Suggestions or advice for pressure and expansion tank caps

There is always a rubber washer on the sprung end. The rubber washer under the cap is so overflow coolant goes down the vent tube. Remember the old movies with cars overheating and blowing coolant out the sides of the cap. If it fits it's right, within reason. Make sure the tank is the highest it can be.

Pressure tank OEM cap was 13 pounds, but most of us are using 16 pounds.

Expansion tank does not take a pressure cap, just a plain cap, often found on the cheap plastic overflow tanks sold at auto stores.  

yes, some of the tanks did have European necks which generally need European caps to properly hold pressure. But if your car has previously been running fine with the caps you are replacing, just find matches for those caps.

this article has good information on the different types of caps:

https://icemancooling.com/2011...ator-caps-continued/

If you use the search tool on this forum, you will find more information.

Larry

I have been using matching 16 pound caps by Stant along with a modification to the cap for the overflow tank.  The modification is easy.  Simply remove the rubber from the lower seal and it stops working as a pressure cap and becomes only a sealing cap.  In the photos below the swirl tank cap is on the left and the overflow tank cap without the lower seal is on the right.  The second photo shows that they look identical from the top.  Be careful to install them on the correct tank.

My tanks both have US filler necks.  The Euro filler necks can be replaced with the US version.

What are the diameters of US and Eropean version, please?

Hi Rene…

I believe the diameters of both are the same, it’s the depth that is different, The American caps aren’t long enough to seal against the bottom of the deeper European necks….

Someone will correct me if I’ve got that wrong!

Rocky

Rocky is correct.  The diameters are the same and depths are different.  The Euro neck is taller than the US neck.  I just measured the depth of two different US tanks and they 3/4" deep.  I am guessing that the Euro neck is between 1/8" and 3/16" taller.

Since the correct fitting Euro pressure caps are essentially unavailable in the US, an easy workaround is to use a higher pressure cap.  Maybe use a 24 pound cap instead of 16.

Also, I added some holes in the modified cap for the overflow tank to help insure it does not act a pressure cap.

I’m not sure a 24lb cap is any deeper ( or taller) than a 16lb cap? I’m thinking that has to do with spring pressure?  My car still has the original tanks with European taller necks. My current 13lb cap has a rubber washer under it to help it seat, because it is a shorter US style cap. That’s just monkey stuff right there.  I’d like to find a 16lb cap that is designed to fit the longer neck? Someone mentioned an Alfa Romeo cap? Surely I’m not the only Pantera with original European neck tanks?

Where can I get the replacement US style filler necks? That would be an easier solution. I’m assuming they would need to be welded into place?

There is no depth difference between the 16 pound and 24 pound. What is happening is that a poor-sealing 16 pound cap will bypass sooner than a poor sealing 24 pound cap.

I have never seen any testing to confirm this but it seems to have some validity in theory.

Larry

Any radiator shop can install a US neck on your pressure tank.

Further to Larry's comment above; it's important to replace your rad cap on a regular basis. The rubber sealing surface compresses and deteriorates from the first moment that you use it. I'm not suggesting you replace it annually but every 2 or 3 years is a good idea. Your cap might hold 16 lbs when it's new but it sure won't after a few years of use. This is even more important if you're using a US rad cap with a European neck.