Coolant tank (radiator) cap sizes

Dr.Jeff

Dr.Jeff

True Classic
The cap for the coolant recovery tank (or "radiator cap" as most sites list) appears to come in two sizes (types). I've seen references to a 'short neck' and 'long neck', or 3/4" and 1" versions. For the purpose of this discussion I am only referring to X1/9 applications, with factory tanks, and only the size (dimension/fitment) of the cap...NOT the quality, brand name, pressure rating, color, price, origin of manufacture, personal preference for, etc. :)

Most of the part suppliers I've checked only offer the shorter (3/4") cap, except on Vick's site he only lists the taller (1") one. Many of the other catalog listings don't specify which one they are offering but only have one. Also the application charts from cap makers seem to only list one, but again they don't specify any dimensions (that I've found).

On Obert's site he states the original metal/stainless tanks use the 'long neck' cap, while the plastic tanks use the 'short neck' cap (however he only seems to offer the shorter one). Yet when I measure my S/S and plastic tanks' necks, they both are the same dimensions (there is no cap on the S/S tank to compare).

Furthermore, the shorter (3/4") version is readily available from most aftermarket cap makers (with commonly found part numbers) however I do not find any aftermarket part numbers for the long (1") version.

So what's the story on cap sizes?
 
Oh man... Last April I went through this same issue and came away with at least three different conclusions.
1. Some caps come defective from the manufacture
2. The "7" on the cap from Vicks actually means .7 Bar which is 10 PSI
3. The metal tank that I have came with a 1" cap and yet it is the same depth as the stock tank which is a short neck..

Here is the thread: https://xwebforums.com/forum/index.php?threads/stants-cap-fyi.32156/

Hope this helps,
Ed
 
Hey Ed, thanks for responding...I still don't know the answer.
I already read your referenced thread a couple times before (and again just now), but unfortunately it really does not give me a clear answer. However your experience with the metal tank and tall/short cap comes the closest to anything I've found. Which cap did you end up using, the short (3/4") or tall (1") one? Frankly at the moment I'm not concerned about pressure ratings or cap quality until I figure out which SIZE cap is correct.
Please clarify for me which cap size you found to be best for the stainless tank. Thanks
 
So I have been using the .7 BAR / 10 PSI cap that came with the Vick tank. It is a longer neck, but lower pressure than the 13 PSI Stant 10229. The Stant cap is the replacement for the Gates 31527 which was listed in that other thread.

My theory is as follows. When you put the cap on the tank you are looking for 2 things.

#1. Do you have to press the cap down against the spring to get it to begin to ratchet into place? If not, then the cap is not hitting the inside lip of the tank, so it is too short.

#2. When you press the cap down and feel resistance are you compressing the spring the entire way? If so, your cap is too long and won't function properly.​

So the goal for me was to have a cap that covered the lip inside the tank, had some pressure required to start it closing and had enough travel that I was sure it could release pressure..

Ed
 
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Since I have duplicates now of the different caps, I figured I would post photos with what I learned. On the left is the Vicks .7 Bar cap, on the right is the Stant cap. Both clamp down on the tanks just fine. If you look at the rubber surface area of the Vicks cap you will notice that it is much thinner than the Stant. That means if you don't get it locked down and centered, sometime it will leak. If you look from the side angle, you will see that they are almost the exact same height. The Vicks is slightly taller and has longer and better travel than the Stant.
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Thanks Ed, great posting. I follow what you mean about the "feel" of how each cap fits securely or not. However without having both size caps on hand that is not possible to compare, and the one cap I do have (a very old "short" one) is so worn that it's spring has little tension so it doesn't offer much help.

What I find really interesting about your results is the "long" cap fit the metal tank better. This corresponds with my interpretation of a vague notation Obert's site has hidden in a obscure location of a related part...tall cap for metal tank. HOWEVER, when I carefully measured and compared the necks of my metal and plastic tanks, BOTH ARE COMPLETELY IDENTICAL...in every dimension, shape, and form. That leads me to think the long cap is the better fit for BOTH types of tanks. Although every site other than Vick's only lists the short cap for all applications. And Vick's listing indicates it (the long cap) is for all applications. Is Vick the only one that got this right? Maybe this is a major reason why there has been so many issues with cooling systems? Everyone that did not buy one from Vick has been sold the wrong cap? Do I see a conspiracy theory at large (just kidding)?

