Immersion heater meachanical flange - how to remove / tighten with no access to rear?

I've got an immersion heater mechanical flange similar to that supplied by Toolstation.
<http://www.toolstation.com/shop/Heating/Central+Heating/Mechanical+Flange+2+14/d230/sd2708/p38614?>
The flange is slightly loose, the cork seal leaking, and I have next to no access to the rear, certainly not enough to get a spanner in. I am sure the flange was fitted before the tank was welded together and while there is a hex on the outer diameter of the inside of the fitting it is only just feelable with my fingers, the bore of the internal part is smooth and there appears to be no way to hold it in place while I apply a loosening action to the external hex.
Any bright ideas on how to slacken it off so I can fit a new washer?
Before anyone suggests it I have considered using an angle grinder but the flange is in a stainless steel tank and even though it is vented and operating at atmospheric pressure I am really reluctant to start hacking access holes in it.
P.S. Do they make 'internal' stillsons ? :)
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The Other Mike formulated on Monday :

I can only suggest the angle grinder solution :-)
Cut a hole in the tank (with the angle grinder) a suitable distance from the flange to enable you to get some stillsons on and hold it. Once the job is done, get a piece of stainless made up to bolt in place and a gasket to reseal it.
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Harry (M1BYT) (L)
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We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember The Other Mike
Not that I know, but I've just thought of a tool. A long pair of tongs with the gripping heads turned out and backwards, so that you can fit them through the hole and grip the outsides of the internal hex.
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On Tue, 13 Apr 2010 01:50:44 +0100, Grimly Curmudgeon

Possible mileage in that one, been also thinking about an expanding blind bearing remover but all the ones I have are too small, or a piece of metal, maybe a tube to reduce the heat needed, temporarily soldered into the internal diameter with a suitable means of holding it.
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On Mon, 12 Apr 2010 17:58:47 +0100, The Other Mike wrote:

I wouldn't suggest hacking a hole in the cylinder but if you cut two slots in diametrically opposite sides of the hex you can split it and remove it, and fit a new flange. (Standard procedure for removing bath wastes that have been welded on by decades-old boss white.) I'd use something a little more delicate than an angle grinder, though - at least for the final couple of millimetres, so you don't damage the cylinder itself. A multitool would be my choice (because I've got one! :-))
--
John Stumbles -- http://yaph.co.uk

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The only problem with that approach is that there is no way of getting a new internal ring into the tank. The biggest opening is where the immersion heater fits, the largest other opening is around 1.5" diameter.
Rather than destructively splitting the ring, maybe a couple of pieces of brass silver soldered onto the inner ring that I could hold with a piece of flat bar would work. Why no one thought to provide some means of holding this ring stationary from the outside I'll never know, maybe the designer somehow thought that it would never leak!
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We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember The Other Mike

As is moulded in to many smaller tank fittings. Annoyingly, some of them don't, either.

Lack of foresight and the saving of a few grams of brass.
Another idea, involving an angle grinder.
Cut a 6mm deep slot across the inner (this might entail also cutting the outer, but no real matter) so that you have a pair of detents that you can fit a flat bar into. The bar is cut just long enough to sit into the inner but not the outer. Grip it firmly or weld on a handle and undo the outer.
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On Tue, 13 Apr 2010 12:19:44 +0100, Grimly Curmudgeon

There is next to no thickness on the diameter of the inner ring to provide any useful support , it's basically the thread depth and 'a bit' , maybe 1/8 inch thick in total, which given the maximum diameter of the immersion element is no more than about an inch is yet another bit of brilliant design.
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We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember The Other Mike

Oh, it's a small one. Damn, I thought it was a 2" jobbie.
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On Tue, 13 Apr 2010 11:01:04 +0100, The Other Mike wrote:

How did they get it in in the first place?
Rather than trying to unscrew it I'd be inclined to try to fill the gap that the washer is now failing to do with some sort of thread (or wire) and adhesive/sealant mixture, so that the thread progressively pushes the surfaces apart (the way winding string or wool around your hand, even loosely, ends up uncomfortably tight), while the adhesive/sealant - er - adheres and seals. The question then becomes what materials to use?
--
John Stumbles -- http://yaph.co.uk

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By welding it together after the flange was fitted. Short of the aid of scotty and a beam me up machine it has to have been done like that. Foldable memory metal that looks and acts like brass hasn't yet been invented :)

I've smeared LS-X on it and that is ok for a temporary repair for a few days but it leaks again with any movement with heat and vibration. I suppose I could dig the cork seal out completely and then wind in some hemp and boss white but I'd prefer a permanent repair that I know will last or at least a modification to the inner ring so that it can be removed easily in the future.
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On Thu, 15 Apr 2010 10:35:35 +0100, The Other Mike

Finally got it removed.
I soft soldered a couple of 10mm lengths of 6mm diameter brass on the inner bore. I held the inner ring in place with a suitable tool (a jammed in copper and hide mallet!) and used a G clamp to rotate the outer part of the boss - my stillsons didn't fit, and the immersion sided hex is behind the circular flange the immersion element fits into. Just thankful the threads were clean and not full of compound so very little effort needed to undo.
On removal I discovered the inner ring had only two spanner flats and was thinned down to about 2mm thick on the tank side. There was an offset full width 3mm high slot cut across the stainless tank circular cutout and so the inner ring could be retrieved to the outside.
I suppose the tank might have been welded before the flange was fitted, how it was held is anyones guess, but it sure wasn't designed for easy maintenance.
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