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Discuss Repairing a damaged back box thread. in the DIY Electrical Forum area at ElectriciansForums.net

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It looks like the upstairs in the house has had some electrical work done in the past and I've been left with a couple of blanked off back boxes.

On one of them, the cover plate is loose and looks a little unsightly. While attempting to tighten the screws up, I found the threads on each of the bushing are buggered....

I am going to try to tidy the thread up by retapping with either a M3.5 or M4 tap.

If retapping with the M3.5 does not work, will I need to re drill/clean the holes with say a 3.5mm or 3.6mm drill before trying to run the M4 tap through it or should the M4 tap clean it up ok?

Thanks in advance,



Electrician's Arms
or replace the back box?

Gavin John Hyde

Electrician's Arms
Backbox saver has saved me a lot of time previously. The small cost outlay is saved in not having to replace a backbox.
With some you can insert new screw plates that hook inside the back box.
Another trick is put a blank plate on front and insert a long thin rod with a bit of paint on back and push until you reach the rear of box. Then drill this mark. Insert a plug and you can then use the long uncut machine screws to fasten socket. Switch etc
I don't really want to remove the back box as it might damage the decorating although I might remove and fill it at a later date.

I have a tool similar to the CK one for doing this sort of thing, but the shaft is double sided, one side M3.5 and one side M4...As I recall M4 threads are used on the screws and bushings on metal conduit?

Anyway, I'll give it a go running the 3.5mm through first and then if that does not work, I'll try the M4 tap....

When I managed to get the existing screws out, it looked like somebody had tried to use screws with the wrong thread...they were close but not close enough.... the screws were also a fraction too long and were bottoming out against the back of the patress...


Admin and gender confused
Staff member
You could also use clinchnuts (sometimes called rivnuts) but I've only seen them in M4, not M3.5. I'd go with the squishing the existing hole and retap to 3.5 advice. You could also put a blob of 2-pack epoxy glue over and into the hole, give it time to set then carefully drill the epoxy and retap to 3.5mm


In echoed steps I walk across an empty dream.
Staff member
Used a Back Box Saver the other day bit fiddly but did the job.

Des 56

Forum Mentor
You can play around re tapping and good luck to you

90% of the time,if you follow the solution given by camerabloke it works just fine


Electrician's Arms
Backbox saver has saved me a lot of time previously. The small cost outlay is saved in not having to replace a backbox.
I've just looked up the price of them and spat my coffee out all over my keyboard, they aint cheap...
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