Metric screw thread measurements? FAQ

he diameter and ended up with a replacement too thin.

1. =A0I've since measured the original with a calliper and got 3.75mm. =A0T= he replacement M3.5 screw is 3.25mm on the calliper. =A0What gives?

ntical in diameter to the incorrect replacement M3.5, which my calliper say= s is 3.25mm.

are known to kick."

It's probably not metric. Try BA.

diameter and ended up with a replacement too thin.

I've since measured the original with a calliper and got 3.75mm. The replacement M3.5 screw is 3.25mm on the calliper. What gives?

identical in diameter to the incorrect replacement M3.5, which my calliper says is 3.25mm.

It's M4. I got some from a neighbour.

So I now have M3.5 and M4 screws (they were labelled as such in the packets) that are both 0.25mm thinner than expected. And I checked with his callipers too.

The second link above suggests that they would be 0.1mm too thin. I don't see why they should be thinner at all.

Basically, metric screw sizes refer to the clearance hole needed. Most of the common ones are a full mm so you just go to the next full mm above what the measurement is. 3.5mm, of course, being the odd one in common use as it's for electrical sockets, etc.

You have to be careful with computer etc fixings. Some are actually american, rather than metric sizes.

All nominal screw sizes come in both coarse and fine threads. One big difference is that the height of the ridges are higher in coarse threads - there's less taken off. The measured diameter will be larger, Apparently the angle is slightly different as well. Among other things. However AFAIAA all nominal sized screws - male threads will fit the equivalent nominal size nuts - female threads. However coarse and fine threads may be used for different applications based on their properties. Apparently coarse threads thread faster while fine threads are less subject to loosening as a result of vibration. Yet all are interchangeable and will fit onto the equivalent size nut or bolt whether fine or coarse. Which is the most imprtant test regardless of what a digital caliper or any other measuring tells you.

I think this is correct, but just to be sure you might like to ask at the customer service desk next time you're in B&Q or Wickes.

michael adams

...

So you are not an engineer then Michael? What a load of rubbish.

Baz

I'd imagine that much could be deduced from my saying "I think this is correct".

Care to explain why ?

As Your answer will presumably be of intest to the OP as well.

As he'll finally get his question answered.

michael adams

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Would 'bollocks' be the best way to describe this?

Fine or coarse refers to mainly the number of threads per inch. (TPI) It may be possible to force different types of thread (but the same diameter) with the same or very close TPI to mate. But then why would you?

Please tell me you are joking.

Partly correct for some thread series

No

No

Total bollocks

I don't think he has worked out how to use Wiki yet:

I think one phrase in it was broadly correct, but the rest was mostly bollocks

but just to be sure you might like to ask

cost you only have a 4BA nut and an M3.5 threaded screw

I had the strangest dreasm this morning. I dreamt that Barack Obama gave me a 6mm screw.

Why on earth would I dream that?

Wasn't it Clinton who was better known for giving people...

Which is correct according to numerous sources.

Yes. That was incorrect.

The OP was trying to discover why the diameters of various metric scews of the same nominal size appear to differ.

Do you yourself have an explanation for this ?

As it would clearly be of use to the OP.

michael adams

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trying to discover why the diameters of various metric scews apppear to differ by small amounts, perhaps you would be kind enough to explain why this is.

michael adams

That was how the dream ended. I said 'you know Mr president, what te headline would be'

And gave it back.

Too much Savile discussion is probably at the root of it, and The Election. Perhaps my wife had the news on..

They almost certainly weren't all metric screws.

At or around 3.5mm is M3.5, 4BA, and something like 5/32" UNF. I domnt think Ive seen UNC or whitworth done that size, but there you go,

In addition there is BSP, and another standrad thread whose name escapes me.

All in all there are probably 10-20 thread forms in use worldwide, or which have been in use in the last 60 years. None of them fit each other.

They're likely not both metric screws as I hinted at in my answer.

Some do, depending on your definition of 'fit'. Some UNC and some metric sizes being the obvious one.