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MG MGA - 1500 Losing water

..almost certainly through the radiator cap/overflow pipe. The cap does not seem faulty but neither does it seem to put much pressure on the radiator inlet.
Anyone been there/got the T-shirt can advise?
The result is the water seems to settle about 1Litre short of full.
Roger Walker


Most MGA's without a recovery system will deposit water through the overflow pipe if they have been overfilled. If it doesn't loose any more water it might have found its own level.

If you fill the radiator up to the neck its overfilled.

Also make sure you have the correct radiator cap which is longer than a lot of the aftermarket ones.
John Bray

Thanks for yours here. I wondered about the cap (GRC103). It doesn't seem to 'engage' as tightly as the cap on our V8. I do have another but that's the same length. Perhaps I should compare lengths with another owner.
Roger Walker

I had the same problem. Take a look at the tongue on the top of the radiator where the cap screws into. Mine just needed tapping down a touch, so it engaged tightly. Should be easy enough to see.
I am assuming of course you have the long reach cap.
Good luck, Graham
Graham V

Yes, that has certainly made a difference. It does at least, now feel engaged. The acid test will be on the road and I must take care (John's advice) not to overfill her.
Thanks for your advice.
Roger Walker


As you have a 1500, I seem to recall that the radiator neck is 1 inch deep, and cap is 1-1/8 inch deep from top seal to bottom seal. The cap will compress 1/8 inch when assembled to apply the preload on the pressure foot.

Caps are available as either 4lb or 7lb, if the cooling system is in good condition the 7lb cap is fine and will still allow water to be displaced if the system is overfilled.

Later model MGB's have 3/4 inch deep neck and 7/8" deep cap, also compressing 1/8 inch when assembled.

See link below by Peter Tipping which is on Barney Gaylord's excellent resource site on topping up the radiator.
John Bray

Thanks for the details of the length of the caps. I now have a 7lb cap to fit. Peter Tipping's article is useful too.
Roger Walker

This thread was discussed between 16/08/2015 and 20/08/2015

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