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Hello all,

I am very new to biking and was hoping to get help with an issue. Any input is much appreciated.

I was recently riding my bike, and my crank arm fell off. I grabbed the pieces that fell to the ground then headed home and put everything back together (or so I thought) the next time I went riding the crank arm fell off within 5 minutes of riding.

Im guessing maybe I'm missing a piece? Maybe a few? I really don't know. Thanks in advanced.
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· Elitest thrill junkie
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When you ride with a loose crank arm, it destroys the interface between the spindle and the crank. It will constantly loosen up thereafter in 98% of the cases.

If this was a new bike, the bike shop did not torque the crankarm correctly or check the torque. If it's been a while, it's something that you'd be responsible to be checking. These press-on spindles take a lot of force to get set correctly and some inexperienced riders are not used to what that means and how much torque it takes.
 

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Is that a tapered bottom bracket spindle? Judging by the amount of taper that is showing in the photo of the crank-spindle interface, the crank hasn't been tightened nearly enough. BUT if it was ridden loose, the crank arm spindle hole might have been damaged to the point that it can't be tightened securely.
 

· since 4/10/2009
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I agree that crank arm is probably trash. And also that it hasn't been tightened enough. Torque spec for crank bolts on that type of crank is no joke. I agree, if it's new, then take it to the shop. If not, then you're going to need a new crank arm. and I'd also not be trusting that the drive side is appropriately tightened. I'd put a torque wrench on it and make sure it's around 50Nm.

I'll bet there's supposed to be some kind of plastic cap over the crank bolt to help keep dirt out of there. But it's not mission critical. It would just keep dirt out of the threads (which are for a crank extractor). Probably lost it awhile ago.
 

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You think OP would be able to find the right one on Ebay and then figure out the mechanics and torque it properly? I'd take it to a shop myself.
How much do you think a shop would charge and how much do you think that bike is worth? A shop may have a spare left square taper arm in it's parts bin, but if not, you're probably looking at fairly hefty bill relative to the cost of that bike. (Assuming it's not warrantied.)

It's about the easiest thing to put on a bike. Simple instructions:
  • Remove the crank arm. The OP likely doesn't have a crank removal tool, but it was already falling off, so that should be easy. If it doesn't come off easily after removing the crank bolt, step on the arm or ride around carefully until it falls off. Confirm that it's a square taper interface on the bottom bracket and crank... otherwise these instructions need to be adjusted.
  • Remove the pedal. It's a left hand thread, so righty-loosey. Assuming it doesn't have a hex end, an adjustable wrench might work. Check now if it will work. If it's too thick, buy a pedal wrench off ebay ($12.95 for a Park 15mm) at the same time as you order a crank arm. It might be easier to remove the pedal before removing the crankarm (you can step on the wrench).
  • Look on the back of the crank arms and see what the length is. Probably 175, 170, or 165. If not stamped, use a metric ruler to measure from the center of the crank hole to the center of the pedal. It's not a Euro or track crank, so the square tape is probably a JIS, which will be the most common, especially for cheap replacement arms. It's a left arm, so you don't have to worry about chainline, just that it doesn't hit the chainstay. It's a curved arm that gets wider from the bottom bracket end to the pedal, so get a replacement that looks like that.
  • If it's 175, this will likely fit for $17.95 shipped. It's JIS square taper, 175, and curved. If you need a 170, this seller and others also has that (search "170 left crank arm square taper black)

ORIGINAL! ALLOY LEFT CRANK ARM 175MM SQUARE TAPER DESIGN BLACK. | eBay

  • When installing, lube the crank bolt threads, and torque the crank bolt down hard. It needs about 35 ft-lb. So if you are using a 4-inch long 8mm hex key, you'll need to step on the end and use much of your body weight (~100lbs worth).
  • Put the pedal back on, lefty-tighty. It also requires a decent amount of torque, about 25ft-lb.
 

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Hard to say whether those steps will be useful to the OP. Perhaps. We don't know much about the cost of the bike originally but there is a value to having it done quickly and correctly by someone who can just order the part the same way the OP would.
 

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I discovered a simple solution to this issue 10 years ago and I have been riding the same bike, with no issues, since.

Get some Teflon tape. It's white (typically) tape used by plumbers on threads to seal them. Wrap it on the spindle and get a decent build-up, but where the crank arm can still press on. Try to wrap some of the tape over the spindle end a little to grab and hold while the crank arm presses on, but leave the hole open for the screw. I think what is happening is the crank presses down and the Teflon tape squishes into the small spaces stabilizing the crank arm and works kind of like a spring, putting pressure on the screw like a lock washer. I'm not completely sure why it works, but it works. I even tested it on a slightly warped crank arm that I rode wobbly on and it held fine.

I tried all of the other solutions, thread locker, new crank arms nothing else worked for me.
 
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