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Leaking cylinder around immersion heater

Hi.
My hot water cylinder has a small leak. It's from the immersion heater where it screws into the side of the cylinder. I removed it, fitted a new o ring and put it back. It still leaked slightly worse if anything. So I bought a new heater the same make and model as the original and fitted that.
It still leaked but it took 2 days before I could feel any water. I checked it regularly and thought I'd fixed it but then I felt a bead of water. I managed to get a tiny bit more tightened on it and it stayed dry for a few hours but then started leaking again.
The problem is as can be seen on the photos I can't see exactly where the water is coming from as it's hidden by the nut on the heater. I'm beginning to wonder if the cylinder is the problem, as I'd have thought a new heater with a factory o ring would cure it.
When the heater was out I could see the stainless steel threaded boss looked in good shiny condition. Feeling all around it there were no burrs or damage. Because it's surrounded by plastic I can't see where it's welded to the cylinder. So can't be certain that this isn't the problem.
Is it worth putting maybe some kind of silicone type of sealant around the o ring or something on the threads? Is leaking like this a common problem with this type of stainless steel cylinders? The immersion heater isn't used. Any ideas much appreciated. Thanks.
 

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Given it's got an insulated case it's quite likely the leak is higher up or at the top and water is leaking down under the cover and coming out at the first hole in it.

Check all the threaded comnnections that are higher than the heater and not just directly above it.
 
Given it's got an insulated case it's quite likely the leak is higher up or at the top and water is leaking down under the cover and coming out at the first hole in it.

Check all the threaded comnnections that are higher than the heater and not just directly above it.
There's no leaks anywhere above it.
The water doesn't seem to be coming from where the heater meets the cylinder's threaded boss. The boss sticks out about half an inch so I can get my fingers behind It. It feels like it's at the back, around the grey plastic box where the boss meets the cylinder.
Also if I dry it no water appears immediately, it takes a short while. But if I get hold of the heater and apply a bit of upward force it starts to come out. I don't think that would happen if it was leaking around the O ring. So it's starting to look like it's the cylinder that is leaking. Where the boss is welded to the cylinder.
I think I'll have to drain it down again and try and remove the grey plastic box. Then I might be able to see if it's leaking from behind the box.
Thanks for the replies
 

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