I have a Shimano FC-M131 crank arm and I want to use a bolt for the axis. I have tried an M20 bolt but it is too loose. I have measured the inner diameter and it is 20.8mm, slightly larger than the M20. Does anyone know what kind of bolt do I need to use?

I have included some pics to clarify the problem. (Click images for larger.)

Close-up of pedal end of crank showing the FC-M131 model number Blue-anodized aluminium Meroca crank cover screw [Picture of calipers3 Close-up of the drive-side crank bolt Close-up of the non-drive-side crank bolt with an arrow pointing to the dust cap thread

  • What's the bolt that says "KB"? Or the bolt with hole in the last picture?
    – ojs
    Aug 31, 2021 at 6:46
  • The KB bolts are the same in both pics. I refered to the outer part (metallic grey with threads) or the arm crank (arrow in the pic) Aug 31, 2021 at 6:54
  • 1
    Ok. That part is called dust cap, and it's not really required. The standard thread is 22x1mm.
    – ojs
    Aug 31, 2021 at 8:02
  • Thanks for the information. I will try to find some. Aug 31, 2021 at 8:06

1 Answer 1


As discussed in comments, the part in question was not crank bolt but dust cap. The standard dust cap thread is 22x1mm, but often they are plastic parts that are just pressed in or even more commonly, part of the crank bolt.

  • So, I could replace the hexagonal bolt of the crank by another that has the dust cap included (as a part of the bolt) right? Aug 31, 2021 at 8:23
  • Yes. AFAIK all square taper crank bolts have the same thread
    – ojs
    Aug 31, 2021 at 8:44

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