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I’m assembling something bought from the store. Neither screw is labelled, and the manual says one of them is M5x10mm and the other is M6x10mm. I cannot tell which is which. Please help.

photo of both screws

isherwood
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HullBreaker
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3 Answers3

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The number after M is the nominal diameter of the threaded portion in millimetres. So M6 is "thicker" than M5. In the photo in your question, M6 seems to be the one on the left, but holding the two threaded ends right next to each other will tell you which one is which with absolute certainty.

TooTea
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  • However, note that we don't know if there's an M8 screw involved as well, in which case the pictured screws could be M8 and M6! But assuming there are only the two sizes, your answer is correct. – Huesmann Feb 16 '23 at 15:58
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The screw on the left side of the photo is the M6. It is visibly larger than the one on the right. The right side bolt is M5 by default.

mikes
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    Your answer doesn't really explain anything. In some fastener systems, numeric values are guages, with inverse relationship to size. Also, you second sentence is ambiguous. I read it backward the first pass, interpreting it as "It is ... the M5 by default", with "it" meaning the same thing in both cases: "The screw on the left". – isherwood Feb 16 '23 at 14:12
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Unlike some items' measurements, which bear little logical reasoning, metric setscrews, and basically all metric stuff will reflect the actual physical measurements.

Thus, the one with a larger diameter thread must be M6, the smaller, M5. Along with that measurement go two others, the fairly obvious length of the screw, and more importantly, the tpi - the number of threads per inch. Except in metric (no surprise) the number of threads per unit isn't inches!

The 10 represents the length of the screw, as can be seen, both are the same 10mm long.

Tim
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    While it is true that M6 has a measurement which is 6mm, it is the "nominal diameter", and not anything you can measure with a caliper. Also M-series screw threads don't use "threads per anything" - instead they standardize the thread pitch (the inverse measurement). M6 has a thread pitch of 1mm and M5 a thread pitch of 0.8. – Martin Bonner supports Monica Feb 16 '23 at 12:18
  • @MartinBonnersupportsMonica - there is, as in a lot of threads, coarse and fine. It's also the case for metric - which is measured as the distance between two consecutive threads. – Tim Feb 16 '23 at 14:06
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    That's true, but in neither case do they have a TPI or TPCM as part of their specs. – isherwood Feb 16 '23 at 14:08
  • Metric thread pitch table https://monsterbolts.com/pages/metric-thread-pitch#:~:text=Metric%20Thread%20Pitch%20is%20calculated,Thread%20Pitch%20of%201.25mm. – Michael Harvey Feb 16 '23 at 14:22
  • @isherwood - well, it wouldn't be tpi! But specs do say the distance between one thread and its neighbour - which could easily be translated into tpcm, I guess. – Tim Feb 16 '23 at 16:17
  • @MartinBonnersupportsMonica While being technically true that you can't measure exactly 6mm anywhere on an M6 screw, the diameter will be very close to 6mm, so taking a caliper to the screw and getting something very close to 6mm will be enough if you know the screw is metric. Which you can be if you live virtually anywhere outside the US. – arne Feb 17 '23 at 09:08