Radiator Cap using overflow tank
Thread Starter
Burning Brakes
Joined: Feb 2001
Posts: 929
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From: Marietta GA
Radiator Cap using overflow tank
I have '68 427/390hp. My car originally did not come with an external expansion tank or an external overflow/collector tank. Years and years ago I added an external overflow/collector tank but was never able to properly secure it. Recently, I got around to purchasing a different style overflow/collector tank and secured it to the inside of the right fender. I thought it would be a good idea at this point to replace my radiator cap but I want to make sure I am using the right one for use with an external overflow/collector tank.
The stock replacement cap as listed in the Stant application guide is either a #230 or #330 cap which is 16 lbs. Stant lists this as "for radiator overflow reservoir systems". A very similar cap #331 (also 16lbs) is listed also listed as "for radiator overflow reservoir systems" but also lists the cap as being "vented".
Do I need to purchase the vented cap to allow the coolant from the overlow tank to be drawn back into the radiator as it cools and the pressure drops? Will the stock replacement (non vented) cap serve the same purpose and allow coolant recovery back into the radiator? I am leaning toward the need for a vented cap to allow coolant recovery but can someone please confirm. Thanks.
The stock replacement cap as listed in the Stant application guide is either a #230 or #330 cap which is 16 lbs. Stant lists this as "for radiator overflow reservoir systems". A very similar cap #331 (also 16lbs) is listed also listed as "for radiator overflow reservoir systems" but also lists the cap as being "vented".
Do I need to purchase the vented cap to allow the coolant from the overlow tank to be drawn back into the radiator as it cools and the pressure drops? Will the stock replacement (non vented) cap serve the same purpose and allow coolant recovery back into the radiator? I am leaning toward the need for a vented cap to allow coolant recovery but can someone please confirm. Thanks.
Melting Slicks
Joined: Mar 2006
Posts: 3,342
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From: Connecticut
Originally Posted by BB68Vett
I have '68 427/390hp. My car originally did not come with an external expansion tank or an external overflow/collector tank.
If it diddnt come with the tank then I would guess it's not a closed system.
Can somebody else shed some light on this topic?
Safety Car
Joined: Dec 2004
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From: Jersey Shore Exit 98
I have an overflow/refill tank on my 66. There is no filler on the radiator itself so I fill from the overflow tank. I use a Napa 14lb rad cap w/the flip top lid to release pressure. No problems so far.
Team Owner
Joined: Aug 1999
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From: Ontario
If you have a rad cap on your rad you have an overflow tank. That cap on the rad is vented and not pressurized. If you do not have a cap on the rad, you have a expansion tank with a non-vented and pressurized cap. This one has no rad cap and is under pressure.
Race Director
Joined: Mar 2006
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From: Florida
Originally Posted by BB68Vett
I have '68 427/390hp. My car originally did not come with an external expansion tank or an external overflow/collector tank....
The tank you added is an "overflow bottle" vs an expansion tank as the ones that came on cars with a radiator with no filler neck.
The overflow tank you has uses the overflow tube of the rad neck to expell excess coolant as it expands into the overflow bottle.
You can use either cap 330 or 331, with the non vented or full pressure cap being the more reliable. A V in the part number should mean vented. Here is why
The vented cap does not have a spring on the vent valve and the non vented has a light spring on the vent valve.
It has been found that the vented cap doesn't always close so you have a system that is not fully pressurized and can expell coolant and/boilover, increased pump cavitation and boiling hotspots in the head.
When using the none vented type make sure you have most of the air out of the system because it takes more heating/cooling cycles to purge air out of the system than does a vented cap.
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by paul67
If you have a rad cap on your rad you have an overflow tank. That cap on the rad is vented and not pressurized. If you do not have a cap on the rad, you have a expansion tank with a non-vented and pressurized cap. This one has no rad cap and is under pressure.
