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It is extremely nice to be able to use a caliper to measure screw/bolt major diameter (and length) to find correct replacements. However I am struggling to come up with a good way to determine accurate threads per inch (TPI; eyeball counts are difficult for me to get right, especially for smaller screws/etc).

What would folks here recommend for accurately determining TPI? I would prefer something other than prebuilt thread gauges, which based on my research are relatively expensive, dedicated-use, and of surprisingly variant build quality/accuracy (e.g., folks have complained about such gauges not being tapped correctly!!!). Thanks for any advice.

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    I've never seen single-use thread gauges. I prefer the reusable kind. – jwh20 Jan 19 at 14:27
  • lol thanks for that. edited to "dedicated-use" – bjsdaiyu Jan 19 at 14:29
  • Do you intend on using the results with taps/dies? If so, make your own thread gauge, and when it wears make another. – Criggie Jan 19 at 23:14
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    Note that if you have a bolt of the proper (and known) thread pitch, the threads of it will tightly interlace with the threads of a bolt of matching thread pitch if you press the two together side-to-side. It's important to closely examine how the two sets of threads interconnect, however, when using this approach. – Hot Licks Jan 20 at 1:51
  • For most purposes once you have the diameter of the bolt there are just two standard thread counts to choose from and they are not very close. For example, if you find the bolt is 1/2 inch, it will either be 1/2-13 or 1/2-20. Those are far enough apart that just counting with a ruler is sufficient. Telling 1/4-20 from 1/4-28 is a little harder, but if you have either nut on hand it will fit or not. They are so common that a glance is usually enough. I am not familiar with metric standards, but I would be surprised if it is different. – Ross Millikan Jan 20 at 5:02
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A thread gauge is not tapped. A tapped gauge is attempting to show both size and threading, and can be limited by being too thin to correctly discern close metric/inch threads, for instance.

A thread gauge lets you worry about size, while it concentrates on accurately determining threading. Of course, you need to buy one that's not a hunk of poorly made junk...

You might "prefer something else" but your title asked for best practices. This is a problem with a tool that solves it; that is the best practice.

Mitutoyo thread gauge - picture from MSC Industrial

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  • yeah, so in other words there is really no process involving a general (non-dedicated use) tool such as a caliper that can be applied to the problem (with better results, i'd argue, than using a screw size gauge for determining screw/bolt size). I'll look into this (inexpensive) tool then, thanks! – bjsdaiyu Jan 19 at 16:59
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    Well you can measure the pitch of external threads with a caliper: it’s just the distance between two adjacent threads. The inverse of thread pitch is threads per inch (TPI). Of course, this is more challenging with finer threads. – canadianer Jan 19 at 17:15
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    You can certainly apply a caliper to the problem, but that is subject to more sources of error - small measurement errors making large differences if reading a single thread, or miscounting if measuring 10 threads, etc. A thread gauge is fairly inarguable - it clearly fits, or does not, even when the differences are slight. – Ecnerwal Jan 19 at 21:08
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Go to your local big-box home improvement center and purchase a variety of nuts of different diameter & thread pitch. Carefully label each one before you throw them all in the little bag to take them up to the register. Also, pick up a piece of 1x lumber and a tube of epoxy.

Once you're home, epoxy each nut to your piece of lumber and use a marker to label each bolt for diameter & pitch. Voila! You've created your own pitch gauge and it's of the exact quality you desire (by buying nuts of the quality you've determined is acceptable).

The best part, is that if you come across a new screw or bolt that doesn't match anything on your board, you can take it to the store, try it against the nuts there to figure out the diameter/pitch, and buy a nut. Bring it home, add it to your gauge, and you've upgraded for just a few cents!

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    Some stores will already have a nut selection glued on something, in store to size bolts, so making/buying your own might not be needed, unless you have a big box of mixed sizes of bolts. – crip659 Jan 19 at 14:45
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    Exactly, @crip659. However, OP seems to be looking for a way to do this at home, and I get it. Sometimes, you grab a bolt out of your "random assortment of bolts" bin and need to find a matching nut. It's easier to do the test at home than it is to go to the store to find the right size, then take all your nuts back to the store and figure out which one will fit. ;) – FreeMan Jan 19 at 14:50
  • Beware of metric/imperial mismatches (or rather false matches) with this system if that's a concern. I can't remember the exact combo but there's one in the 4mm range where you can thread an imperial nut on a metric thread (or vice versa) and it seems to fit, because the mismatch isn't apparent with so few threads engaged – llama Jan 19 at 22:58
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Are you saying one of these is too expensive and not accurate?

enter image description here

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  • Or one of these. – canadianer Jan 19 at 16:18
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    I am, yes (wrt inaccuracy). The plastic ones wear over time due to mis-fittings and lose accuracy, assuming they're even accurate to begin with. I can't claim to tested every (more expensive) metal gauge on the market, but am definitely wary of the prospect of spending money several times over in search of a gauge that actually produces correct results (and continues to do so over time). @canadianer's recommendation definitely worth exploring though. – bjsdaiyu Jan 19 at 17:06
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    A metal version would be much more accurate and durable. For example: amazon.com/Horizontal-Gauge-Thread-Checker-Metric/dp/B00OM1HUJ2 – computercarguy Jan 19 at 23:31
  • Most hardware stores I've seen have something like this mounted to a shelf near where they sell nuts and bolts. So if you can bring one with you to test against it, you don't need to purchase your own. – Darrel Hoffman Jan 20 at 17:46

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