electrical socket screw problem

[BufferParticipant @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

[Bill PhinnParticipant @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.

[WingoParticipant @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

[BufferParticipant @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.

 

[BufferParticipant @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

[Buffer]

On 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.

[BufferParticipant @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

 

[Michael GilliganParticipant @michaelgilligan61133]

From what I have read here, the best option must surely be to purchase a 3.5mm tap.

MichaelG.

[DC31kParticipant @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.

[Wink HackmanParticipant @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

[File HandleParticipant @filehandle]

I have often used a treading tool similar to the above. Has always got me out of trouble.

[DC31k]

On DC31k Said:

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 […]

Not forgetting to note the thread forms … the angle is the devilish detail in this situation.

MichaelG.

[BufferParticipant @buffer]

Wink

That’s a great bit of advice thanks.  I would never have thought of looking for a tool like that.  Perfect if my 4ba screw doesn’t fit and there’s a screwfix with it in stock just 4 miles away if I’m stuck.

 

 

[VicParticipant @vic]

You can buy M3.5 taps with a screwdriver handle at Electrical Suppliers, I have one. Handy to have in my electrics box. I suspect most electricians have one?

 

[Wink HackmanParticipant @winkhackman25989]

I would forget the 4BA screws and stick with standard M3.5.  You can get them in various lengths at any electrical wholesaler.

[Bill PhinnParticipant @billphinn90025]

Several people here have tried to bring clarity to the OP’s problem.

Can I try to clarify things further?

We know we’re dealing with the metal tab variety of female threads.

We don’t know whether they are M3.5 or 4BA.

We know an M3.5 screw won’t go in but we don’t know whether this is because it is an M3.5 female thread and it is damaged or because it is a 4BA thread, which may itself be damaged.

Recutting a 4BA thread to M3.5 isn’t really doable, for reasons DC31K tried to point out. Hence, if it is 4BA, advising you to get an M3.5 rethreading driver isn’t very helpful advice. Even if it is M3.5 all along there is no guarantee, in my experience, that a very short, already compromised M3.5 thread in back boxes or pattresses can be made good again with just an M3.5 rethreading tool. Same applies to 4BA, i.e. if the thread is 4BA you have chance of rethreading it with a 4BA tap,  but it may be too reamed out and messed with for this.

This is really why I suggested the T nut repair earlier. You can easily make one yourself in 4BA (or M3.5 if you had a tap) and end up with a secure fix that won’t need revisiting. Bear in mind that when plugs are removed from sockets the face plates can be subjected to quite a strong pull and any weakness in the tiddly faceplate fasteners is likely to be found out sooner or later.

[JourneymanParticipant @journeyman]

There are various types of back box repair clips available. Use pliers to make the lug flush with the side and push the repair clip in place.

Short video of repair

John

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