My older GM cars had a tube running off the fill on the rad that basically went to the ground
My newest vehicle is a 99 Explorer V8
It has a cap and an expansion tank as does my 73' Corvette.
Can you please elaborate because you have me scratching my head big time
Thanks
Bob
Thread Starter
Burning Brakes
Joined: Feb 2001
Posts: 929
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From: Marietta GA
Thanks for all the responses. I do appreciate it.
So according to Noonie, I can use either a vented or non-vented cap and either will allow recovery of the coolant back from the overflow bottle into the radiator as the engine cools. Have I misunderstood? I was under the impression that a non-vented cap would not allow recovery of the coolant from the overflow bottle back into the radiator. If I can use either vented or non-vented then what is the difference between a vented and non vented cap from a functional perspective.
By the way, Noonie is right on regarding the non-vented cap having a spring on the vent valve and the vented cap has no spring (and actually this is what raised a question in my mind as to whether I am using the right cap)
Just to clarify, I do not have an expansion tank plumbed into the heater hoses. I merely added a coolant overflow bottle so when my radiator expels coolant as the engine heats up it can be recovered back into the radiator as the engine cools. The overflow bottle just has a plastic cap on it, not a pressurized cap or anything like that.
I apologize if I am a little thick here.
So according to Noonie, I can use either a vented or non-vented cap and either will allow recovery of the coolant back from the overflow bottle into the radiator as the engine cools. Have I misunderstood? I was under the impression that a non-vented cap would not allow recovery of the coolant from the overflow bottle back into the radiator. If I can use either vented or non-vented then what is the difference between a vented and non vented cap from a functional perspective.
By the way, Noonie is right on regarding the non-vented cap having a spring on the vent valve and the vented cap has no spring (and actually this is what raised a question in my mind as to whether I am using the right cap)
Just to clarify, I do not have an expansion tank plumbed into the heater hoses. I merely added a coolant overflow bottle so when my radiator expels coolant as the engine heats up it can be recovered back into the radiator as the engine cools. The overflow bottle just has a plastic cap on it, not a pressurized cap or anything like that.
I apologize if I am a little thick here.
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Team Owner
Joined: Jul 1999
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From: Huntersville NC
BB68vett - Same situation here, same engine (68 L-36). Just attach your overflow hose to the BOTTOM of the coolant overflow bottle. Actually, I replaced the overflow hose with a clear plastic hose of the same size. I didn't change the radiator cap and it works just fine. Truth is, it doesn't overflow any more, but when it did the coolant would get sucked back into the radiator when the engine cooled off.
The overflow bottle that I bought actually came with a "special replacement" radiator cap, but I never used it. Just kept the old radiator cap and it worked fine.
68 overflow hose dumps directly to the ground
Even if your cooling system works great and "never" overheats you don't want to find yourself stuck in traffic on a 100 degree summer day and start dumping coolant on the street. Or pull into a parking lot and shut your engine off and start puking coolant in the parking lot. Good insurance to have an overflow bottle! 
The overflow bottle that I bought actually came with a "special replacement" radiator cap, but I never used it. Just kept the old radiator cap and it worked fine.
68 overflow hose dumps directly to the ground
Even if your cooling system works great and "never" overheats you don't want to find yourself stuck in traffic on a 100 degree summer day and start dumping coolant on the street. Or pull into a parking lot and shut your engine off and start puking coolant in the parking lot. Good insurance to have an overflow bottle!
Last edited by PRNDL; Sep 7, 2006 at 11:57 PM.
Race Director
Joined: Mar 2006
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From: Florida
Originally Posted by BB68Vett
So the stock replacement non-vented cap allows recovery of the coolant back into the radiator or does it just catch the coolant as the radiator expels it?
The original cap should not have the vent valve at all, so it cannot recover coolant back into the rad.
I use the 330. It has a lower failure rate of keeping the system fully pressurized than the "no spring" cap.
Oem's use the 230 because it purges the system much quicker and it is cheaper. Both the 230 and the 330 ultimately do the same job.
