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When my 10 year old chain snapped, I realized that I needed to replace my rear cassette and crank, due to wear. Noted for the future! I ordered a 9 speed cassette from Shimano (same brand and number of gears as I had), and a 3 speed front crank (again, same amount of gears and same brand, different type though: Shimano Altus M371). I thought I had taken all the factors into account, but of course I was wrong :). I noticed that I could not adjust my derailleur to smoothly shift gears without rubbing. I didn't know about the concept chainline, and I think it is very likely that my new crankset has a different chain line (50 mm) than my old one (unknown).

The problem that I have is that I can't find what chain line my old crank had. I think my options are either:

  1. find out what chain line my old crank has, and try to find something similar. -> This seems very difficult, because most cranks I can order have 50mm (the same as the one I just bought)
  2. play around with spacers. I am afraid this gets me into a rabbit hole. I don't exactly know what I need and I am afraid I will ruin something when I have to dissassemble my bottom bracket.

A picture to give it a little bit more context: Situation with new crankset

Is this the moment to go to a bike shop? Or is there an easy fix that I could apply? I have a three piece crankset (I think it is Shimano Octolink).

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You have a square taper bottom bracket, which means there are no spacers that can help you. What you need is a bottom bracket with a longer spindle. Shimano recommends a BB-UN300 with a 123mm spindle for your crankset, so that's what I would get. Don't be afraid of changing your bottom bracket, it's not a complicated procedure and there's not really anything you can damage if you're careful and not cross-thread the BB into the shell.

enter image description here

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  • +1, although "there's not really anything you can damage...". Not really accurate- plenty of people have come to grief as the left hand side is left threaded.
    – mattnz
    Commented Jun 29, 2021 at 23:24
  • Thanks for the elaborate response! I think I can manage this repair :). Did they change this (recently?) for a reason? Perhaps I should stock up on cranks before the next time they decide to change a standard :) Also, Is there a way to be sure my frame is compaitible with a axle length of 123mm? Naturally, my current bottom bracket is more narrow. Is there a system that keeps it from sliding to the sides?
    – Erik
    Commented Jun 30, 2021 at 12:11
  • The frame is compatible with any axle length, as long as the proper chainline is maintained (which is a function of spindle length and crank construction). Neither the axle nor the crankarms can slide to the side, if you tighten the crank bolt correctly.
    – Victor Stanciu
    Commented Jul 1, 2021 at 4:51
  • Speaking of the crank bolt, the torque spec on the bolt is 40Nm, which is a quite a lot. If you don't own a torque wrench, you at least need a tool that can provide a lot of leverage: media.alltricks.com/hd/5166682271a21.jpg
    – Victor Stanciu
    Commented Jul 1, 2021 at 4:51
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    Thank you! I fixed it today :)
    – Erik
    Commented Jul 12, 2021 at 17:34

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