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Metric Bolt Sizes – Coarse vs Fine Thread (Quick Reference)

Andrew Wade

I often find myself double-checking metric bolt sizes when switching between coarse and fine threads, particularly on older equipment or mixed-standard assemblies.

As a quick reference:

- M6 → 1.0 (coarse), 0.75 (fine)
- M10 → 1.5 (coarse), 1.25 (fine)
- M12 → 1.75 (coarse), 1.25 (fine)

In general:
- Coarse threads tend to be more robust in typical applications and more tolerant of damage or contamination
- Fine threads are useful where higher tensile strength or finer adjustment is required

I’ve also noticed head sizes and tolerances can vary slightly depending on the standard revision being used.

I’ve put together a more complete set of tables (M3–M36, including dimensions and tolerances) here if useful:
www.roymech.co.uk/.../Hex_Screws.html

Interested to hear if others have a preferred rule-of-thumb when selecting thread types in design or maintenance contexts.

  • 0

    "Sorry this page is missing or not available, please use menu above"

    I think your link is broken.

  • 0 in reply to Simon Barker

    the links should be: www.roymech.co.uk/.../Hex_Screws.htm

  • 0 in reply to Andrew Wade

    Do be aware that for larger  sizes there are more than one 'fine' thread in the metric series - e.g. M10 *1, M10 *1.25 and then of course the  'normal' coarse M10 *1.5. M12 is similar..

    Personally I tend to stick with the coarse metric series and specify nordlocks or loctite for vibration, depending on severity, and just specify more fasteners if the strength is in any way in doubt.
    This makes it easier for maintenance teams, who often only have the coarse metric in stock. 

    Then on older kit there is the unhelpful  proliferation of BSW BSF UNC UNF etc. very much still in use.

    Mike.

  • 0 in reply to mapj1

    Good point — it’s worth noting that for larger sizes there are often multiple “fine” thread options in the metric series.

    For example:
    - M10 → 1.0 and 1.25 (fine), plus the standard 1.5 coarse
    - M12 → 1.25 and 1.5 (fine), with 1.75 coarse

    In practice, I tend to default to the coarse metric series unless there’s a specific requirement for fine threads. For vibration, I’ll usually specify Nord-Lock washers or a threadlocker (e.g. Loctite), depending on severity.

    It also simplifies things for maintenance teams, who typically only stock coarse metric fasteners. If there’s any concern over strength, adding more fasteners is often the more practical solution than switching thread type.

    Of course, older equipment adds another layer of complexity with BSW, BSF, UNC, UNF etc., which are still very much encountered in service work.

    Interested to hear how others approach this — particularly when balancing design optimisation against maintainability.

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