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I noticed that in some (many?) countries, screw sizes are listed as "M+number", e.g. M3, M4, M5 and so on.

What does this size designation mean?

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    This has nothing to do with location (countries). This is just standard nomenclature for metric screws as opposed to imperial sizes.
    – tnknepp
    Commented Jan 29, 2021 at 15:02
  • @tnknepp: Where I live, the typical packaging doesn't say "Mn". It just says "A mm x B mm" for some values of A and B.
    – einpoklum
    Commented Jan 29, 2021 at 15:05
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    Well, I don't know what you mean by "typical packaging", but the Mx nomenclature is pretty standard...and that's what you asked about, hence my comment.
    – tnknepp
    Commented Jan 29, 2021 at 15:08

2 Answers 2

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There are a few more numbers for a complete description. It's pretty much a world-wide system described by several ISO standards, but there are occasional oddities (slight differences between ISO and DIN standard sizes on a few items for instance)

You asked about M3, M4, etc - that describes (only) the outside diameter of the screw - or the size of an unthreaded hole the screw would slip through.

A full description is like M6-1X25 Which describes the outer diameter (6) , the distance from one thread to the next (1) (pitch) and the length (25) (all in millimeters.)

An M6 could also be an M6-0.75 (fine thread pitch) X whatever length. When not specified, the coarse thread is generally assumed, but sometimes the assumption is wrong...

Coarse and Fine pitch for the sizes you asked about:

  • M3-0.50 and M3-0.35
  • M4-0.7 and M4-0.5
  • M5-0.8 and M5-0.5

Thread charts are easily found if you need to know about more sizes.

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  • So the "fine" and "coarse" are defined within the ISO standard by screw diameter? Interesting to know. I'm used to SAE sizes like 1/4-20 (1/4" diameter, 20 TPI) or 1/4-32 (1/4" diameter, 32 TPI), and have seen other 1/4-x sizes listed as well.
    – FreeMan
    Commented Jan 29, 2021 at 15:06
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    1/4-32 would be an oddball, or "special" - 1/4-28 is the "standard fine thread" (UNF rather than UNC 1/4-20) & yes, the pitches are standardized (except for evil manufacturers who get special non-standard screws made to force you to buy their overpriced replacement parts) (and sloppy ones who's parts are so far off the standard they are supposed to be that they don't work with actual standard parts, I guess I should include...)
    – Ecnerwal
    Commented Jan 29, 2021 at 15:10
  • #TIL. Thanks! (and maybe it was a 1/4-28 not 1/4-32 and I just misremembered...)
    – FreeMan
    Commented Jan 29, 2021 at 15:17
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    Correction: 1/4-32 is UNEF - so it's standard, but you'll need a really good hardware store (or McMaster) to actually find any that size. I used to use a bunch of stuff that was 1/2-28, which is also UNEF (Extra-Fine) en.wikipedia.org/wiki/Unified_Thread_Standard
    – Ecnerwal
    Commented Jan 29, 2021 at 16:20
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    I can find M9 taps and dies at metalworking suppliers, so I doubt they are all THAT custom, though they might well be uncommon. A quick look indicates that both pitches of M9 are in the "fine thread" section of ISO 724. So you might miss them if you only look in coarse.
    – Ecnerwal
    Commented Jan 30, 2021 at 1:28
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Mn for a number n means that the diameter of the screw, projected onto the transverse plane, is n milimeters - including the threading.

The "M" stands for "Metric" (as opposed to British-imperial units).

Illustration:

enter image description here

Remember the Mn designation does not tell you what the length of the screw (along its axis) is.

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    Me too, @Ecnerwal. Just making sure it was obvious to others. An additional note is that the M indicates a metric size description, just to make 100% sure that everyone knows that it's not being measured in any sort of imperial unit.
    – FreeMan
    Commented Jan 29, 2021 at 15:03
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    @FreeMan: You know, you could have just edited my answer to reflect this information... this is a collaborative Q&A platform after all :-)
    – einpoklum
    Commented Jan 29, 2021 at 15:34
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    It is. I could have. I tend to not edit answers other than for formatting/spelling/grammar issues. Right, wrong, or indifferent, that's my take on it.
    – FreeMan
    Commented Jan 29, 2021 at 15:44

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