Electrical box problem

My son tried to replace the cooker switch/socket with posh one. Problem is he overlooked the fact that the new posh version has a much thinner face plate, the screws hit the back of the metal box in the wall, he carried on tightening and has stripped the threads in the little metal tabs in the box.
Can anyone offer a suggestion how to overcome the problem without replacing the metal box? We don't want to have to dig the box out as it is buried behind tiles that we do not have replacements for.
Mike
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Muddymike wrote:

Threads are usually 3.5mm. You can buy a 4mm re threader http://www.tlc-direct.co.uk/Products/BHM4.html and use some 4mm screws.
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Dave - The Medway Handyman
www.medwayhandyman.co.uk
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Thanks for the suggestions guys. The tapping out to 4 mm would not work as the holes in the posh face plate are too small and it came with matching screws my son prefers to retain. I have come up with a cunning plan!
I have cut the tabs and some of the surrounding metal from a spare box and bent them into a "U" shape which will clip over the damaged tabs with the threaded hole directly behind the damaged holes. I plan to fix them on with Araldite so that they hold in place whilst getting the screws started. Once started they should then hold themselves in place and act like a captive nut behind the damaged tabs.
Wish me luck.
Mike
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Will this hold someone yanking on the socket part?
The ideal solution (indeed the ideal design!) is to have a lug with a threaded rivet-insert. Unfortunately I never recall seeing M3.5 and only M3 or M4 which is obviously not what we want here (M4 is a much bigger head than many wiring accessories as you have found - although surface button-head torx might look ok :-)
Have you considered just replacing the box?
1 - Tiles fixed by PVA-based adhesive - Stanley knife to cut around the box - Thin screwdriver to fold in the long side a little - Needle nose pliers to further fold the long side - Fold in further - Stanley knife to remove bottom tile's grout Then if tiles cut very close to the box... - Widest wallpaper stripper to pop tile off - Tilt the box out - Replace box
2 - Tiles fixed by miserable epoxy/cement - Stanley knife to cut around the box - Tin scewdriver to break seal all round - Drill box across the middle - Fold in sides further - Flex metal under box collapses (easy) - Replace box
You remind me I've a cooker backbox to replace which is wobbling at one end. The screw has stripped the thread, this is common on tiles because of the glazed surface and people overtightening. A dab of sealant works wonders.
Examining closely there are no more than 1.5 threads in the lug, a good box only "spikes" a hole in the lug then taps the "burst edges" so as to obtain a long threaded hole. There are some real rubbish backboxes out there re mounting tabs, front edge literally rust encrusted within 5wks of wet plaster, earth screws spinning, out of shape preventing some tight SFCU designs fitting between the lugs.
Ironic that plasterboard boxes often get decent threaded brass inserts, metal backboxes get the most pathetic "knock a nail thro and create an oversize tapped hole in 1mm sheet. Ah, MK Grid Flush backboxes get a proper tapped tab - however just 3 cable entry holes top/bottom on 2G rather than 4 on conventional 2G... that's got to be a French design at work to spoil your day, it really has :-)
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wrote:

As my improvised captive nuts are behind the original tab it will be as strong as the original

I agree but like you am not aware of such a thing

That was my first thought. Sadly the tiles are old, well fixed and protruding over the edge of the box with no replacement tiles available. Another box I dug out of the wall in the house was simply set in some kind of mortar and it was one hell of a job to get out even with only one 2.5 cable coming in from above. This one has 2 x10mm cables (coming in the top and back out the bottom) so removing the box would have involved a lot of work and re-tiling.

Definitely not PVA

lump hammer to cut.
Anyway, you will be pleased to hear its done now (my son certainly is) The only modification to my plan above was to stick the "U" shapes in place with Epoxy resin adhesive just to keep them in place whilst starting the screws.
Mike
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wrote: <snip>

I think these are what are being referred to. I used some once to replace stripped threads on a PC case.
http://tinyurl.com/ykhf4jg
<snip>
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Tinkerer




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wrote:

Yes, but I've not seen M3.5 unfortunately.
The OP has what sounds like a cemented-in box and cement-bonded tiles. For the record; HSS drill the back and sides with overlapping holes until the box collapses. Then use a mini diamond core bit in a cordless drill to grind out the material in depth and sideways so a new box will fit. The core drills are sold on Ebay from Hong Kong - about 10 for a 20mm sintered diamond core drill, it has long tabs on the front. They can be used to create backbox holes when you don't have an SDS or don't want the noise or risk of damage to the other side. Sintered rather than surface encrusted with diamonds means they will do about 500 3/4" deep holes in brick before worn down.
Just remember 6mm twin and earth if close to the CU has a low enough current to cause a high fault current - so it can be a big bang (and blow the main BS1361 fuse). So if a cable does get damaged, replace it - which you can do under SI2006.
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