Around 2/3 months ago I put a new crankset on my bike including new square taper bolts and now every time I ride the non drive side bolt comes loose as I’m riding until it falls off. It does this eventually no matter how hard I tighten the bolt. Do I need a new crank arm or could it be the bolt or bottom bracket? Thanks in advance.
by _fistpunch
8 Comments
You need a new crank arm. Sorry but it is damaged.
Did you use a torque wrench when you first assembled it? Crank bolts, as you have discovered, are the most important bolt to check torque on.
When a crank falls off you need to replace it. The square taper portion gets rounded slightly and will no longer have a secure fit on your bottom bracket. The bottom bracket should be fine as it’s made of steel so it’s harder than the cranks. When you install your new cranks tighten them down really tight. So tight it almost feels like it’s going to break the bolt.
That crank arm is toast! It has been ridden while being too loose. This has caused the square taper to deform. No matter how tight you get the bolt now it will come loose after some time.
The crank arm is deformed and will never stay tight. Typically the arm deforms, but if the crank spindle is deformed it will ruin another arm. Fit a new arm on the spindle (clean the grease off the spindle first) and check for a good solid fit. If it’s wobbling, it may be a sign of a worn spindle. Tighten the arm to 30-40 ft lbs. Stand on it with cranks horizontal (both forward and back) and check for any movement. Any movement is a bad sign. Tighten the crank bolt again after a ride or two – if you didn’t do this last time, it may be why the arm loosened and deformed. Check you other arm as well.
This is quite normal when you ride the crank loose until it falls off. The square taper shape gets damaged and it’ll never stay tight again. You need to replace the crank arms, but usually the bottom bracket is fine since it’s steel which is a lot harder than the aluminum arms.
I’m going to differ from the other posters here, your crank arm is fine. It looks like you have the crank bolt broken off in the spindle of the bottom bracket. There’s a couple ways to tackle this, get a new bottom bracket or drill and easy-out the bolt stub in the non-drive side and get a new crank bolt.
The crank arm can round out, but it doesn’t look like yours is.
If you can’t wait for parts or can’t afford parts, try putting some plumbers ptfe tape on the square taper shaft, tighten the bolt, ride a few minutes, tighten, ride, tighten, until it stays tight, then tighten it once per ride.
The good news is you get to shop for some new cranks!