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MG MGB Technical - Coolant Loss

Hey Folks, the '73 B uses a couple of oounces of coolant every 200 miles or so. Should I re-torque the head or pour in some stop leak? Thanks, Tom
Thomas McNamara

Neither, that sounds quite normal to me. Remember the MGB does not have a sealed cooling system like more modern cars so some coolant loss is to be expected.

Head gasket failure on the B is quite rare and if it does happen it can be detected by water getting into the engine oil - look for 'mayonnaise' on the filler cap and a distinct smell of oil in the radiator. On a cool engine at tickover you will likely see bubbles being blown up through the radiator if the gasket's leaking/blown (for obvious reasons don't try removing the radiator cap with the engine hot). Any leaks in the system should be easy enough to see (look for reddish/brown staining aside from obvious leaks) and if found are best sorted by repairing/replacing the faulty component rather than by using goo.

How full are you filling it? If it's filled too high the excess coolant will go out the overflow and usually stop overflowing when it reaches a lower level. Usually around 1 1/2" below the top is where it settles.

If that isn't the problem I would get a coolant pressure tester to find where it's leaking. You can rent or borrow a tester from some auto parts stores.

I wouldn't add a stop leak product, they tend to plug some areas you don't want plugged. You can also re-torque the head bolts. Sometimes it helps. Just don't over-torque them, they do sometimes break unless you have ARP bolts/studs.

Clifton Gordon

Tom - See the article Expansion Tanks, Coolant Recovery Systems and How the Cooling System Works in the Other Tech Articles section o my web site at: for an explanation of normal coolant loose from the cooling systems in older cars. Cheers - Dave
David DuBois

A cooling system pressure tester will tell you if and where your problem lies. It could be as simple as the fact that you're overfilling the radiator and it is seeking its own level. The most likely cause is that your radiator cap is no longer sealing the cooling system and is allowing coolant to escape at a lower pressure than the cap is rated at. RAY
rjm RAY

A weak pressure cap usually reveals itself on switch-off, when heat soak raises the temperature of the coolant above its normal running level. You get a gurgling and coolant splashing on the ground from the overflow pipe a few seconds after switching off a fully up to temperature engine. A 73 should have the radiator cap directly over the core tubes so it is easy to see whether you are overfilling, unlike the earlier angled filler tube type. As long as you have coolant about an inch above the tubes that is enough. In either case you can check to see if you are losing coolant via the overflow by putting the overflow tube in a catch bottle. If you see coolant in there immediately after a run it is either pushing a little bit out constantly, which could indicate head gasket problems, or overfilling. Incidentally you would only get oil in the coolant if the area of the gasket around the oil supply to the head is leaking, compression leaks would more likely *not* include oil. You can also get air into the cooling system from a leaky pump seal, or slack bottom hose clamps. If you get the same amount in there regardless of the length of the run it is more likely overfilling than gasket, pump or hose clamps. Note that if you leave the engine to cool down it *should*, if you have the correct cap, suck it back in again, but that is no argument to fit a catch bottle permanently, it isn't needed. If nothing is going in the catch bottle then a loose hose clamp elsewhere is the most likely cause, and surely the first thing to look for. Be aware that even if a clamp is tight it doesn't mean it is sealing the hose properly, as old clamps can seize and as the rubber under it deforms the seal is gradually reduced in effectiveness. See if you can unscrew and retighten them. That's why continuous-pressure spring clamps are better than screw type.

Incidentally I've long suspected I may have a source of air being pumped into the cooling system on my 73, but it's been doing it for 20 years and I rarely have to topup. The symptom is that on hot switch off I can hear air coming out of the overflow pipe. Then as the engine cools it slows and stops, then a bit later starts pulling air back into the system. In theory the system should heat up, pressurise but stay well below the level of the cap, then cool down again, all without anything passing the cap in either direction. As mine seems to be blowing air all the time it is running it must be coming from somewhere, but like I say it doesn't cause any problems so I have left it alone. Note that if this happens on the later system with the remote pressurised reservoir air can continually push coolant into the reservoir and out of the overflow, emptying the engine and radiator. BT, DT as well on my V8 when the pump (almost certainly as I couldn't find anything else wrong) was pulling in air.
Paul Hunt 2010

I have a coolant recovery system on my 1970 GT. See photo. It is very simple but effective. I have no coolant loss issues and the coolant level rises and falls in the plastic bottle with engine temperature.

Prior to having that setup, I was never sure if there was any coolant loss.

J Tait

Hey J Tait, I would like to duplicate your effort. Can you tell me where the container came from and what cap you are using?

BTW, I keep the coolant topped up to about 1/2 inch above the tubes and find the tubes just exposed when checking after a couple of hundered miles.

I wil be putting some kind of a catch bottle in at my first opportunity. Thanks for the input.

Regards, Tom
Thomas McNamara

Hi Tom,

Like JT I too fitted an overflow catch bottle to my 66 B.
I used one of those drink bottles with carrier, for bikes. Remove the centre holding bolt from the radiator shroud and drill two holes to take the bottle holder. You will need a sealed cap, otherwise coolant will still escape from under the cap.

See photos.


Herb Adler

Pic 2


Herb Adler

pic 3


Herb Adler


Herb's solution is the one I used on an MGB I had in the 90s except I bought a generic overflow bottle from a car accessories shop.

The bottle and its cap (as shown in the photo above) for mine is off some Brit built car - found it at a swap meet some time ago. That cap has a flat top with a simple flat rubber washer to seal off the filler area to reduce evaporation loss and keep wild animals out.

The radiator cap is a coolant recovery cap and has a sort of big rubber bung in it to seal off the filler tube/throat area. The rad cap is only 7 lb (should really be 15 lb for that model car) but it works beautifully.

J Tait

Later model "B"'s came with a coolant recovery system installed.

The authentically "duplicated" container (almost identical to J Tait's photo)is available from Moss.

The system works a treat.

JR Ross

"Later model "B"'s came with a coolant recovery system installed."

Not really, that was the remote pressurised header tank/reservoir with the pressure cap, and a sealed filler plug in the radiator or thermostat housing. A coolant recovery system is an unpressurised bottle with the overflow pipe dangling in it and reaching the bottom, the pressure cap is still on the radiator, and the recovery bottle is open to atmosphere. The radiator pressure cap needs to be the type with a low-pressure inlet valve as well as the main pressure seal or it will not be able to suck coolant back in. It can be added to either the early or later cooling systems, but shouldn't be needed in either case. If you do find coolant going into the recovery bottle when hot, sucked back when cooling, then either you are overfilling the cooling system or there is some other problem.
Paul Hunt 2010

This thread was discussed between 20/06/2010 and 23/06/2010

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