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Doesn't matter what I do, I can't seem to push the arm all the way, even though the bolt is tight and it feels stable:

enter image description here

I just replaced a worn out and damaged arm but they look similar enough to not have this gap (old on the left, right is new):

enter image description here

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Crank arm looks like it is actually fully on the tapered part of the axle. Square taper cranks don't mate up to the frame bottom bracket shell, there's supposed to be a gap there.

See: Sheldon Brown's page on square taper cranks/bottom brackets.

The gap does look a bit large to me though. Are you replacing crank arms? (Crank arm in pic looks new.) Is there a difference in fit of new vs old arms?

Update after pic of old crank added.

Wow! You really did let that crank get bad! Old crank arm that badly damaged would sit further onto the axle because the taper has been enlarged.

I'd take the bike to your local bike store and ask them to tighten the crank bolts up to the proper torque with a suitably large torque wrench (they'd likely do this for free or for a nominally small charge) to avoid recurrence of the issue.

Update 2: for reference here’s an old BB and crank I dug out. Bolt is not torqued down, but it shows where crank sits on the taper.

enter image description here

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  • Yes I am. See edit.
    – PIXP
    Commented Jun 6, 2018 at 12:05
  • There is a possible issue regarding JIS and ISO, you should mention that. sheldonbrown.com/bbtaper.html or bicycles.stackexchange.com/questions/10186/…
    – StefanS
    Commented Jun 6, 2018 at 12:24
  • @StefanS Yes I was thinking about that. but I believe ISO and JSI share the taper dimension, but ISO is longer at the thin end.
    – Argenti Apparatus
    Commented Jun 6, 2018 at 13:15
  • Thank you for the information. I had to do a lot of kilometres back home with the bad one, tightening it mid-ride. So basically what you are suggesting is to tighten the screw with enough force?
    – PIXP
    Commented Jun 7, 2018 at 6:42
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    @PIXP Have you checked that the taper is still ok? Riding with a loose crank arm may not just damage the crank arm itself, but also the taper it's rubbing against. It's better not to put any load on a crank arm after it's loosened, even if that means riding only with the power of one leg.
    – cmaster - reinstate monica
    Commented Jun 7, 2018 at 19:53

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