creative solution required...

[Robbox]

Our house has a couple of light switch boxes where the adjustable threaded lug has fallen out, it's impossible to get it back in (seriously bad design!) The boxes are set deeper than usual and are the shallowest type, to get them out and replace is a big mess.
Had an idea to drill small hole in the back of box and thread it with a rethreading tool I saw at Screwfix. They do two, M3.5 and M4. I looked at faceplate screws and they all seem to be M3.5.
Is there any value in going oversize with M4?
Secondly, what drill size do I need to give the tool a chance to work?

 

[conny]

I would try scraping a blade around the exterior of the box first, then remove any fixing screws. With the aid of 2 small right angled levers try to jiggle the box forward out of the hole. Replace with a suitable alternative. If you want to go for a re-thread 3.0mm should be ok for 3.5mm thread but make sure not to overtighten the screws. Bear in mind you will need to drill past the box enough to allow the tap, (re-threading tool), to complete the thread.

 

[rsgaz]

Is there any value in going oversize with M4?

Not really, useful for re-threading the fixed lug if it won't hold and you don't want to replace the box, but stick to M3.5 so you can use normal screws.

what drill size

Officially, the pilot for an M3.5 thread is 2.9mm, but if you have a 3.0mm, just use that.

 

[sparkwright]

An extension stud may be useful to fit to the box first, then fit the screw.

May be easier than fitting a long screw, may not be.

 

[333rocky333]

When the lug falls out i have often found you can usually bend the two tabs out put the lug back in then close the tabs up