Correct Expansion Tank Cap

[Mark W.]

What is the correct replacement cap for my expansion tank? I have a '66 327/350 hp, without AC. The cap that has been on it since I've owned it the past 5 yrs is RC15 15lb, however, when I look in different catalogs for replacement, I see others listed ( RC 26-15lb and AC 307 13lb). Which is correct?

[Scott Marzahl]

Re: Correct Expansion Tank Cap

Hi Mark,
Joe L. suggested that I use the RC 26-15lb for my '67 327/350 car. It is GM part number 6410206 and I picked up from my local GM parts counter.

Good Luck,
Scott

[Joe L.]

Re: Correct Expansion Tank Cap

Mark----

The original cap was an AC 307. The AC R-26 replaced that cap many years ago and that's the one that I suggest you use. The AC RC-15 is NOT the correct cap for your application and it should not be used since it has no provisions for galvanic corrosion protection and could cause other aluminum components of the system to suffer accelerated corrosion. At this point, though, I wouldn't worry about what's been done; just change it out.

Incidentally, for many years GM P&A Catalogs were very ambiguous about correct radiator cap applications for Corvettes. That's likely how the RC-15 got on your car.

[Gary B.]

Correct Expansion Tank Cap: Clarifiation needed

Joe,

I'm confused by your answer. The '66 TIM&JG says the original expansion tank cap for all '66 SB cars was the RC-26. Can you clarify what you mean when you say the original cap was the AC 307 and an AC R-26 was a replacement cap? I've never heard of either.

Thanks,

Gary

[Joe L.]

Re: Correct Expansion Tank Cap: Clarifiation neede

Gary-----

I always thought that the '307' cap was used in PRODUCTION from 1963 through, at least, 1966 for small blocks without C-60 and without K-19 applications. That might not be correct, though. It might be that the last year for the '307' was 1965 for small blocks without C-60.

[Duke W.]

Re: Correct Expansion Tank Cap: Clarifiation neede

The '63 models were originally equipped with a 13 psi cap. I don't know what year it was replaced with a 15 psi cap, but the 15 psi cap can certainly be retrofitted to a '63.

Since the coolling system components are the same, the additional pressure potential will do no harm.

Duke

[Gary B.]

Watch out for stainless NOS caps on e-bay

Mark,

Don't be tricked by the NOS RC-26 caps that show up on e-bay. Typically there say "Stainless" on them and while they may be NOS, they are not what was used originally in '66.

Gary

[Mark W.]

Thanks

Thanks for all the info and suggestions. I ordered one from my local Chevy dealer today and should have it not later than Tuesday.

[Peter L.]

Re: Correct Expansion Tank Cap

Mark - The RC-26 was used on all 1966 and 1967 sb Corvettes w/ and w/o air conditioning. The cap top has the AC logo stamped in a circle by one ear and on the other ear 15# and below that RC-26 are stamped. An earlier version, pre-1966, of the RC-26 cap was used on Corvettes equipped w/ air conditioning but these had different information stamped on the cap top. The original equipment RC-26 expansion tank cap tops were zinc plated steel. Later versions have a stainless steel cap top as others have pointed out.

The 307 cap was used pre-1966 sb Corvettes w/o air conditioning.

Pete

[Tony H.]

[Joe L.]

Re: Correct Expansion Tank Cap: Clarifiation neede

Duke-----

Plus, the 13 lb cap was replaced for SERVICE by the RC-26 15 lb cap. So, that would indicate that the change from a 13 to 15 pound system has GM's full approval for Corvettes originally equipped with the 13 lb '307' cap.

[Peter L.]

Re: Correct Expansion Tank Cap: Clarifiation neede

The '65 TIM&JG states "air conditioned cars use a cap rated at 15 psi with an AC logo and RC-26." I expect the '64 Corvettes equipped w/ C60 also used the 15 lb RC-26 cap. I checked the C60 sections of the '64 and '65 Corvette AIMs but didn't see a call out for the 15 lb cap. In '66 and '67 all sb Corvettes came with the RC-26 cap. Since all the cooling system components are basically identical for the years '64 thru '67 and '63, there should be no problem in using the RC-26 in place of the 307 cap. Pete