Radiator Cap Operation

[Joe C.]

Re: Radiator Cap Operation

Just making ya work a bit, and glad to hear that your car will suck even more now.

G110 is not in the cards this year, the stars are not aligned despite best efforts.

Too bad.
Well................not really, cause that leaves more fer me to eat and drink.
See you next year.

Back on topic:

The AC RC33 and the AC RC27 are (seemingly) identical caps, with the exception of the absence of the upper gasket on the RC27. Here's a link to a RC27, same exact as mine:

http://cgi.ebay.com/ebaymotors/GM-NOS-AC-Radiator-Cap-RC-27-Corvette-Camaro-Cadillac-/120579538507

Here's a test question:

When I add the gasket to the underside of the cap on my RC 27, assuming that the compressed thickness of the gasket is 0.039", how will that affect the preload on the relief spring, how many pounds pressure will the difference be, and in what direction will the relief pressure go, higher or lower? The design pressure of the RC27 is 15#.

This is worth 15 brown nose points toward a red judging hat, or if you already have one, then a .50c gift certificate for Hooters.

BONUS POINTS (Added after examining the ebay ad): If you examine the ebay ad, you'll see something that makes much of this discussion moot.

[Michael W.]

Re: Radiator Cap Operation

Here's a test question:

When I add the gasket to the underside of the cap on my RC 27, assuming that the compressed thickness of the gasket is 0.039", how will that affect the preload on the relief spring, how many pounds pressure will the difference be, and in what direction will the relief pressure go, higher or lower? The design pressure of the RC27 is 15#, without the upper gasket.

This is worth 15 brown nose points toward a red judging hat, or if you already have one, then a .50c gift certificate for Hooters.

BONUS POINTS: If you examine the ebay ad, you'll see something that makes much of this discussion moot.

Trick question, no fair. You didn't specify the vertical height differential of the coolant recovery tank. I presume you're not trying to factor in distortion of the cap or the rad neck flange?

[Clem Z.]

Re: Radiator Cap Operation

I have an RC27 cap on my 1965 Corvette's reservoir tank, which is a 15 pound cap. I bolted a plastic coolant recovery tank just behind the reservoir, and want to ensure that the system will function correctly. The goal is to keep the reservoir full to the top, and function as a "recovery" or "closed" system (not a simple "catch can").

What must I look for in a rad cap? Is the RC 27 OK? What is the difference, if any, between a cap used for an "open" and "closed" system?
Thanks in advance.

if your corvette has a aluminum rad that is a "expansion" tank since the rad does not have a reservoir tank for the hot water to expand. the reason you only fill the "expansion" tank half full is to allow for the water expansion when hot. if you fill the aluminum tank full you will force the coolant into the plastic tank you installed so your plastic tank has to be fed from the bottom so the aluminum tank can "suck" the coolant back in as it cools and you need the correct cap that will allow this to happen.

[Joe C.]

Re: Radiator Cap Operation

if your corvette has a aluminum rad that is a "expansion" tank since the rad does not have a reservoir tank for the hot water to expand. the reason you only fill the "expansion" tank half full is to allow for the water expansion when hot. if you fill the aluminum tank full you will force the coolant into the plastic tank you installed so your plastic tank has to be fed from the bottom so the aluminum tank can "suck" the coolant back in as it cools and you need the correct cap that will allow this to happen.

Clem,

That's right and that's how I did it.

The ancillary benefit of this (not correct, God forbid) modification, is that the system gains about a quart of additional coolant, which gives it somewhat greater cooling capacity.

[Clem Z.]

Re: Radiator Cap Operation

Clem,

That's right and that's how I did it.

The ancillary benefit of this (not correct, God forbid) modification, is that the system gains about a quart of additional coolant, which gives it somewhat greater cooling capacity.

as long as the cap is correct,not NCRS correct, because i would guess the original 1965 cap would not work that way because i believe the original setup just had a tank overflow hose to the ground

[Joe C.]

Re: Radiator Cap Operation

as long as the cap is correct,not NCRS correct, because i would guess the original 1965 cap would not work that way because i believe the original setup just had a tank overflow hose to the ground

Clem,

It would if you added a gasket to seal against the radiator neck. Of course, you'd have to add the recovery tank as I did.

Read this thread entirely through, and carefully.

What I learned over last few days is that all pressurizing caps function exactly the same. The only diff, is that when (if) you properly seal the underside of the cap to the radiator's fill neck by adding an upper gasket to the cap, then when the cap works in reverse, during cool down, it will siphon coolant back from the recovery tank rather than sucking air from the overflow tube and the incomplete seal around the rad neck.

If the underside of the cap is not sealed, as in the pre 1973 Corvette systems, then there will be 2 differences in operation:

1. If the boilover is violent, some coolant MAY escape directly out the underside of the non gasketed cap, while the bulk of it is discharged to the ground thru the overflow tube.
2. On cooldown, the negative pressure inside the radiator will cause the relief valve to open, but the leaks around the radiator neck will draw air rather than coolant from the recovery tank.

[Michael W.]

Re: Radiator Cap Operation

You just learned this? Where you been the last 37 years paisan?

[Joe C.]

Re: Radiator Cap Operation

You just learned this? Where you been the last 37 years paisan?

Reminds me of the old joke about the pilot of a lost transatlantic airliner, who gets on the intercom to inform his passengers of their plight. He announces: "Ladies and gentlemen, this is your captain and I have some good news and some bad news. First the bad news.........we're lost. Now the good news...........wherever we're going, we're getting there fast!"

[Clem Z.]

Re: Radiator Cap Operation

Clem,

It would if you added a gasket to seal against the radiator neck. Of course, you'd have to add the recovery tank as I did.

Read this thread entirely through, and carefully.

What I learned over last few days is that all pressurizing caps function exactly the same. The only diff, is that when (if) you properly seal the underside of the cap to the radiator's fill neck by adding an upper gasket to the cap, then when the cap works in reverse, during cool down, it will siphon coolant back from the recovery tank rather than sucking air from the overflow tube and the incomplete seal around the rad neck.

If the underside of the cap is not sealed, as in the pre 1973 Corvette systems, then there will be 2 differences in operation:

1. If the boilover is violent, some coolant MAY escape directly out the underside of the non gasketed cap, while the bulk of it is discharged to the ground thru the overflow tube.
2. On cooldown, the negative pressure inside the radiator will cause the relief valve to open, but the leaks around the radiator neck will draw air rather than coolant from the recovery tank.

sucking air into the system would be the last thing you would want to do as this will cause heating problems. on race cars we would pressurize the system cold so there would be even more pressure when hot to keep out any air