'68 L79 radiator cap question

[Peter L.]

Re: Joe - one more question

Dave - The new RC-26s have the SS cover. As far as I know, except for the stampings on the cover noting what cap it "really' is and the gasket, the new RC-26s are the same as the new RC-15s. Pete

[Dave F.]

Re: Joe - one more question

Part of the issue for me is functionality. If the 26 is SS, and the 27 is SS and not suitable for contact with my aluminum tank, what makes the 26 suitable for my aluminum tank ? And is the 15 suited for aluminum tank contact?
My head is starting to hurt

[Joe L.]

Re: Joe - one more question

Dave-----

It's really pretty simple-----the RC-26 is the ONLY current AC radiator cap which is available and approved for all 1963-72 Corvette applications using an aluminum supply tank. This cap is designed and insulated in such a way as to reduce galvanic (dissimilar metal) corrosion.

The currently available version of the RC-26 does NOT match the configuration of the original RC-26 used in the 65-72 period. For one thing, the current cap uses a stainless steel cover; the original was zinc plated steel. Second, the ears on the current cap are much smaller than the ears on the originals. Third, the script stamped on the cover of the current cap is not the same as the originals. However, FUNCTIONALLY, the current RC-26 is just as good, if not better, than the original style.

ALL of the current AC radiator caps, be they RC-26, RC-27, RC-32, RC-33 or whatever, have the stainless steel cover. This is the only way the caps have been manufactured for the last 15 years, or so.

There are "reproductions" of the original RC-26 available. However, based upon my cursory inspection of them, I don't think that, functionally, they're made to the original standards as far as galvanic protection is concerned. Mostly, they've concentrated on making the cover similar to the originals and using an "aluminum" rivet. These 2 things do not make an RC-26 an RC-26.

[Patrick H.]

Re: Joe - one more question

1) The current RC-15 has stainless inside, so will work fine on an aluminum supply tank. The dissimilar metal problem has disappeared on the new RC-15 caps. I'd be interested to know if the current RC-26 is stainless inside too. I'm not that far into the caps yet, so I'd have to ask Tony.

2) My current RC-15 has ears the same size as my original RC-15. I can't comment on the RC-26 currently available as my partner Tony is working on those.

3) The commonly available reproduction has a gasket between it and the tank as does the original, which should work as well as the original regarding corrosion.

So, I wonder how close the RC-15 and RC-26 have come in configuration.... wait and find out.

Patrick

[Joe L.]

Re: Joe - one more question

Patrick------

1) Except for the galvanic corrosion inhibition provisions, the RC-26 and RC-15 are functionally identical; they always have been. The RC-15 was never used in PRODUCTION for a Corvette with an aluminum supply tank. Also, in the last year, or so, the RC-15 has been discontinued and has been replaced by the RC-26. An RC-26 has always been able to be used in replacement for an RC-15, but not vice-versa. The reason for the 2 caps was that the RC-15 was less expensive than the RC-26 due to its lack of anti-galvanic corrosion provisions. At this point in the SERVICE life of the cap, I suppose that GM figures that part number consolidation is more important than the price difference between the caps; folks will be happy just to get a new cap for their 30+ year old Corvette. However, you will note that they didn't replace the RC-26 with the RC-15; they replaced the RC-15 with the RC-26. The stainless steel construction should have little to do with affecting the anti galvanic corrosion qualities of the caps;

2) Assuming that your current RC-15 is one produced in the last 15 years, or so, if it has the same size ears as your original, then your "original" is not original. The RC-15s used in PRODUCTION (and, available in SERVICE for at least several years afterward) had the large ears and a zinc plated shell. The change to the stainless steel shell occurred just about the same time as the change to the small ears. I've never seen a large ear cap with a stainless shell. I'm not saying that none were ever made, but I've not seen one. Certainly, I don't think that ANY caps with stainless shells and/or small ears were used in PRODUCTION on any pre-73 Corvette;

3) Which gasket are you referring to? All caps, RC-15, RC-26 or otherwise, have the gasket beneath the spring-loaded pressure relief mechanism which seats on the annular surface about an inch, or so, below the filler opening.

[Dave F.]

Re: Joe - one more question

The RC 27 I have has gaskets which isolate it from the aluminum tank. The only metal to metal contact is where the ears clamp around the lip of the tank opening.

[Joe L.]

Re: Joe - one more question

Dave-----

All I an say is that the RC-27 was never a cap which GM specified for use on 63-72 Corvettes with aluminum radiators and/or aluminum supply tanks. As Far as I know, this cap was never used in PRODUCTION or catalogued as a SERVICE replacement. Presumably, the RC-26 differs from the RC-27. Both are 15 lb caps, so that can't be the difference.

[Dave F.]

Re: Joe - one more question

Joe, I understand completely. I certainly am not challenging what you are saying. As you saw in an earlier post, what is inscribed on the 27's cap meets the need of my '68's system regarding it being 15 lb and for a closed system. I will be comparing the two tomorrow when I get the 26 in an attempt to find the difference between the two. Again, thanks much for your input. I had no idea this subject could become so complicated , but I have enjoyed the exchange !

[Patrick H.]

Re: Joe - one more question

Joe,

You're right - small ears on the new one. Went and checked. Oops. I've been looking at way too many RC-15s.

But, the RC-15s also have that disc immediately under the cap made of stainless steel, not brass as it used to be. That is why I would think the latest RC-15 could be used in any application. The rubber gasket is there, but the RC-26s and RC-33s I've seen have a fiber (?) gasket under the cap mechanism itself.

Patrick

[Tony H.]

RC-26 Fiber gasket

Sorry to chime in so late. Patrick and I will be writing a series of articles for the Restorer on radiator caps. We hope to cover as much as possible beginning with the RC-15 followed by the RC-26, then others.

I believe that the role of the fiber gasket is eliminate contact between the stainless steel spring and the aluminum. The contact of any two dissimilar metal (even stainless steel and aluminum) have the potential for galvanic corrosion. The extent depends highly on their respective positions in the galvanic potential series. Aluminum is anodic to stainless steel (cathodic) in this case. In plain English, aluminum will dissolve or corrode when in contact with stainless and in the presence of moisture.

PS. If any of you out there are willing to loan your original caps for this series of articles, please contact me via email.

[Dave F.]

Just picked up the new RC 26, and...

The cap has larger clamps than the 27 I have. And the gaskets are a little more "pliable". The quality of the 26 (the AC box it came in says" Made in India") stainless steel appearance is inferior to the 27. However, the guts of the cap appears to be identical to the 27. At least it was only $11.

[Craig S.]

Re: '68 L79 radiator cap question

Thx Joe - that makes sense, I always wondered how the little rivet made much of a difference but the insulation explanation makes much more sense. All the more reason to use the correct cap when the DeWitts goes in....Craig