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Identifying a hanger screw

siddy1972siddy1972 Posts: 180
edited January 2019 in Workshop
Hi all. I need to get hold of a replacement gear hanger screw for an Alchemy Helios. The one that came out is 2.8mm diameter according to my digital callipers.
Does anyone have any idea what this equates to?
I was expecting an M3, but I thought that would measure 3mm?


  • Vino'sGhostVino'sGhost Posts: 4,129
    why not ask Alchemy?

    as it happens its not unusual to have major diameters that are a little smaller than the "nominal" size so your caliper is probably right. I havent time to check the chart but id say its M3

    it's most likely to be a metric size so will need to know the pitch size (the distance between the the top of two threads on a metric screw). Imperial or whitworth or American style is TPI or threads per inch.

    Download a thread identification chart, that will give you the pitch / TPI for screws and bolts. Measure as best you can, consult the chart and the closest is it. (its not unusual to lose count when counting tpi since it all makes your eyes cross) likewise for measuring the distance between two threads for metric pitch measurement) especially in small sizes. for the various imperial standards the chart will identify what youve got exactly. but id be surprised if it were not metric.

    then you will need to measure the length of the screw if its a countersunk screw that sits flush, it's the whole length of the screw. If it has a hex or torx cap then its the length of the screw not including the cap.

    is it threaded all the way down? measure as appropriate.
    is it a constant diameter? measure as appropriate.

    When youve done that call the supplier and youll have all the info you need. (if its a bolt and there are markings on the top make a note)

    They’re cheap as chips

    Or just ask the lbs to sell you one. Or Alchemy.
  • siddy1972siddy1972 Posts: 180
    Many thanks for the detailed reply. I think you’re right, it’s M3. I did a double check of an M6 screw, and it came up 5.8mm, so I suspect the digital calliper may be a bit less accurate than I thought.
    Some M3 by 8 ordered.
    Thanks again for the reply.
  • figbatfigbat Posts: 680
    Your caliper is probably fine (I would be worried about something that is up to 7% out, assuming you zero it before use). The 'M' number refers to the nominal bolt diameter. Coincidentally I was also measuring a bolt the other day with digital calipers and found it to be 5.8mm with a 1mm pitch, thus an M6 bolt.
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  • Vino'sGhostVino'sGhost Posts: 4,129
    yes the caliper is probably right its normal for the bolt or screw to be slightly smaller than the nominal size or else youd have a job getting it in and out. The best thing is one of those bolt finder templates. they cost a fiver and youll know what size, what thread and everything else you need to know without being exposed to the detail. look for one that shows two numbers below each metric size eg under 6 it will say 1.0 and .75.

    Those are the standard and fine thread pitches for an M6 screw

    if its lump less than that then its superfine. There is a superfine thread for eg in the derailleur hanger (connection between derailleur and hanger). The superfine threads are used where you have a bolt that is subjected to vibration or maybe you need to make accurate small adjustments or where you need maximum sheer strength for a given size bolt. It's used in the hanger because any slop or movement would adversely affect the gears. its a 10mm 1.0 whereas the the std and fine are 1.5 and 1.25. Superfine threads are far more sensitive to material damage of the thread hence the need to use a tap on occasion if you're regularly building, maintaining or repairing bikes
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