Left Cranks


New Member
Jun 14, 2014
I need some help. I have a mongoose mountain bike whose left crank arm recently loosened an eventually fell off. I re tightened only to find that it would not stay tight again. It held for only 30 seconds or so then fell. I then bought a new arm as I heard that it may have had a taper. The new one was pretty easy to get on, and after a minute of riding it fell off. I'm extremely frustrated and want to fix my bike, but I have no idea. If someone could help, that'd be great.
Thanks for the photo. It helps.

Your new crank looks like it's starting to meet the same fate as the original, but I can't say for sure if the crank and bottom bracket taper are both lost causes. I'm inclined to give them one more chance. Here's how.

Clean the crank and bottom bracket tapers thoroughly with a good degreaser, and then dry. Insert the crank dry, no grease at all on the tapers. Grease the threads on the crank bolt, thread it into the hole, and tighten the hell out of it with the proper crank bolt tool or a thin-walled 14 mm socket. Then ride it around for a while and tighten it some more. And while you're at it, tighten the bolt on the other side.

If it still falls off, it's probably because the tapers or threads are damaged beyond redemption. Replace the bottom bracket, crank arm, and bolts, and try again.
Square taper cranks HAS to be mounted with the correct torque. If they begin to wiggle, the fit will get destroyed very quickly. And with a messed up fit, they'll never stay on again. Easy fix is to buy a new left crank. Get the length right, and the hole orientation right. Can be either Square or Diamond. And since you've had troubles, see if you can get someone with either experience or access to a torque wrench to help tighten it down. The good news is that the BB is hardened steel, a lot harder than the aluminium alloy in the crank arm. It'll most likely have survived just fine. So unless the threads are damaged(possible but rare), you don't need to mess with the BB.

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