psi in a rad shop tank, but I wanted a goal of about 25 psi. This is a high horse, very high chrome engine which we could not even get to 170 degrees running without the cap leaking! I did order a replacement tank today but refuse to admit defeat easily on this one.

At what pressure is the top gasket designed to hold to? It should be the same for all pressurized automotive caps regardless of the lower gaskets ratings, correct?

Thanks,
Angelo
Original Post
quote:
but I wanted a goal of about 25 psi.


First off, I'm not even sure I understand what you are referring to.

Top gasket versus lower gaskets? HUH?

Next, why do you want a pressure capability of 25 pounds?

Standard for our Panteras is 13 or 16 pounds.

We have many hose clamps in areas not easily checked, and running a 25# cap serves no purpose that I can see...?

Larry
There are 2 gaskets on our 16 lb radiator pressure caps, one (the lower one) unseals when the cap opens at 16 lbs. The other (upper that seals at the top of the neck) SHOULD hold to a substancially higher pressure point for safety reasons (This is the pressure I am looking for) I realize our systems would/could have multiple failure points above 22 lbs and am not running this system above a 16 lb cap. This system is failing at the upper gasket at the temps stated above for clarity. In an overheating condition, idealy the overflow would happen in the overflow tank then down the overflow hose to the ground.

Thanks,

Angelo
quote:
The other (upper that seals at the top of the neck) SHOULD hold to a substantially higher pressure point for safety reasons

Why?

Once the lower one unseals at 16 pounds, the system starts to vent that 16 pounds of pressure into the overflow hose that carries the coolant into the overflow tank.

As the system pressure is thus vented, it will not continue to climb above the 16 pound point.

So the upper gasket sees no more than that 16 pounds, and only for an instant.

So why does it not sealing above 22 pounds worry you?

I am surely no expert on cooling systems, and you did mention you had been to a radiator shop.

Did they think the top gasket failure at 22 pounds was a problem?

I am very open to learning why what I see as a non-issue, is seen as a problem by you - and the radiator shop?

Please explain so I can get my head wrapped around something I may be missing.

Thanks,

Larry
If you think about it the upper seal is not really a pressure seal, 22 psi on the upper seal is more than enough. If you don't seal the bottle o/flow what pressure does it hold? It should obviously be 16 psi. I postulate that you have more going on than a cap with an upper seal that is not sealing;

a) If the cap is leaking then the lower seal/spring is not sealing or you have exceeded 16# pressure in your system. What I suspect is the neck may be too long on your bottle for the cap you are using. This is a common problem with oem bottles as they are euro necks and caps here are US length.
b) when the pressure seal opens it allows coolant to flow to the o/flow bottle, which is a non pressurized bottle. You don't have a pressure cap on the o/flow bottle do you?

Let us know what you find.
Julian
Angelo, you may have a few problems. First, brass radiators balloon the water tubes in the core somewhere around that pressure, closing off air flow thru the radiator. This can happen as the cap's pressure relief is not designed to flow much volume, only small amounts. We usually recommend no more than a 16-lb cap. Welded all-aluminum radiators (no epoxy to seal tubes) will handle a little more pressure- maybe up to 25 psi intermittently, but only NASCAR uses 22 psi (or higher) caps- and that's only because they don't have rad-fans at all, so their pit stops are always engine-heaters.
Second, the cap or tank neck top may be distorting and leaking under that much pressure, or the cap may have been defective out of the box. Cap-tabs are easily bent with a pliers if the sealing pressure needs to be adjusted. Finally, I've seen leaks from the weld or whatever fastens the small overflow tube to the tank neck. Leaks there are often hard to see and are sometimes mistaken for cap leaks. As Julian said, we ARE talking about the small surge tank, not the larger overflow tank, right? The big overflow tank is only supposed to have a cap with a splash-guard gasket, not a pressure cap.

For everyone: it's good to eliminate even small rad-cap leaks; water from the cap on a surge tank in the stock location runs down the tank, down the inner fender and right into an oval hole cut into the lower right subframe's top. So if the car's frame rails haven't been drilled on the bottom for the recommended 4 per side drain holes, the frame fills with water and rusts! Some cars will hold water for weeks after a short drive in a surprise rainstorm.
quote:
Get off the computer and get that engine put together.

quote:
Get off the computer and get that engine put together.

D'oh! You're right. I'm feeling the stress of time running out. Gotta get it done. Thanks for the kick where I need it.
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