Screw sheared off in pattress - what to do

[mark.bluemel]

As part of a redecoration project, I've been tasked with replacing the rather cheap and nasty light switches with nice brushed steel, concealed screw ones.

Not that that's awfully relevant. The point is that when refitting one, one of the screws holding the plate to the pattress jammed and when I tried to unscrew it, the head sheared off :-(

Is there a way to resolve this without recourse to a cold chisel and replacing the pattress?

Note - I have a limited range of tools, and little desire to get many more, so recommendations involving angle grinders would probably necessitate getting someone in...

[Tim Watts]

A drop of oil on the thread and pliers on the stub? Is there enough screw left to get hold off?

[Adrian]

With the front off, you can probably unwind the screw with a pair of pliers.

[GMM]

If the switch is stuck in position because of the way the screw has sheared unevenly, and there's no chance of getting any purchase on the screw, you should be able to drill it to the point where it slips through the hole with drill diameter the same as the hole size. The biggest problem is getting the drill started on the rough surface but if the screws are concealed, that will hide any little accidents. Alternatively, it may be necessary to wreck the new switch, which would be a pity if it's an expensive one.

[Jeff Layman]

Sounds to me as though it become cross-threaded for some reason. If you can grip what's left of the bolt with a pair of pliers, first try turning it a quarter of a turn as though you were trying to tighten it. That might possibly loosen it from the pattress.

If it does loosen, you have two choices. If there is enough room, you could continue "tightening" it until it goes all the way through. Or if not, unscrew it very carefully trying to keep it absolutely straight (or you risk cross-threading it again). If it doesn't loosen, and what others have suggested doesn't work, I would use a Dremel to cut it off and drill it out, but from what you've said you don't have that available, unfortunately.

Even if you can remove the screw, there is a possibility that the pattress thread has been damaged, and even a new screw might get stuck or not grip properly.

[Chris French]

If the thread in the box is then a bit knackered, you could try using a re-threading tool to tidy it up. e.g.

This is for 'modern' 3.5mm threads, if it's an old box it might have imperial screws, I forget what thread they are, even though lots of the boxes in this house have them

[meow2222]

If its so stuck the head sheared off, I dont think theres much hope of getting a new screw to work in there. Replace the pattress. Chip out around it with chisel - or even a screwdriver - and fit anew. Fill.

NT

[spuorgelgoog]

4 BA for faceplates 2 BA for BESA boxes

IIRC

Owain

[Capitol]

3BA I think.

[Ian Jackson]

In the absence of a proper tap for that size, a poor man's tap can sometimes be made from a new screw, with a short more-or-less lengthwise slot cut in it with a hacksaw (at the 'point end', of course) - or maybe even a shallow flat filed on it.

[harryagain]

Drill out and use self tapping screw. Obviously use a drill size suitable to the self tapper.

[Chris J Dixon]

Faced with a stripped thread, some years ago, I rummaged in my trays of assorted screws, and found a long thin woodscrew which managed to work as a self-tapper.

Chris

[meow2222]

Yes, just the right size does work even when the pattress thread is trashed. But it doesnt look right when the screw is visible.

NT

[Dave Plowman (News)]

First, try and establish just why it's sheared. If it has corroded badly, you're not going to be able to unscrew what remains.

Might be best to use small right angle grips and screw it out clockwise gripping it underneath, as it were.

If all that fails drill it out and superglue a nut to the back of the lug. The size is 3.5mm - you'll get them on Ebay.

[meow2222]

I can just see the brittle superglue bond breaking as the screw is tightened. Then when you come to unscrew it one day, it won't, it just goes round & round.

NT

[JimK]

/If all that fails drill it out and superglue a nut to the back of the lug. The size is 3.5mm - you'll get them on Ebay./q

If this really is a pattress box that won't be an option....

Jim K

[Dave Plowman (News)]

You'd need to explain what you mean by a pattress box, then.

Most used for surface mount plastic boxes these days - in which case easily changed. Unlike a flush steel box I'd guess is meant.

[Tim Watts]

I'd use some chemical metal aka isopon/plastic padding.

Drop of oil smeared on the thread so the bolt doesn't stick to the gunk.

In fact the red grade of Plastic Padding could actually have a machine screw thread formed in it this way (I did that once) - ie by burying half a screw that was lightly oiled into a blob of the stuff.

[Dave Plowman (News)]

Use a 3.5mm rivnut, then. Good luck in finding one.

[JimK]

/You'd need to explain what you mean by a pattress box, then. /q

The generally accepted definition, obviously. Why what did you think it meant?

Jim K