Just make sure you have it plumbed as PRNDL said. Some of the tanks don't have a bottom outlet and use a 2 hole cap with a vent and a long hose or tube to the bottom. Also make that hose is good if it applies.
Race Director
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From: Foxfield CO 1970 Convertible
Originally Posted by Bob Onit
Im sorry Paul but I do not understand your post at all
My older GM cars had a tube running off the fill on the rad that basically went to the ground
My newest vehicle is a 99 Explorer V8
It has a cap and an expansion tank as does my 73' Corvette.
Can you please elaborate because you have me scratching my head big time
Thanks
Bob
My older GM cars had a tube running off the fill on the rad that basically went to the ground
My newest vehicle is a 99 Explorer V8
It has a cap and an expansion tank as does my 73' Corvette.
Can you please elaborate because you have me scratching my head big time
Thanks
Bob
This link should show a picture of the radiator. Notice the small tube at the top for the expansion.
Race Director
Joined: Nov 1999
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From: Foxfield CO 1970 Convertible
Originally Posted by BB68Vett
So the stock replacement non-vented cap allows recovery of the coolant back into the radiator or does it just catch the coolant as the radiator expels it?
Team Owner
Joined: Aug 1999
Posts: 30,995
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From: Ontario
Bob,
I should have been more precise in my language. What I was trying to say is that I do not have a cap on my rad. The pressurized cap (15psi) is on the expansion tank.
This is the overflow tank on my 1979. The rad does have a pressurized cap and the tank is not under pressure. Basically it just accepts hot coolant that in earlier times used to flow straight to the ground.
I should have been more precise in my language. What I was trying to say is that I do not have a cap on my rad. The pressurized cap (15psi) is on the expansion tank.
This is the overflow tank on my 1979. The rad does have a pressurized cap and the tank is not under pressure. Basically it just accepts hot coolant that in earlier times used to flow straight to the ground.
Last edited by Paul L; Sep 8, 2006 at 06:45 AM.
Thread Starter
Burning Brakes
Joined: Feb 2001
Posts: 929
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From: Marietta GA
Paul67, you are up early in the morning.
I have installed a system just like your 79. I want to make sure that in addition to the overflow bottle catching the expelled coolant, that the radiator cap will allow the coolant to be re-introduced into the radiator as the engine cools.
So what I am trying to figure out is whether I need a vented cap to accomplish this. If either a vented or non-vented cap will accomplish it then what is the functional difference between the two different types of radiator caps.
BTW, thanks to all for the replies so far.
I have installed a system just like your 79. I want to make sure that in addition to the overflow bottle catching the expelled coolant, that the radiator cap will allow the coolant to be re-introduced into the radiator as the engine cools.
So what I am trying to figure out is whether I need a vented cap to accomplish this. If either a vented or non-vented cap will accomplish it then what is the functional difference between the two different types of radiator caps.
BTW, thanks to all for the replies so far.
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 99
From: Ontario
The words "vented" and "non-vented" confused me but after checking the Stant web site I think I have things sorted out. But people will pipe up if I am wrong.
My conclusion is that either one can be used. Both have a return valve/seal to allow coolant flow back to the rad when cooling occurs and negative pressure develops. One cap is sealed (non-vented) and must be opened to relieve pressure. The other has a vent (see red handle in my pic above) to relieve pressure without removing the cap. I have used both on my 1967 although in that system there is no return valve/seal. This may be a simplistic and erroneous conclusion but we shall see.
My conclusion is that either one can be used. Both have a return valve/seal to allow coolant flow back to the rad when cooling occurs and negative pressure develops. One cap is sealed (non-vented) and must be opened to relieve pressure. The other has a vent (see red handle in my pic above) to relieve pressure without removing the cap. I have used both on my 1967 although in that system there is no return valve/seal. This may be a simplistic and erroneous conclusion but we shall see.
