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I measured the maximal diameter of this counter-sunk bolt.

enter image description here

And tried to match it with the metric bolt dimension chart:

enter image description here

But I did not find any match. I find the difference between dimensions M,N and R,S to be mysterious and suspect that the answer to my quandary is hidden therein.

My guess is that this is an M4, since the diameter without the thread could be 4mm and perhaps in some countries the standard specifies the inner diameter instead.

I am almost sure this is a metric bolt, since it is from a EU-made machine.

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    @FreeMan Sorry, I chose a weird angle. The bolt is not parallel with the jaws, but slightly rotated so that the bolt head is behind them. Mar 3 at 12:10
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    No worries! I do see that now that you've pointed it out. In the future, I'd suggest a nut/bolt sizing gauge. A series of holes & threads punched in a piece of aluminum, steel, or plastic that have all the common metric & Imperial sizes for screw/bolt size & thread pitch. No need to worry about measurements & charts, just see which hole the bolt goes through and which threads it matches up with and read the size off the gauge.
    – FreeMan
    Mar 3 at 14:58
  • N is the outer diameter of the threads and should be easy to measure with the flats of the caliper. It is a "nice" number. Heads can come in many sizes. Mar 3 at 21:47
  • Any idea how old the machine is? EU and less than 40 years old is more likely metric, but if its older than (about) 40 years then the chance of SAE, Imperial, Standard, etc goes up.
    – Criggie
    Mar 4 at 21:45
  • what makes you think it's metric?
    – njzk2
    Mar 5 at 22:27

6 Answers 6

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If it's metric, it's M5, of a poor fit class (or with measurement error contributing) meaning the thread fit is somewhat loose compared to a better fit class - common for non-critical hardware where "it fits in the nut and grabs" is adequate. It could not possibly be M4, it simply won't fit.

Put an M5 nut on it and see if it will thread all the way, or if it gets stuck. If it gets stuck it's not metric.

S is not mysterious - it's the amount of the screw that's solid inside the threads. Though actually it's being given as the tap drill size (how big to drill the hole before putting a tap in to cut the threads.) That's usually a hair bigger than the actual minor thread diameter, as cutting "100% threads" is difficult and prone to breaking taps .vs. "95%" threads, say, with very little change in performance. I'd use a real drill chart to verify that the size is correct for that application...

Neither M nor R appear to be given in the chart.

Standards-wise, one of the most world-wide standard things around - mostly ISO standards, though the Germans do throw in a couple of DIN standards that are slightly different. Quality-of-execution-wise, all over the map, and the fastener may well be imported rather than made in the country the machine is from.

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    Although, if it is not metric, then it is awfully close in size to a #10 bolt.
    – Glen Yates
    Mar 3 at 17:07
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    I get a pitch of ~0.82mm by roughly scaling that image, which fits with the standard M5 x 0.8. Mar 4 at 16:45
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    @SpehroPefhany - #10-32 bolts would have a pitch of 0.79mm (25.4/32), so that doesn't really rule it out...
    – IronEagle
    Mar 5 at 0:34
  • @IronEagle Good point! Mar 5 at 1:12
  • It is 10-32. Easy to check in GIMP. I cropped and scaled the image to the width of the caliper measurement. The length of four threads is 1/8". (i.e. 4/32").
    – YoeyYutch
    Mar 5 at 5:54
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It is 10-32. Easy to check in GIMP. I cropped and scaled the image to the width of the caliper measurement. The length of eight threads is 1/4". (i.e. 8/32"). 10-32 screws have a pitch of 1/32" so it matches up perfectly. Note the measurement in the bottom corner of the image.

enter image description here

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  • That's... an interesting way of going about it! Very clever, outside the box thinking. A bit more time and effort than just holding the screw up to a sizing/pitch gauge, though. :) Also, did you do the measurements to determine it's a 10, not an 8?
    – FreeMan
    Mar 5 at 14:05
  • Thank you kindly. The measurement of 4.78mm is what tells me that it is likely a #10 screw. The nominal diameter of a #10 is 4.82mm (.190"). Honestly it took me longer to write this than it did to make the measurement. Fun fact about numbered screws sizes: #0 has diameter .060". Each successive number is .013" larger than the last. So a #10 is 10 x .013 + .060 = .190. An 8 is .164". Kind of SCREWY eh?
    – YoeyYutch
    Mar 5 at 15:06
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The dimensions chart you're looking at is only the base nominal dimensions, and does not take into account tolerance classes. Normally, nuts (internal thread) are 6H which ideally matches the nominal exactly, while bolts (external thread) use 6g or another class that may be undersized to leave clearance. I use this site for getting the real dimension ranges for a particular metric thread size in a particular tolerance class: https://amesweb.info/Screws/metric-thread-dimensions-calculator.aspx

For M5 in class 6g, the outer diameter range is 4.826 to 4.976, which your measured value is close to fitting in; in particular it's larger than the maximum pitch diameter of 4.605 for a nut, which means it will almost surely catch and hold at least half-decently. This is of course assuming it has matching pitch.

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To know bolt size, you need a few things:

  • Major diameter, e.g. on top of the threads.
  • Minor diameter, e.g. bottom of threads.
  • Thread pitch
  • Thread form.

Thread forms (By Jeremy Riley - Own work, CC BY-SA 3.0)

The two first one is relatively easy to measure - at least the first one. And it will give an indication, but it's not enough to know what threads you have.

To measure pitch, the easy way is to get a thread gauge, which easily let's you find the pitch.

For metric, the pitch is the distance in millimeters between threads. For imperial, it's specified as number of threads per inch. To measure without a thread gauge, measure a distance, and count the threads across that distance.

The thread form we can normally ignore, but there's a few differences worth knowing. Metric always has 60° thread angle. UN threads has 60°. British Whitworth has 55°. Bot UNC and Whitworth are imperial threads measured in inches, and the only difference is the thread angle. You can normally mix Whitworth and UNC, at some reduction in strength.

So - to figure out what bolt you have:

  1. Figure out the outer diameter
  2. Figure out the number of threads per inch, or the pitch. A thread gauge will tell you this with certainty in seconds. Without one, this may be difficult for small bolts.
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It's too large for M4. An M4 bolt must fit inside an M4 hole. That's kinda the whole point.

Similarly, any size X bolt will fit inside a size X hole. Longer bolts aren't threaded all the way; they have an unthreaded shank for some distance. The shank, also, will fit a size X hole.

0
4.78mm is 0.188" 
4.7625mm is 0.1875" which is 3/16"

I think you have an older thread form that predated metric, called imperial or SAE or standard.

The next tool you need is a thread pitch gauge, to figure out how many TPI or threads per inch. Or one of those organised box-sets of fasteners and just try different nuts till one fits.

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    Specifically, it might be a #10 bolt (3/16" is actually uncommon), which has a diameter of 0.190" (4.826mm). A #10-32 bolt would even have a similar thread (pitch of 0.79mm vs 0.8mm for a M5).
    – IronEagle
    Mar 5 at 0:38

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