electrical socket screw problem

This topic has 9 replies, 6 voices, and was last updated 2 September 2026 at 17:04 by Wink Hackman.

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2 September 2026 at 13:23 #863285
Buffer
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@buffer
Hi

I have just taken the screws out of an old looking MK twin electrical socket and now I cant get them back in. The ends of the screws are all worn away and wont pull up tight.

It seems as though the old back box may have 4ba threads and someone has forced m3.5×0.6 screws in which is why they are chewed up.

I have tried new 3.5mm screws and they also get messed up and instantly tight so no hope there so my question is. Do you think I could just tap the 4ba back box 3.5? Or is it just wishful thinking? Maybe I can tap 4ba onto a 3.5mm screw.

The other thing I’m thinking is to solder a piece of rod onto a shortened screw and thread the extension piece 4ba.

I don’t really want to change the box as it would involve knackering the plaster on the wall.

Any ideas or advice is greatly appreciated especially if you’ve also had this problem. Thanks
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2 September 2026 at 13:56 #863292
Bill Phinn
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@billphinn90025
What sort of female thread arrangement is inside the back box? Those thin bendy metal tabs or a brass insert inside the plastic moulding?

Assuming it’s the first, and you’ve got a supply of replacement M3.5 screws of the right length, you can use what’s called a back box saver. It’s a T nut basically.

For mangled examples of the second type, some trickery with JB Weld will be necessary, unless someone has got a better solution.
2 September 2026 at 14:37 #863300
Wingo
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@wingo
Hi, How long are are the 4BA screws required?

I do have some ‘in stock’ from time gone by…

I might have what you would need.

Michael
2 September 2026 at 14:38 #863301
Buffer
Participant
@buffer
Thanks Bill I had not seen a back box saver I might need that. Its a thin bent tab no plastic anywhere. I have just drilled into the end of a 4ba cap screw and silver soldered it to a turned down 3,5mm screw. I’m hoping this will work first but thanks again.
2 September 2026 at 14:40 #863303
Buffer
Participant
@buffer
Michael they are 45mm long countersunk slot head.

I’ll try this first and as long as the tabs are knackered it should work. I’ll try running a tap down the hole first.

Thanks
2 September 2026 at 15:44 #863318
Bill Phinn
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@billphinn90025
On 2 September 2026 at 14:38 Buffer Said:

Thanks Bill I had not seen a back box saver I might need that. Its a thin bent tab no plastic anywhere. I have just drilled into the end of a 4ba cap screw and silver soldered it to a turned down 3,5mm screw. I’m hoping this will work first but thanks again.

Have you established what pitch/standard the female thread is though, i.e, that it’s definitely 4ba? Whichever it is, it sounds like the female thread may be compromised, so some re-chasing or more drastic thread repair may be necessary before you can just thread in your Frankenstein screw.
2 September 2026 at 16:09 #863320
Buffer
Participant
@buffer
Bill I haven’t established it but a 3.5mm won’t go in. I did google it and the old standard is apparently 4ba so I thought i’d try that first. I don’t have a 3.5mm tap but I do have 4ba so I might just see if that will go in if the Frankenstein screw won’t. Annoying thing is the socket is a 1hr 10 min drive away so I’m trying to cove every option.

Thanks
2 September 2026 at 16:49 #863323
Michael Gilligan
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@michaelgilligan61133
From what I have read here, the best option must surely be to purchase a 3.5mm tap.

MichaelG.
2 September 2026 at 16:50 #863324
DC31k
Participant
@dc31k
Would it be useful to write down the diameter and pitch of both threads on a piece of paper?

Then look at the numbers and think about the implications, keeping in mind how the female thread is formed, how deep that thread is and the likely tolerances applied to it for the application.
2 September 2026 at 17:04 #863331
Wink Hackman
Participant
@winkhackman25989
Mangled backbox threads are a daily occurrence for sparkies. If they’re undersize then it’s not hard to fix. One of these should sort you out:

https://www.screwfix.com/p/c-k-re-threading-tool-m3-5-x-0-6mm/59312
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