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Any idea what kind of tool should I but to fix the loose left crank on my bike? I found this https://www.amazon.co.uk/Park-Tool-CCW-5-Crank-Wrench/dp/B0012Q5XAS but not sure if it's gonna suit. Or is it enough to buy only nut driver?

Crank pohoto

  • Aside - don't ride the bike till this is resolved. If you do, it will make things worse. – Criggie Oct 18 at 7:11
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The Park tool will work. A 14mm socket and a ratchet will also work. If the crankarm has been loose for some time there is a chance the movement has ruined the crankarm by distorting the square hole. I would attempt to tighten the bolt then see if the arm still wiggles on the crank axle. If there is no movement, ride for a few days then recheck the bolt tightness and for wiggle. If everything is good recheck in a week. then after 30 days. If everything is still tight you should be all set. It is a good idea to check the drive side while you are at it. As the bike looks fairly new I would recheck all the hardware, stem and bar clamps, pedals, wheel QR etc.

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    +1 for the point about a loose arm messing up the hole; that bit me once... – DavidW Oct 17 at 22:16
  • Most workshops will have the socket set already, and depending on the nut, a 9/16" may fit (though it may be a poor fit) – Criggie Oct 18 at 7:13
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If it is just the crank arm that's loose, then tightening the retaining nut at the base of the arm should be enough. A regular socket (of the appropriate size, probably 14mm) is all you need for that.

You do want to make sure that it's just the crank arm and not the bottom bracket. (Or the pedal itself.) If you hold the other pedal and only the left crank moves, you're good, but if they both move the bottom bracket may need servicing.

It's a bit hard to tell from the photo, but it looks like your pedal may not have an 8mm hex driver, in which case you'll need a pedal wrench if it's loose.

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You have a '3-piece crank'. Each crank is attached to the axle on a square or splined tapered interface. The central bolt pulls the crank onto the taper.

The bolt is just done up with a standard wrench or socket, but it really needs to be tightened to the proper torque which requires a torque wrench. You can ask a local bike repair shop to do this for you.

As others have said when a cranks gets loose on the axle it can deform, will not grip the taper properly and repeatedly comes loose. The only solution is a new crank arm.

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