Radiator overflow catch tank

I purchased an overflow catch tank to install with my new aluminum radiator. I noticed that the tank isn’t vented. Is it not supposed to be vented? Or does the build up of pressure help push the antifreeze back into the radiator?

For Healey use, the tank needs to be vented. On my ex-MG brass tank, I had to remove the guts of a standard radiator cap so as to prevent it from sealing on the tank neck. You can also leave a stock cap loose on the tank.

If its an auto parts store plastic tank, just make sure there's a vent hole in or near the cap.

Your Healey radiator cap will need a gasket sealing against the radiator neck lip in order to create a vacuum as the radiator cools - this is what pulls the coolant back from the overflow tank.

Easy enough to drill a hole in the cap. Here's a pic of the cap I’m going to use, looks correct. How would a non vented tank even work?

aero3113 said:

...How would a non vented tank even work?

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As (un)luck would have it, I just replaced such a tank on my mom's 2000 Lincoln LS with a 3.9L V8. I believe this is a British-designed and manufactured engine as, supposedly, it is the same as used in the S-Type Jags of the era. As this engine is British-designed, what should have been a simple 20-minute R&R turned into a multi-hour torture test.

I haven't had the plastic cover off in a while, but IIRC the radiator has no top tank. Presumably, this is to allow for the low front profile of these cars--they do have excellent visibility over the bonnet--so this plastic bottle serves as the top tank/reservoir/pressure bottle of the radiator. The cap notes it is '16PSIG,' and it does retain a lot of pressure when hot. It is also called a 'degas bottle' in the shop manual, but all the parts houses call it an 'overflow tank' (or bottle). The Lincoln sprang a coolant leak, and as an uncle of mine's wife had a similar car he told me the bottle was likely the problem, so I set about what should have been a simple fix (removing the coolant in the tank was going to be messy, no matter what). The tank is bolted-in near the scuttle, but getting to it was not an issue; the problem is that the half-inch return hose was at the bottom of the tank, was cut to the shortest length possible, affixed with one of those PITA spring clamps and routed under the brake reservoir and booster (I suppose this would not be an issue with a R/H drive car). After finally wrangling the tank out the problem was obvious: it was a plastic tank made from two parts with a seam about the middle, but had developed a 6-inch crack height-wise (not too surprising at 85K miles and 18 years). As bad as getting the tank out was, getting a replacement back in was thrice as bad; I finally gave up on the spring clamp and used a good ol' worm drive hose clamp (note even this would not have been possible had I not borrowed a set of those extra long needle nose pliers from a friend). As the highest point on the engine cooling system sits higher than the radiator, there is a cap on the top of the engine in which coolant must be added after losing some, and after a heating/cooling cycle (I call this 'burping' the engine). The car does have one nice feature: there is a hose to, presumably, the highest point in the heater core, through which you can bleed air from the core (you open a valve until hot coolant sprays in your face and and all over the engine then scramble to close it).

So, to answer the question, a non-vented tank would probably be used in lieu of a top radiator tank. As the manual called it a degas bottle coolant from the radiator comes in in a smaller hose at the top of the tank, and the larger hose returns it to the radiator (gas would percolate to the top of the bottle). There was a third hose fitting, but it was not connected to anything; I presume this is to allow coolant to spray all over the engine should the pressure exceed 16PSI and the cap relieves it. This issue is related to a problem a lot of newer cars have: the radiators may have a metal--usually aluminum--core, many have plastic tanks on either or both sides or the top. Given the expansion/contraction cycles, and the fact most are manufactured in two parts and somehow bonded together almost all of them will crack and leak eventually. I've replaced a couple before. I don't think you'd want a conventional overflow tank without a vent, when the engine cools the coolant is sucked back into the radiator as gasses cool, and without a vent in the tank you'd get a vacuum in the tank preventing the flow back into the radiator.

aero3113 said:

Easy enough to drill a hole in the cap. Here's a pic of the cap I’m going to use, looks correct. How would a non vented tank even work?

View attachment 56376

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Your pic looks like a brand-new cap. Even has the needed upper gasket in place. If it's a 7 lb of the appropriate reach, why not use it on your rad and disable an old cap for the overflow tank. This is a stock 7 pounder I cut the lower rubber and valve off. Looks correct on the MG tank.

If drilling, you only would have to drill through the lower part as the MG neck halfway up has a vent pipe to atmosphere (yellow circle).

Yes Steve, that cap is the one I’m going to use on the radiator. I was talking about drilling the cap on the overflow tank. Here is a pic of the tank and cap I’m using. Not sure if I should drill the cap or drill a hole on top of the tank and use a barbed fitting and hose?

Nice tank. Billet!

Does it have a pickup in the bottom? What you want is a connection in the bottom for the hose from the radiator neck. That and a vent in the top.

The MG tank I show has a pickup near the neck, but internally that tube reaches down to the bottom. I always start with a pint or so of coolant in the bottom of the tank.

Thanks! Yes, it has a hose from the bottom of the tank to the radiator neck. I just have to put a vent in.

Just "thinking out loud" here - was looking at the billet recirculating tanks online (Jegs, Summit). If it has 2 pipes coming out the bottom, does one of the pipes feed from the bottom of the tank and the other from the top? If so, attach your radiator overflow hose to the bottom-feeder excitement and leave the top-feeder open for your vent.

No, this tank only has one hose hook up on the bottom.

It must have a vent, then. If it seals tightly with the cap down, what's the point, it won't fill with overflow.

Hi all,

Since my overflow tank is out of obvious sight, I simply used a vented radiator cap with the overflow tube directing the overflow through a 1/2" hole drilled in the cap of a plastic container mounted to the passenger side of the radiator. When pressure builds, the vented cap allows excess coolant to flow through the tube and into the plastic container. Upon cooling, the vacuum of the tank pulls the fluid back into the radiator.

The overflow tank is not pressurized and filled with coolant to 1/3 capacity for the tube to be maintained below its surface so as not to pull air into the system on retrievel. I appreciate it is not as elegant or visually appealing but it is relatively out of sight, works rather well, and the cost of the system was the cost of the vented radiator cap. Although the cap is a non-common item, they are available for both LBCs and older American classics through NAPA (Balkamp 703-1411) and a number of other suppliers.

I appreciate you already have your container and a pressurized overflow will be needed if you intend, or have, mounted the unit any distance from the radiator. However, the container you have purchased could probably be used if mounted to the side of the radiator and rigged as I have described.



Keep Cool,
Ray(64BJ8P1)

I just installed a vent, should be good to go now!

Ford bought out Jaguar round about 1995 the 3.9L V8 is Ford designed and produced can't blame the British for that one!

blue62 said:

Ford bought out Jaguar round about 1995 the 3.9L V8 is Ford designed and produced can't blame the British for that one!

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https://www.wardsauto.com/news-analysis/ford-motor-co

It's a good engine; I rather enjoy driving the car. They got a bad rep from defective coil-on-plugs, which could cause the engine to shut down, and an SB on its goofy hydraulic--it uses the same type of pump as the power steering--engine fan but once we got those sorted out it's been reliable (except for the aforementioned problem with the 'degas' bottle). The automatic trans even performs admirably. My mom loves the car, is comfortable with it and at 84 probably couldn't adapt to anything new.

I started up the car today and took it for a drive. Everything worked as it should with the overflow tank!