Honestly I'm not sure what to believe. I suppose it is also possible there are two types each for both the plastic and metal tank necks. I will buy a cap and see what happens...knowing the other one is available to try if needed. But it would be nice if there was an actual (correct) answer for this.
Thanks for your input and great pictures.
 
I can clarify one thing.. "Metal tanks" apparently have two different neck sizes. It sounds like your metal tank is the same as mine which is the same as the plastic tanks. I saw a metal tank on another X1/9 that had a long neck and there is no mistaking it. I am fairly confident that both the caps I have were designed for the same size neck and are just slightly different in height.. Neither of them are for the longer neck... Gonna search the web for a photo of a long neck tank...

Oh and I just ordered a CSF #4501 off of ebay.. Apparently this company makes good stuff and the 4501 is the replacement for the Stant 10229..

http://csfimports.com/radiator-pressure-caps/
 
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ecohen2 said:
Neither of them are for the longer neck.
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Ahhh, my misunderstanding...I thought you were dealing with one of each size.
Between that clarification and your sighting of two types of metal tank necks I think it pretty much solves the mystery (as far as I can tell). Most tanks (plastic or metal) likely use the short cap, but there are some (seemingly some metal ones) that have a different neck and therefore need the longer cap. I wonder if the difference in metal tank necks is between the stainless ones and the steel ones? Anyway, I should be good now. Thanks
 
So I found the SAE standard for radiator caps... It looks like the diameters are small, medium and large.. Ours are medium. The industry standard is .75 inches but there are some legacy caps that are 1" in depth. The only real variable seems to be the pressure which has a huge range. I am going back to the 13 PSI cap just because on super hot days the 10 PSI blows out the vent hose...
 
What I found in practice:

OE cap for plastic resv. worked fine on OE tank. On my SS tank, it would "vent" at normal operating temps. !" cap (sourced from IAP (whose inventory was split between MidwestBayless and AutoRicambi) worked properly on SS tank. As 1" caps were more common on older Spiders, their inventory may have gone to AutoRicambi.

The Alfa inventory from IAP went to an Afla parts house, so they may have them as well, whoever they are.
 
Kevin, please clarify for me. When you say "OE tank" and "my S/S tank", is your S/S tank NOT an OE item? There are aftermarket S/S and OE S/S tanks so I'm not certain which you refer to. Please re-read your post with that question in mind and see if you get what I'm confused about.

Just to be specific, as I understand it there are different tanks as follows:
OE plastic
OE metal (not S/S)
OE S/S
Aftermarket S/S
And likely aftermarket plastic?
Furthermore ecohen2 says, ""Metal tanks" apparently have two different neck sizes", so I guess there are OE metal long neck and OE metal short necks

So it gets confusing which ones you are talking about you say, "OE cap for plastic resv. worked fine on OE tank. On my SS tank, it would "vent" at normal operating temps."

Thanks for the input.
 
Thanks buddy. So you found the 1" cap worked for your '78 OE S/S tank? Did you ever notice if the neck on it was any different than the neck on your OE plastic tank? Was the S/S a 'long neck' version?
 
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While I was researching cap sizes I ran across this company called CSF. They claim to make a high quality cap for a reasonable price. I ordered one that should fit my steel tank and it arrived the other day. It is considerably better constructed and both rubber pads are thicker than the Stant version of the same cap. I am going to give it a try and see how things go.. The part I ordered is cap #4501...
 
Dr.Jeff said:
Please let us know how it works out, thanks.
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So far so good... No real hot weather yet.. Although it is made in China it really is much better built than the Stant. Upon closer inspection I noticed that the plunger piece spins freely, so when you put it in the tank, it is not twisting the rubber. The center shaft does not wobble and the spring tension feels far stronger than the 13 from Stant. The only concern I have is that you don't feel any resistance until you start to torque the cap down.
 
I recognize the brand name as one used by the 'American-style' racing guys (I hate to mention the word NASCAR because I don't want to affiliate my name with it :)). Looking at the picture I can see the difference in thickness of the rubber, etc. Likely the reason you don't feel a lot of resistance when installing is due to the "free spinning" of the center piece (your not rubbing the rubber face against the seat on the radiator neck).

Interestingly they make their caps in four basic sizes (e.g. "small, med. large") and each size fits a long list of applications. The 4501 for example also fits some 1700 models, mainly Fords, Jeeps and Toyotas.

I'm pretty sure you said this is for the "short neck" style tank (i.e. 3/4", not 1")?
 
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