I'm amateur and got this stationary bike. The crank arm, I think that's what it's called, fell off. I can put the bolt back (the one inside the red circle) and turn manually but can't tighten it. There was a cap on top of it. What tool do it need here?

It's completely round and not hex shape enter image description here

  • 1
    Make and model stationary bike? Nov 2, 2020 at 6:13
  • Not quite sure what you're showing in this picture. Is the black arm going to lower-right your crank? That looks like one half of those fancy 2-part washer pairs called a Nord Lock, or a "wedge washer". Are you sure there isn't another part missing ?
    – Criggie
    Nov 2, 2020 at 7:38
  • 4
    If you take the nut back off and turn it around, do you have any hex shape? just a hunch. More important might be why it fell of and if that means any damage to the crank or spindle
    – Swifty
    Nov 2, 2020 at 8:04
  • 2
    It's a $100 one from a Chinese company I got on Amazon. abdnChap is correct about the wrong way of the nut
    – Motoko
    Nov 2, 2020 at 16:35

1 Answer 1


That nut was put in the wrong way around.

This is what the nut looks like:

enter image description here

The hex end is what you should tighten with a approriate socket.

This is what it should look like when installed correctly:

enter image description here

If it ever get's super tight as it is now, to get it off, the easiest way is probably to cut it with a tremmel and chisel it out.

Good luck!

  • If you tightened it manually you probably remove it manually as well. If not manually you could try with tongue-and-groove-pliers (carefully!)
    – Carel
    Nov 2, 2020 at 15:13
  • 2
    Thank you, you are right on. This really gave me a Homer Simpson d'oh moment. The mystery is solved. It's tighten now. Will try working out on it again tonight
    – Motoko
    Nov 2, 2020 at 16:37

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