The mystery of the disappearing olive...

Noticed a water leak by the mains stopcock which cuts off the feed to the header tank while allowing the rest of the mains system to still be used. It's in a dry (or was) cupboard. Turned out to be leaking from the stopcock itself, and at quite a rate of knots. Rather than try and fix it in situ, I decided to change it for a new one I had lying around. On removing it, the cause of the leak was obvious - no olive in the input side. So where has it gone - some kind of electrolytic action? There's no way it's been there for many years with no leaks without one being fitted in the first place...
The old stopcock was near welded onto the input pipe, although the compression nut came off easily enough. I had to use a fair bit of force to free it, but both the pipe and tap body seemed corrosion free where they met. Just no olive.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Dave Plowman wrote:

If it wasn't you posting this Dave I'd think it was a troll...
What was there where the olive should've been? Anything? A dent in the pipe? A ring of corrosion?
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I'd tend to think the same thing - but there's no way the olive could have come off while I was dismantling it - I noticed it was missing as soon as I slackened the nut, before removing the (stuck) stopcock.

There was a slight change in the colour of the pipe where it had once been. But no actual corrosion on the pipe or part of the stop cock body where it mates to it.
I've had a slightly similar thing with a near twenty year old central heating pump - the threads between the gate valves and the cast iron pump body were near corroded away, and the nuts had to be cut off. I suppose that's more to be expected with a mixture of cast iron and brass, though.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Dave Plowman wrote:

Hmmm...
Thing is, if the olive was copper then you'd expect whatever corroded it to have corroded the pipe. If it was brass, you'd expect the fitting to have corroded.
Here's a thought - were there ever such things as aluminium olives? Maybe during the great copper shortage?
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It's not that old. I've a feeling I installed it when I had the attic conversion done - the header tanks all had to be moved and this was a way of having water in the kitchen and outside loo while that was happening. So late '80s.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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A metric pipe into an imperial fitting might be tight, and I could imagine the olive wouldn't fit in the pipe.
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They were both metric.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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wrote:

And the olive isn't stuck to the end of the stopcock ? Weird !!! Maybe a mouse is using it as a hoola hoop now.
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Heh heh - a virtual hoola hoop then, as there wasn't the slightest sign of it. It can't have simply fallen out when I was dismantling it either. There's no way it would have sealed without one on a mains pressure feed?
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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On Thu, 22 Jan 2004 00:28:10 +0000 (GMT), Dave Plowman

If you really tightened the nut down and there was some sealant in there?
I have seen that on a low pressure system and the joint was fully continent for years.
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No sealant, and not even my later trick of using PTFE round the olive - it would have showed on the upper one.

Yes. I'm beginning to think this must have been the case with mine - I simply left it out, and the hard water sealed the joint.
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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