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Bicycle Mechanics Broken bottom bracket? Tacoed wheel? If you're having problems with your bicycle, or just need help fixing a flat, drop in here for the latest on bicycle mechanics & bicycle maintenance.

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Old 04-07-07, 04:19 PM   #1
travisthomas
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crank, bolt keeps coming loose

Hey, I'm relatively new to bike mechanics, and generally figuring it out by doing things wrong and then trying to fix them.

I overhauled my bottom bracket about three months ago, and just last night my left crank came loose while I was riding. I'm still suspicious that someone unscrewed the bolt, because I park my bike the street, delivered to someone's door around the corner, came back, and immediately noticed the crank was wobbly. I got a socket wrench and tightened it down, but less than five minutes later the crank was loose again. I figured I hadn't tightened it properly, and tried again. Same thing a few minutes later.

I took off my cranks today and put the bolts into the spindles without cranks and wiggled them around--the left side seems no more loose than the right, so I don't think I stripped the bolt or the inside of the spindle. The inside of the crank has wear, and slightly more on the left crank than on the right, so I'm thinking that may be my problem. Also, the washer inside the left crank won't come out anymore. It's been pretty worn for a while, but this is the first time I've been unable to get it out of the crank.

I think that the crank is probably just worn enough on the inside that it's working itself loose. Any other thoughts? Thanks.
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Old 04-07-07, 04:40 PM   #2
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yea, sounds like taper on the crank has rounded out

when you get those type of cranks its good to periodically check them to make sure theyre not loose...cuz once theyre loose theyll round out the crank and you can drag them out a bit...but eventually youll need new ones
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Old 04-07-07, 05:06 PM   #3
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did you recheck/retighten the crank after the intial installation? it's a very important step to do, and the most common cause of ruined (usually left) crankarms. cranks tend to "settle in" after installation, meaning the crank bolt is actually not as tight as it need to be any more, causing the arm to loosen and eventually wallow out on the spindle.
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Old 04-07-07, 06:11 PM   #4
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Yes, do not forget that since both sides use clockwise threads, the left-side crank fixing bolt is self-loosening, like the lugnuts (lugbolts in my case) on the left side of your car.
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Old 04-07-07, 06:26 PM   #5
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Originally Posted by dafydd
did you recheck/retighten the crank after the intial installation? it's a very important step to do, and the most common cause of ruined (usually left) crankarms. cranks tend to "settle in" after installation, meaning the crank bolt is actually not as tight as it need to be any more, causing the arm to loosen and eventually wallow out on the spindle.
i honestly don't remember at this point, but i'm aware that i should check it, and will make sure to check it next time.

thanks for the help! time to go get a new crank, i guess.
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Old 04-07-07, 09:25 PM   #6
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Hold your horses on the new crank. First try putting some Loctite Threadlocker Red on the threads, just follow the instructions on the package. This stuff could just stop the problem for way less then a new crank...unless you want to spend the jack for a new crank!
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Old 04-08-07, 10:19 AM   #7
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Beware that aluminum crank arms are far softer than steel spindles (I'm not sure from your post if these are aluminum.) If you ride on a loose arm, even for a few street blocks, you will round out the square hole in it, and it will then be unuseable. I rode on a loose crank arm for about 5 easy miles and ended up having to replace it. If it keeps loosening on the spindle (as opposed to just the bolt), then you've probably worn it too much already.
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Old 04-08-07, 10:36 AM   #8
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Originally Posted by froze
First try putting some Loctite Threadlocker Red on the threads,
Don't use red loctite unless you never plan on unscrewing the thing.. Ever. Red is for big bolts that allows you to use either an impact wrench or huge leverage to undo.
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Old 04-08-07, 01:22 PM   #9
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Don't use red loctite unless you never plan on unscrewing the thing.. Ever. Red is for big bolts that allows you to use either an impact wrench or huge leverage to undo.
a little heat fixes that. with that said, if the OP can see the deformation in the taper, chances are loctite will do little more than keep the crank bolt from unscrewing as the crankarm rocks back and forth.
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Old 04-08-07, 03:22 PM   #10
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Sorry, but the Loctite won't help. It's not the bolts that hold your cranks on the spindle ... it's a proper press fit. Having ridden on loose cranks means that you've lost the proper press fit. As another poster noted, the soft aluminum of the cranks deforms very quickly under the load of your legs against the hard steel of the spindle. Once you've rounded the inside of the crank holes, you're done. I did that once before after not properly tightening the bolt down and riding less than 5 miles. I was never able to get the cranks on solidly after that.
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Old 04-08-07, 06:51 PM   #11
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Originally Posted by s1nglespeed
Sorry, but the Loctite won't help. It's not the bolts that hold your cranks on the spindle ... it's a proper press fit. Having ridden on loose cranks means that you've lost the proper press fit. As another poster noted, the soft aluminum of the cranks deforms very quickly under the load of your legs against the hard steel of the spindle. Once you've rounded the inside of the crank holes, you're done. I did that once before after not properly tightening the bolt down and riding less than 5 miles. I was never able to get the cranks on solidly after that.

Yeah, I was the one who said that. Why do I get the feeling this specific post is attracting a lot of us who have learned through experience.
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Old 04-09-07, 09:37 AM   #12
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You need to replace the cranks.

Here's an interesting article on the issue: http://www.sheldonbrown.com/brandt/i...ng-cranks.html
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Old 04-09-07, 09:47 AM   #13
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Always tighten crankbolts to the specified torgue of the manufacturer. Anything less will jeapordise the the integrity of your crankset.
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Old 04-09-07, 03:41 PM   #14
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Always tighten crankbolts to the specified torgue of the manufacturer. Anything less will jeapordise the the integrity of your crankset.
Yes, there's no way you can develop the proper 25-33 lb*ft of torque required by most cranks with short allen-keys of the tiny double-sided Park crank-bolt wrench. Only an automotive-style 3/8" torque-wrech with 12" handle will allow you to get that bolt tight enough.
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