Radiator overflow tank

Has anyone successfully fitted a recovery tank to the radiator overflow pipe of a Mk IV or V?

Hi Peter, no I have no experience with radiator overflow recovery for Mark V or IV. But since I also never had a need for such as I have not experienced overflow as an issue. What is your motive for seeking recovery, is it regulatory?

I put one on my XK120. It is from Speedway Motors, a specialist for the hot rodding hobby. Most hot rods have them now. Even rat rods will have a Coke bottle for overflow.
overflow tank
It’s nice, when the coolant is hot the excess goes in the tank, and when it cools off the excess is sucked back into the radiator.
However, it needs a sealed pressure cap in order to work right.
For the SS, Mark IV and V, you would need to have a good sealing ring on your cap and run without a mascot.

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Apart from the general expectation that nothing should drip from a car, I’m not aware that there are any specific regulations referencing coolant. Oily wet engines and transmissions attract a failure at the annual test, and I’m sure coolant would be classed as an environmental contaminant.
A tester once commented on the rusty water overflow stains on a different old car but didn’t take it further.

My enquiry was more of curiosity and whether anyone has considered adding this to do ‘the right thing’. For the early non-pressurised systems, they would only overflow if overfilled or boiling. The pressurised systems overflow down the pipe until the volume stabilises to suit the operating condition.

That’s interesting Rob. I’ll probably look into this for more information and possibilities. I’ve noticed that a small % of classics at show days are fitted with overflow additions, with attempts at positioning them discretely.

Hello Peter, thanks for your further thoughts. My Mark V is used in California, which has no vehicle inspections or tests on cars of this vintage. Having said that, I bring a cardboard to receive oil drips on trips where the droppings would trouble others. At home I use three oil pans under the car.

For the radiator overflow question, it does seem recovery versus retention as separate end games.

For retention, a bottle of some sort to collect anticipated overflow volume would be possible. My experience is basically no overflow observations on Mark V cars I have seen operating as per the factory intentions (excepting those which were too full when cold). I have seen overflow from overheating on cars where the thermostat bypass remains open after the car has reached operating temperature. I have seen cars where the thermostat has been removed without plugging the bypass line, these owners do not enjoy drives longer than a few kilometers.

For recovery, perhaps a little caution is worth including in any re-engineering steps pondered here. The Mark IV and Mark V engines operate at atmospheric pressure at the top of the radiator. I’m unsure what static pressure increase in the coolant system would be tolerated throughout if a pressure cap were to be considered. Perhaps only a good seal on the present cap would be enough? If the retention container has radiator overflow line to the container bottom, then fluid would cover the return path when overflow occurs. A pressure differential of about 5k newtons per square meter, or less than 1 psi would be needed to pull fluid back into the radiator when engine cooled.

One purpose of pressurized caps is to raise the boiling point of the coolant so an engine can run more efficiently at hotter internal temperatures. Since the engines in question here were designed for unpressurized cooling, I’ve never pursued a pressurized system.

Several times I’ve driven my MK IV on a tour for a couple of hours, then parked slightly nose-down in a row with other tour cars. Coolant immediately began flowing out the overflow. Not boiling. Not overfilled. Just a typical tomcat marking his territory.

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I’ve been thinking about this. On my '38 SS saloon, which I drive average once a week in good weather, the coolant level when cold is about 2 inches (5 cm) below the overflow tube. In other words the header tank is nearly full, maybe 3/4" to 1" from the top. I haven’t had to add any for a couple of years. If I did, when the coolant gets hot and expands, the excess will go out the tube and I would be back to where I was. The system finds its own best level.

On these saloon cars the bottom of the header tank is about 4 inches (10 cm) above the hottest place in the engine, the return manifold. I don’t know the SS1, 2, 90 and 100 all that well so it may be less with them. But anyway as long as the header tank is nearly full, there is no chance that the coolant level will fall below the return manifold.
If there is say 2" of coolant in the header tank, then all the coolant that can be cooled at any moment is in the radiator tubes.
That’s the best the system can do.

With the tank on my 120, it fills or returns coolant from the bottom, as the pressure cap releases pressure when hot or allows return flow when cold, and lets air in and out from a tube at the top, as modern cars have.

So I’m thinking we don’t really need a sealed system coolant recovery/return tank to maintain the level. We should fill and monitor the system, noting where it falls when cold, and not overfill from there.

Now if you wanted to have a tank to prevent loss on the road or at a show or inspection, you could rig one up with just one input hose. Like the Coke bottles on the rat rods. :grin:

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If you use a glass bottle, make sure it’s safely contained…:grin::wink:

Part of the rat rod culture seems to be to make their cars appear to be unsafe. :laughing:
green string


In truth, I would NEVER use glass…:blush:

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