Radiator recovery tank

Has anyone successfully fitted a recovery tank to the radiator overflow pipe on an S type? Are there any dos and don’ts? I would like to fit it in the front left wing near the vacuum tank.

Yes, Decades ago I fitted one. Used an MGB overflow tank mounted under the LH front fender to conserve space in the engine bay. It is important to use a blanking cap on the radiator and a pressure cap on the overflow tank. That way a slight vacuum created when the engine/radiator cools down from hot it will suck the overflow back to the radiator, thus keeping it full for the next run.

Thanks John, a simple solution. I’m familiar with this equipment from when I owned a wonderful Austin 1800, which used the same system.

I’m planning to do this type of system on my Mark 1, do you think the 4 lb pressure cap on my radiator would prevent the return flow once the engine in shut off ?
Thanks for your thoughts !

You need to move the pressure cap to the reservoir and put a blanking cap on the radiator for this system to work., otherwise expelled coolant cannot return to the radiator top tank.

I did this and did not move the pressure cap. I kept the pressure cap on the radiator and then ran the overflow hose to the bottom of the overflow bottom.

It fills halfway up when hot and sucks the liquid back in as it cools. Have not had any problems with this for years and I can keep the engine totally full of coolant by just adding distilled water to the overflow bottle every now and then.


I’m not sure how returning coolant can get through a pressure cap. I am only going by how this type system is set up on various BMC cars that I own.

John, take a look at this radiator cap. The metal disc in the center is a lightly spring-loaded poppet valve that allows air or liquid to return to the radiator when the radiator pressure is below atmospheric:

It opens like this:

That would normally allow air to be drawn back into the radiator as it cooled, but as you can see in the first picture the underside of the top of the cap has a rubber ring that seals against the top of the radiator neck, so outside air is excluded from the radiator. This cap allows the radiator to “breathe” only through the overflow pipe. If the pipe went to a reservoir it would send hot coolant to the reservoir and then draw it back when the engine cooled, just as TheoS says.

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Theo and Mike’s description is exactly how the tank I added to my XK120 works. I got it from Speedway Motors. They service the hot rod community.
overflow tank

But John’s description is how the tank on my XJ12 works.
It’s under the air cleaner.

XJ12 header tank
The pressure release cap is on the tank.
There is also a non-pressure-releasing cap on the engine.

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You are absolutely correct, Mike, I went and checked a cap on one of my Morris Minors. I wonder why BMC used a flat cap on the radiator on cars like the MG1100 and Austin America which had recovery tanks from the factory…

The old Coke bottle trick looks like a requirement by the show organisers to ensure there is no contamination from unexpected coolant loss. It certainly leaves a colourful stain so it’s not as if you can hide it. Probably a grass killer too. There are the two systems, or approaches, by the manufacturers that seems to have started in the '60s, albeit not universally.

The fully pressurised system means the overflow tank and hose are also pressurised. The radiator has the plain cap and the overflow tank has the pressure cap. The tank needs to be of the right material to take the cyclic temperature and pressure. I’m not sure of the life span for a plastic tank but the Leyland system I’m familiar with are brass.

The other system is the modern type with the pressure cap on the radiator and the overflow tank at atmospheric. It is important that the seals for the pressure and vacuum valves in the cap are sealing properly and a replacement every few years is a good investment.

The most serious downside to spilled coolant that I have discovered is that if your dog or cat drinks it (and it is sweet!), they’re as good as dead. I have installed inexpensive, white plastic overflow tanks on a Series Three E-Type and a Mark IX, and they work great. Here’s a nice article about how it all works:


I have that speedway motors unit on the GTO (aka “The Goat”) and it works a treat!

I converted years ago to rack and pinion with the American style reservoir built into the pump. I converted the fender wall original factory steering pump reservoir to an overflow tank for the radiator. Had to get a hose fitted to do the required match up on the overflow. I continue to use the cap which was on the Jaguar steering reservoir. I rarely add fluid, and if so I add it to the reservoir overflow on the inner fender wall. As is offten the case many modifications were done years ago and the details have become foggy.

I need to re wrap the pressure line from the steering box to the pump. Hard to keep everything tidy these days.

As we have seen with this discussion, either system will work.
Possibly BMC continued to think that a metal tank, made to take the same pressure as the radiator header tank, was still a good idea. But later the blow molded plastic tank idea, as found on all modern cars, while not needing to take any pressure, was also found to be lower cost.

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