How to fill in a tapped hole for a smaller screw?

[sjalloq]

Hi,

bit of a random question but I have just finished a house renovation and sods law, but the only metal back box I left in place and didn't replace seems to have had an M4 screw forced into the M3.5 thread.

I've just bought a couple of M4 screws to find that they don't fit through the mounting holes on the switch plate I've got. Ergh.

So taking out the back box isn't an issue as it's all been plastered and decorated. The tab is removable but it's an old 80's Appleby back box and I haven't seen a tab similar before. I was thinking I could just replace it if I found one matching. So can I solder on a bit of copper or something and re-tap? Or shall I just try to bent a bit of steel to fit?

Thanks, Shareef.

 

[MikeG.]

Bad luck. That's just sod's law. Are you sure they're "M" threads anyway?

Can't you drill out the face-plate to take the larger screw?

 

[Lons]

Drill it out and epoxy in a nut. JB Weld is decent stuff.

 

[sjalloq]

Thanks both, hadn't thought about epoxy so will try that. Mike, the switch plate is a Varilight Ultra Flat so there's only a tiny rivet holding on the faceplate. Drilling it out isn't an option.

 

[AES]

That's "a bit of a B--gger" that is. Typical of electrical fittings! I would also try epoxy, or, possible the thick "Gel" type cyano acrylate ("super glue"). Either way do make sure both the lug and the box areas where the adhesive will go is dead clean (e.g. acetone - nail varnish remover - on a cotton bud).

One other point: Depending on age, the thread may not be Metric at all, it may be BA (probably 6BA, which is very similar - but not the same - as 3.5 M Fine). Maybe worth while to visit the local model shop if one's nearby. They should still keep little packets of 6BA nuts & bolts. If so you can at least try a 6BA bolt into the lug - however you fix it, it will be best if the bolt going into it is the right thread size, so minimising strain on the lug itself, and its fixing to the box, as you screw it up (and of course, do NOT screw too tight, just enough to hold the outer visible plate steady as you operate the switch).

Good luck, horrible little problem and a real fiddle to fix nicely

 

[Setch]

Whack the tab with a hammer so that the hole is oval. You won't have a nice thread, but it'll grip fine for two or three removals and refixing.

 

[Inspector]

Am I missing something. Can't you get a little piece of metal, drill and tap it with the thread of choice, file it to shape.

Pete