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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?

einpoklum
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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 Jan 29 '21 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 Jan 29 '21 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 Jan 29 '21 at 15:08

2 Answers2

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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.

ThreePhaseEel
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einpoklum
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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 Jan 29 '21 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 Jan 29 '21 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 Jan 29 '21 at 15:44
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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.

Ecnerwal
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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 Jan 29 '21 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 Jan 29 '21 at 15:10
  • #TIL. Thanks! (and maybe it was a 1/4-28 not 1/4-32 and I just misremembered...) – FreeMan Jan 29 '21 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) https://en.wikipedia.org/wiki/Unified_Thread_Standard – Ecnerwal Jan 29 '21 at 16:20
  • Worth noting is that manufacturers use M notation for sizes not in the standard; Shimano, for example, uses screws they call "M9" on some bike components; it is 9mm in OD. But there is no M9 on any ISO list. – CCTO Jan 29 '21 at 22:48
  • Another cool thing about the metric standard: thread diameter + thread pitch = approximate coarse clearance hole for the bolt; e.g. an M6-1.0 bolt has a 7mm clearance hole. In some circumstances you may want to get more exact but it's another reason why the the metric system is so superior. Whenever the US wants to transition to metric, and the pigs take flight, I will be ready – Dan A. Jan 29 '21 at 23:12
  • I'm not buying a copy of the standard to check, but wikipedia implies there is a 9mm size in ISO 261 but not in ISO 262. – Peter Green Jan 30 '21 at 00:57
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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 Jan 30 '21 at 01:28
  • There's no such thing as "coarse" and "fine" in metric. Any diameter can be any thread pitch. Also, there's no standard for hex bolt head sizes for any given diameter, so an 8mm capscrew may take a 12mm or 13mm hex depending on maker. They really made a bodge job of it. – Harper - Reinstate Monica Jan 30 '21 at 07:41
  • @ecnerwal: thank you! it seems I've limited my knowledge to Wikipedia; M9 is not on their lists for ISO 261 or 262, and they don't seem have a page for 724. Now that I know what to search for, I see it on third-party reference pages. However, in reference to another comment, all the ISO tables I see do list "coarse" and "fine" standard threads (0.75 and 1.00 for M8, M9 and M10). As for head sizes, applications will have their own constraints, it's not too surprising they leave that to the market. – CCTO Jan 30 '21 at 21:12