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I'm trying to identify several screws. Guides online are fairly straightforward - measure the diameter of the thread part, and pitch if possible. However, none of my screw measurements make any sense. I've ruled out my caliper as the cause.

Here's some of my measurements (Suspected screw: Expected -> Actual):

  • Known M2.5: 2.5mm -> 2.43 mm
  • M3: 3mm -> 3.38 mm, 0.5mm pitch
  • M4: 4mm -> 3.88 mm
  • Known 5/16": 0.3125" -> 0.305", 18 threads/inch

My measurements seem to be off my 2% to 10%. Am I missing something? Is my M3 screw actually M3.5 or M4? I can't find, say, 3.38mm on any screw charts (example)

isherwood
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Aloha
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    Could you provide the threads per inch or cm that may provide us with a SAE corse or fine pitch screw. – Ed Beal Dec 22 '20 at 17:06
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    *Weird size screws* in metric are probably normal size screws in Inch/number. – Ecnerwal Dec 22 '20 at 17:11
  • @EdBeal The 5/16" has 18 threads/inch as measured. The suspected M3 screw has a pitch of 0.5mm. – Aloha Dec 22 '20 at 17:11
  • What problem are you trying to solve? This isn't the type of question we handle here. It's speculative at best. – isherwood Dec 22 '20 at 17:13
  • @isherwood I'm trying to identify screws for a DIY project, yet none of my measurements match charts online – Aloha Dec 22 '20 at 17:15
  • Well I have some 5/16-24 that are used with electrical equipment. Did you know there are different thread pitches for metric? Having been a maintenance type of person for many years I can tell you machine screws do not vary or they will not fit but there are different pitches in both metric and “English”. – Ed Beal Dec 22 '20 at 17:15
  • @EdBeal Good point. Yes, I'm aware. I'm designing in Fusion 360, and thread pitch is required. I've measured them as best as I can. – Aloha Dec 22 '20 at 17:17
  • @EdBeal fine pitch metric screws are rare, and are probably a red herring here – Chris H Dec 23 '20 at 09:00
  • @chris h, I work in a lumber mill and our equipment uses them when I worked at HP we also used them so I would not say rare in industry where metric equipment is used I find them commonly so much so that I have a pitch gauge exactly like monkey posted and plastic screw size templates like ecrnewal mentions. – Ed Beal Dec 23 '20 at 14:27
  • @EdBeal, sure, I've seen them in work (optics lab) where you also get extra fine (M3x0.25) and ridiculously (not an official name) fine (M3x0.2), as well as things like 1/4-80. But for adjustment, not for general fastenings. For a DIY design, or even in work when not absolutely needed, they're best left in the box of odd screws and something normal used (for me, M4x0.7 or M6x1 by default) – Chris H Dec 23 '20 at 15:14

4 Answers4

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Metric screw diameters are always below nominal.

The nominal size is as if the thread came to a sharp point, and it doesn't. For the exact amount see Wikipedia, from where I lifted this drawing (thanks to user Inductiveload).

enter image description here

Note the P/8 (i.e pitch/8) flat on the outside (top) of the external thread. This means the outside diameter (roughly doing the trigonometry) is about 0.2×pitch less than nominal. Taking your M4, with a pitch of 0.7, you'd expect an OD of 4-0.2*0.7=3.86, an almost perfect match to your 3.88. Similarly for you M2.5.

So your 3.38mm OD screw isn't M3. The diameter is good for M3.5, but that has a 0.6mm pitch. I don't know how you measured the pitch, but that's a fairly easy error. Without a thread gauge I'd measure the length of 10 (or more) turns, and divide to get the pitch.

M3.5 is rare, but common in British and some European electrical hardware, usually slotted (e.g. it holds our sockets and light switches to the back boxes). I have seen socket head M3.5 on a metric version of an American product, where a UK engineer would have used M4

Chris H
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It stands to reason that most threads will measure slightly below nominal. That's much less of a compatibility problem than if they're too large. It's probably due to threads being cut or cast using tooling for the actual shank diameter, where slightly too much material is removed either intentionally or due to manufacturing tolerances.

To that point, I'd be willing to bet that your M3 is actually an M3.5. It being so would put it in line with the negative variance of all your other examples.

isherwood
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For machine screws:

You can either purchase a screw identifying plate (with labeled threaded and sometimes clearance holes), or you can purchase known nuts that you keep track of and thread "suspect" screws into them. There are also dedicated thread pitch gauges which are far more accurate than trying to "eyeball count" threads against a ruler.

Aside from your caliper appearing to read a bit on the small side, the most likely cause of confusion is trying to force a metric fastener to be "english" or an "english" fastener to be Metric. This leads to misreading your measurements slightly to fit the bias (and that can be where a thread gauge that simply won't allow you to fake it can help get you on track.)

Depending how deep in the rabbit hole you go, there's also the "Actual 'English' issue" of getting into Whitworth threads rather than SAE "inch & Number" threads. They are a different thread shape, more an issue with antiques than current production now.

ThreePhaseEel
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Ecnerwal
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  • Whitworth screws have one common use: camera tripods are 1/4-20 Whitworth (3/8-16 Whitworth is also used to attach heads to tripods). These are semi-compatible with the much more common 1/4-20 UNC for light duty. – Chris H Dec 23 '20 at 09:18
  • Seriously, @ChrisH? I had absolutely no idea... – FreeMan Dec 23 '20 at 14:39
  • @FreeMan I use both, as 1/4-20 UNC is used on non-metric optics lab kit, but sometimes I want to integrate cameras. If you're gentle it's fine, but the thread on the bottom of a camera isn't always very strong or deep. I'm not convinced all cameras follow the standard, but good ones do – Chris H Dec 23 '20 at 15:17
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Quite frankly, you need a thread gauge:

thread gauge for machine screws

https://www.amazon.com/ChgImposs-Imperial-Whitworth-Industrial-Measurement/dp/B07J9V9JTK/

MonkeyZeus
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