RC 26 Radiator Cap

[Dave B.]

I recently purchased what I believe is a NOS RC26 for my '66. The part # on the box is 6410206.
However when I try to put it on my expansion tank, it will not depress and turn. I have used later service replacement, non AC, RC26 caps that work, but this one will not.
Does anyone have any ideas?
Thanks
Dave

[Craig S.]

Re: RC 26 Radiator Cap

Dave - can't answer your question directly, but I have an original RC26 we can compare yours to when we get together later this week....Craig

[Peter L.]

Re: RC 26 Radiator Cap

Dave - Interesting situation. 6410201 is the correct part number for the RC26; but more important is the cap in the box an AC RC26? If you're going to connect up with Craig, I'm sure he'll be able to work with you on the details. good luck. Pete

[Joe L.]

Re: RC 26 Radiator Cap

Dave-----

"RC-26" is the Delco part number for the 15 PSI relief pressure radiator cap applicable to many 1965 to 72 Corvettes with aluminum radiator and/or aluminum radiator supply tank. For 1965-67 this cap may have been used only with cars with A/C and L-78/L-88. Other aluminum radiator/supply tank mid-year cars used an AC Type 307 cap which was a similar cap but rated at 13 PSI relief pressure. For 68-72, the RC-26 was used with all cars with aluminum radiator and/or aluminum supply tank.

In any event, as with many Delco items, this cap has had more than 1 GM part number over the years. Remember, Delco part numbers are APPLICATION-specific; GM part numbers are PART-specific. The GM part number for the RC-26 for the past 30 years, or so, has been GM #6410206. This cap has a stainless steel shell and small ears, unlike the originals which were made of zinc plated steel with large finger-hold ears.

Another "phenomenon" of the 6410206 is just as you described-----it requires significant "down-pressure" and torque in order to twist it on or off. This is much greater than that required for original caps and, presumably, other aftermarket caps.

[Dave B.]

Re: RC 26 Radiator Cap

Thanks guys for the replies. Joe...you are right on about the cap design. This one is stamped 'AC' and stamped '15#..RC26', but also stamped 'stainless' and has the small ears. It also is darn near impossible to 'push' on and/or turn. Anybody know where to find an original zinc unit with the larger ears?
Craig..see you Thursday. Bring your cap and we'll compare. Regards to all,
Dave

[Dave S.]

Re: RC 26 Radiator Cap

Joe,
As I recall my 70 LS-5 car had an RC-26 radiator cap and if memory serves I recall it being correct for that car. That was a brass radiator and brass expansion tank car. Is it possible the General wanted 26 psi in the big block systems. ????

[Joe L.]

Re: RC 26 Radiator Cap

Dave-----

The RC-26 is a 15 PSI relief cap. The RC-15 which could otherwise have been used on your 70 with 454 and A/C with a brass tank was also a 15 PSI cap. An RC-26 can always be used in place of an RC-15, but not vice-versa.

The cap specified for PRODUCTION for the 1970 454 with A/C application was a GM #861042. This is a PRODUCTION-only part number for the RC-15. However, I can easily see where the factory may have substituted an RC-26 for it.

[Joe L.]

Re: RC 26 Radiator Cap

Dave-----

Reproductions are available of the RC-26 which are very close to the originals in configuration. I expect that they'll be easier to install, too, than the replacement RC-26 you have.

For an original style, you'll need to check eBay or swap meets. On eBay, expect to pay a HUGE price for an NOS original piece. These radiator caps fall into the same category as distributor caps and certain alternator part numbers-----they're "trendy" items that folks will pay ridiculous prices to obtain.

[John H.]

Re: RC 26 Radiator Cap

Dave -

The "26" in RC-26 doesn't relate to the cap's pressure rating; the RC-26 cap is rated at 15 psi.

[Dave B.]

Thanks Joe *NM*