Back box screws size problems

[happyhero]

Hi I wanted to change a light switch, from which the old screws had long gone, (held on by paint and luck) and as far as I know new back boxes have M3.5 screws and threads, but M3.5 screws wont go into these threads. I have many M3.5 socket screws and checked another few just in case, so I suspect the box must have some old imperial thread. What thread did they used to have then?

What is the best way to deal with this? electricans must come across this all the time so what is their most common cure? Would you get away with retapping it to M3.5, or would you do something else? Is there a special tap you buy to deal with this as most tap and die sets don't seem to have any M3.5 taps and dies as part of the set, they seem to jump from M3 to M4.

Got no worries about changing it as I have electrical background but just have not come across this problem hardly ever.

The box is fixed in solid, it is only the front that was in a state.

Any advice would be appreciated.

 

[RF Lighting]

Old backboxes have 4BA threads.

I have a few 4BA screws from old sockets, but the normal thing is to retap the thread.

You should be able to get an M3.5 tap from any electrical wholesaler or online here

 

[luminaire]

I did a Google check and picked up one for <£2

 

[briwire]

M3.5 taps are very easy to get hold of individually. Any engineers whole saler should have them, or RS.

I've even seen a special tool which is an M3.5 tap with a handle for running through blocked or damaged holes.

 

[firegazer]

You could try

http://www.modelfixings.co.uk/machine_screws.htm

They sell the old BA sizes.