Tap for M3.5 Screws

Because they are not long enough or because they are damaged?

Electrical fittings like that tend to have the same M3.5 standard screws in of various lengths and have done since almost forever (certainly since 1977). Some cowboy might have forced the wrong sort of bolt in or got one in cross threaded but you should be able to buy replacements.

The threads in the sockets are on such a small thickness of material that you are quite likely to strip them completely bare with a tap unless you have a very steady hand and get it exactly square on.

If you need a range of lengths it is easier to buy them as a mixed bag since cutting a screw to length requires a bit of skill for it to remain useable afterwards. Pair of nuts on the bit you want to keep and then cut off the excess - but it is time consuming that way.

The most obvious way of differentiating between a bolt and a screw is that a bolt is not usually threaded all the way along its shank as it has a plain portion. A screw, however, is threaded fully to the head.

That's bullshit with most wood screws.

They can be a bit of a pig if you don't get the screw at right angles to the lug in the back box. A slightly longer screw helps because it can give some view behind the faceplate as you position the screw.

You also need some longer screws if the back box is recessed behind a layer of plaster or tiles. If found that on the first sign of the screw getting tight to turn immediately back it off and start again.

If you need to occasionally cut down a long M3.5 screw the tool below works well. It's absolute s**te for crimping but it has a tapped hole for M3.5 screws. Open handle to expose the through hole, wind in the screw, squeeze handle which cuts the screw, wind out the screw which re-cuts the tread at the end.

But long before the 1980's. I can't recall seeing BA threads on electrical fittings at all in the past few decades.

I suspect he wants the opposite, a tool to re-cut the threads in the backbox to accept M3.5 screws

I daresay cheaper versions are available

But the backboxes of that 1980's era should already be M3.5.

Only thing I've noticed is these things tend to be called machine screws. (Not tappered and have the thread running from head to the 'end')

'screws' seem to be tapered at the end usually for wood or plastic or called self tappers for metals .

Bolts don't have threads for the whole part of the shaft, mostly just at the end.

The above is how I describe these things to students for ordering purposes and asking and getting what they want or think they want.

and then by lenghs in mm and M as the size M2, M2.5, M3, M4 & M5 are our standard range.

Personally I recommend getting an electrician in to do an EICR. There are many potentially dangerous faults that you won't find. Can you test for ring continuity for example or transposed live/neutral.

"Imperial Threads" were never used on "modern" wall boxes, I believe that originally they were a 4BA thread which because it was intended for Scientific use is a metric thread. Compare the thread pitch.

Longer bolts (to me a screw cuts its own thread) may foul the back of the box and not tighten.

Dave

They used to be 4BA before metrication

Cleaning nuts with an M3.5 tap is easy, just be sure to get it in straight or it'll trash the nut. Imperial were used a decade before your place so I'd not expect you'd encounter it.

Many thanks for all the replies :-)

I have gone for a thread-cleaning screwdriver and a bag of mixed size M3 screws, I can use the smallest size feasible without standing on my head with a torch in my mouth!

I certainly remember a thread before M3. Whether it was imperial or not I don't know (my dad referred to it as such) but I remember if you did it carefully with a decent screwdriver you could "convert" it to metric by driving a bolt in.

As well as BA screws, the double MK sockets from early 70s had 4 screw fixing not 2, so while they look like modern sockets & boxes they're not really compatible.

But it was all over bar the shouting by 1975 so it is very unlikely that a home built in the 1980's would have any such parts installed. My house extension was in 1975 and there is no trace of pre metric components.

The last time I saw BA threads in electrical installations was somewhere that hadn't been rewired since the 1950's and still had a selection of scary round pin partly broken Bakelite plugs and sockets in daily use.

I have used one of these to restore threads in various back boxes and it has done a good job so far. One or two of the very badly raunched ones have been a bit sloppy after rethreading but all so far have screwed up tight and held OK.

They changed to M3.5 in the late 1960's. Some switches and sockets came with both 4BA and M3.5 screws for a while

My house, bought new in 1971, had all sockets with 4BA screws.

In addition to all the previous posts you can get replacement 3.5mm screws that have a plain, non threaded, end. This allows easy alignment of the screw & helps to prevent cross threading. e.g. -