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@tapped_thread_tal ·

What do the numbers in M3, M4, M5 screw sizes actually mean, and why does the same M5 bolt sometimes not fit?

I keep buying the wrong fasteners and I would like to stop guessing.

I understand that M means metric. What I do not understand is what the number is measuring. It is obviously not the head, because two M5 bolts in my drawer have completely different heads. It does not seem to be the length either, since that is quoted separately.

The part that made me post: I bought an M5 bolt to replace one I had lost, and it would not thread into the hole. It started, then bound up after two turns. Same M5 marking on the packet. A friend said something about pitch and I nodded like I knew what that meant.

So:

  1. What is the number actually measuring?
  2. What is pitch, and how do I tell which one I need without a gauge?
  3. Is there a sane way to buy fasteners so this stops happening?
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  • @clamp_and_wait_cal · yesterday · 2 replies
    @clamp_and_wait_cal ·

    The number is the nominal outside diameter of the thread, in millimetres. An M5 screw has a thread that measures roughly 5mm across the outside of the ridges. That is it — it says nothing about the head, the length, the drive type or the strength.

    So M3 is about 3mm across the threads, M4 about 4mm, M5 about 5mm. If you put callipers across the widest part of the threaded section you will get a number very slightly under the nominal, because the tops of the threads are truncated. 4.8mm on the callipers is an M5.

    The reason two M5 bolts look nothing alike is that everything else is quoted separately: M5 × 20 is 5mm diameter, 20mm long under the head, and the head could be hex, socket cap, button, countersunk or flanged without changing either number.

    Your binding bolt is the third dimension nobody prints on the front of the packet, which is what your friend meant.

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    • @tapped_thread_tal · 2d ago
      @tapped_thread_tal OP ·

      So the callipers reading slightly under nominal is expected rather than me measuring badly. That has been quietly bothering me for a year.

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  • @conduit_connie · 17h ago · 2 replies
    @conduit_connie ·

    Pitch is the distance from one thread crest to the next, along the length of the screw. Two bolts can both be M5 and have different pitches, and they will not go into the same hole — which is exactly what you felt when it started and then bound.

    Every metric size has a coarse pitch that is the default, and one or more fine pitches that are not. For M5 the coarse pitch is 0.8mm and the common fine one is 0.5mm. Written properly they are M5×0.8 and M5×0.5, but shops routinely print just M5 on the coarse one because it is what nearly everyone wants.

    The useful table, because these are worth knowing by heart:

    size coarse pitch
    M3 0.5
    M4 0.7
    M5 0.8
    M6 1.0
    M8 1.25
    M10 1.5

    If a packet says only M5, it is coarse. If your hole is fine-pitch — common in anything automotive, and in adjusting screws where precision matters — you have to ask for it by name.

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    • @idle_hours_ike · 2d ago
      @idle_hours_ike ·

      Adding the identification trick, since you asked how to tell without a gauge.

      Lay the old screw on a ruler with millimetre marks and count the crests over 10mm. Ten crests in 10mm is 1.0 pitch, about twelve or thirteen is 0.8. It sounds crude and it is accurate enough to tell coarse from fine every time, because the two options for any given size are far apart.

      And never force one that binds. A fine screw started into a coarse hole will cut its own way in for a couple of turns and destroy the threads on the way. That is a much worse afternoon than a second trip to the shop.

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  • @clamp_and_wait_cal · 21h ago
    @clamp_and_wait_cal ·

    On your third question, the thing that actually fixed this for me: stop buying fasteners for the job and start owning a range.

    A sorted assortment box of M3 to M8 in coarse pitch, in a few lengths, costs less than three emergency trips and removes the entire failure mode for maybe 90% of household jobs. Keep the removed screw taped to the box lid until the job is done so you can compare directly.

    The remaining 10% is fine pitch, imperial hiding in old or American equipment, and self-tapping screws that are a different system altogether. Those you take the old one to the shop for.

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  • @seasoning_saskia · yesterday
    @seasoning_saskia ·

    One trap worth knowing before it catches you: a hole that takes an M5 bolt is not drilled at 5mm. It is drilled a bit under and then tapped, or it is a moulded thread.

    This matters because people measure the hole, get 4.2mm, and go looking for an M4.2. There is no such thing. If you are replacing a screw, measure the screw. If the screw is gone, take the thing to the shop and try bolts until one turns freely by hand.

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