Xk120 Expansion Tank

Greetings, I’m sure this is another dead horse topic but thought I’d drag it out of the barn again. Has anyone seen any good solutions for adding a radiator expansion tank on a 120? Thanks! Mark S676095

It’ll take 20 characters, but…why?

To retain the coolant and keep it from spilling out when it expands with heat, I have rigged up an expansion tank from Summit Racing.

Here’s one that’s out of sight.
A bit awkward to access.Susp%20-1%20LH%20Front%20Suspension%20and%20Recover%20Bottle|666x500

Are you filling the top tank to the neck when the engine is cold? If so that could be part of the problem. The tank needs to have an air space for the coolant to expand into.

easily done…but…it is normal for a full to the neck radiator to expel coolant…down to what should be the actual normal operating level…this is in a Jaguar Factory Service Bulletin. Note the the radiator cap is low pressure–3-4 lb only. Nick

I have expansion tanks in all my classics, mainly because they all run with Evans coolant which seems to expand a fair bit. Technically it makes no odds, because at running temperature the system will be full whether or not there is an expansion tank, but I’d rather collect the overflow than have it drip onto the road. With the Evans, I use zero pressure caps.
The XK doesn’t even have an engine yet so this is a way off, but I see no reason for running it anyway different from the other cars. I’ve used Evans for 16-17 years now and it’s been great, especially in marginal systems.


The attached shows my solution to the expansion tank situation on my XK120. Unfortunately, it is a bit difficult to see the tank itself, however, it is the white thing with the black cap visible just aft of the rear carburettor and affixed to the front wing side panel.


Hi Chris, that is very clean looking. I think I would try to match something like that.
Thank you everyone for all your responses. All the input from this group is vital and so and helpful.


Hello Dick, That looks good. I like the fact that it’s out-of-the-way.
Thanks to all of you for your responses. This is a great forum.

This is the one I put on mine. The inlet tube is 1/4" and the radiator tube is 5/16" so I had to put in an adapter. We should mention that if your flow is into the bottom, it will siphon back into the radiator when it cools off. There is a pressure relief tube inlet at the top and outlet on the bottom which will drain out onto the ground if you are way overfull. So be sure you connect your radiator outlet tube to the right one on the expansion tank.

Can people explain why they use an expansion or overflow tank in this context?

When the engine is warm the cooling system can be run full, but whatever is in an overflow tank will not contribute to cooling. In this case it seems that fluid in an overflow tank means the radiator system was too full at the start.

When the engine is warm the cooling system can be run full, whatever is in an expansion tank may have the ability to be recirculated and thereby have greater cooling effect?

When the engine is warm the cooling system can be run full, whatever goes into an overflow or expansion tank does not go on the road or floor. This is environmentally beneficial in several ways. For example, race cars may have tanks because puking on the racing line is frowned upon. And ethylene glycol is bad for wildlife.

What are the benefits people have found by adding the tanks? For my Jag, learning what coolant level did not overflow showed me a good coolant level without adding any tanks. Never giving any thought to overflow or expansion tanks since, what other thoughts are there to learn from?

1 Like

On an XK120, and possibly other models, the thermostat is located at the top of the radiator tank, and the bypass directly below it, so there is concern about not ever running with the level too low, and the water pump failing to circulate the coolant. The expansion tank ensures that the level is always up above the thermostat and the coolant always circulating.
The thermal expansion coefficient of ethylene glycol is .00057 / deg C, and water is .000214, so an XK120 system of 29.8 pints US in 50 percent mixture going from 0 C to 100 C would expand about 1.17 US pints or 18.7 ounces.
The Mark V and other pushrod Jags, being non-pressurized systems, would not benefit from an expansion tank, as it would not draw back into the engine on cooling off, unless you sealed the radiator cap. Therefore the method of observing of a normal coolant level would apply.

Hello Dick,

With your expansion tank being mounted in the wheel well I assume it is lower than the radiator head tank. Is the expansion tank supply hose connected to the radiator overflow line that discharges beside the radiator foot tank or did you connect the hose by the radiator neck?

Regards, Geoff

If you are using a double-sealed rad cap, and the small hose from between the seals to the expansion tank enters the tank at the base, it doesn’t matter what height the tank is with respect to any other part of the system.

1 Like

Right, the overflow tube that discharges beside the radiator foot tank is the other end of the tube that connects at the top neck.
The lower rubber seal on the pressure cap opens each way as the pressure differential goes one way or the other.

Correct. The upper rubber seal is necessary to maintain the vacuum to suck coolant back into the radiator, through the lower rubber seal.

Refilling the coolant level to the exact sweet spot where the radiator has the right amount to keep cool, but not so much as to spit out the overflow on a particularly hot day is not always straightforward, particularly if serviced by a mechanic who is not familiar with the right level below the filler neck.

For me I think an expansion tank is preferable to the outlet pipe venting under the car if only to avoid the mess from excess coolant blowing back and drying against the nice clean (new) black chassis paint. Maybe after a few years and some puddles I won’t be so precious about cleanliness under the guards :slight_smile:

We do a lot of European touring in our classics, sometimes in areas with much higher ambient temperatures than the UK. Unavoidably the car will run hotter in these temperatures, especially if prolonged queueing is involved - we once spent 2.5 hours in a bank holiday traffic queue to enter Lisbon, idling in an ambient temp of 42°C, which was… interesting. I’ve done no tests but I suspect the coolant will expand more in these higher temps - as far as can be told from the gauge (it was a '68 Mustang), the engine was idling happily at 110° - so I think the expansion tank serves a purpose. Had the system been completely open, more coolant than usual would have been lost, meaning a reduced level when the temperatures returned to normal. We still had three weeks to go, including some high altitude sub-zero weather, hence my concern - but I could well be wrong. Whatever, it works for me!

1 Like

Cheap and simple : Range Rover Classic expansion tank.

1 Like