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  1. #1
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    Crankset doesn't stay on bottom bracket!

    I have a 1997 Norco Scrambler, as far as bikepedia shows. The original crank is a TX70, I ordered a FC-M191 48/38/28t 170mm crankset to install.

    Techdocs states that my bike uses a bb-un26 bottom bracket, and so would the fc-m191. Some how the left pedal doesnt slide all the way onto the bottom bracket. It seems that the bottom bracket starts smaller than the opening on the left pedal, then is too big for the pedal to slide all the way in and sit comfortably on it. This has resulted in the bolt coming loose on my recent bike trips and the loose crank arm warping the inside of the bottom bracket attachement hole slightly.

    Is there something about the bb-un26 I dont know? If I go buy a new one, will I have the same problem? Are there other BB's I can rely on to work better?

    If I need a bottom bracket tool, can someone point me to the proper park tool for this? Is it BBT-22?

    I believe it calls for a BB that is 122.5 mm. Thanks for your help!

  2. #2
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    Odds are that your BB is fine, but the crank may be toast.

    It seems that you have a square taper bottom bracket. The cranks (not pedals, those are where you put your feet) have a similarly tapered square hole, and mount by being forced onto the taper where the jam fit keeps them locked in place, with the bolt to keep them from loosening. Once they're ridden loose for any length of time (can be very short) the square hole in the crank arm gets distorted and it becomes impossible to tighten them effectively.

    You probably need a new crank arm, but the steel spindle of the BB is most probably fine.
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  3. #3
    Senior Member Homebrew01's Avatar
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    2 reasons I can think of for it coming loose.
    1. It was not tightened properly
    2. Crank arm has a crack where it attaches to the bottom bracket axle and won't stay tight (happened to me)
    Bikes: Old steel race bikes, old Cannondale race bikes, less old Cannondale race bike, crappy old mtn bike

  4. #4
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by FBinNY View Post
    Odds are that your BB is fine, but the crank may be toast. You probably need a new crank arm, but the steel spindle of the BB is most probably fine.
    That happens often enough that, if you're not fussy about having a perfect match, a local bike shop may have a replacement in stock. That's about a $20.00 part. There are 2 things to look for:
    1. The crank arm length will be stamped on the inside near the pedal hole. It'll be something like 172.5 or 175. Make sure that you get a match.
    2. There's squares and there's diamonds. Make sure that your new crankarm matches what's on the other side.

    The torque spec for crankarms is generally around 30 lb/ft. That's more than you're going to get using a six inch long allen key. Imagine a 30 pound weight hanging off the end of a foot long wrench. Tighten it that much.

  5. #5
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    Quote Originally Posted by Retro Grouch View Post
    That happens often enough that, if you're not fussy about having a perfect match, a local bike shop may have a replacement in stock. That's about a $20.00 part. There are 2 things to look for:
    1. The crank arm length will be stamped on the inside near the pedal hole. It'll be something like 172.5 or 175. Make sure that you get a match.
    2. There's squares and there's diamonds. Make sure that your new crankarm matches what's on the other side.
    .
    To clarify, you probably can't get get a cosmetic match, but you can get a perfect mechanical match.

    By squares and diamonds, he means the orientation of the square hole. Most are with the crank centered on the corner of the square (diamond) but some are centered on the flat of the square. A mis-match would have the 45° off from where it needs to be.

    Don't over-tighten the replacement, but at the same time, if you hear any clicking or creaking once per revolution as you pedal, that's your crank coming loose. Attend to it immediately. It isn't rare for newly installed cranks to settle and loosen slightly even if properly tightened, so be ready to deal with it. Usually htis happens only once, sometimes twice, but once settled a crank will stay tight for eons.
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  6. #6
    Can'tre Member 3alarmer's Avatar
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    Quote Originally Posted by FBinNY View Post
    but once settled a crank will stay tight for eons.
    Especially when you want to remove it again........
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  7. #7
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    Quote Originally Posted by polskon View Post
    ...Some how the left pedal doesnt slide all the way onto the bottom bracket.
    If it's square taper, it's not meant to. The end of tha axle should stop a few mm short of the recessed bottom of the hole in the crank.

    Quote Originally Posted by polskon View Post
    ... This has resulted in the bolt coming loose
    Unlikely. Insufficent torque at assembly is the overwhelmingly most common cause of cranks working loose.

    Quote Originally Posted by polskon View Post
    ... the loose crank arm warping the inside of the bottom bracket attachement hole slightly
    That, however, is a common occurrence.

    Quote Originally Posted by polskon View Post
    ... If I go buy a new one, will I have the same problem?
    The BB is probably fine, but the crank is likely shot. Get the torque right and the next crank will do fine.

    Quote Originally Posted by polskon View Post
    ... Are there other BB's I can rely on to work better?
    Not really, they pretty much all have their different issues. In fact, quite a few would hold the square taper design as superior to some of the current designs.

  8. #8
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    Everyone has put such great information in their posts. Thanks to all that responded!

    I ended up going to a local bike shop and they installed a new crankarm of matching length. The mechanic mentioned the bolt on the other side was coming loose too, so he tightned it. I didn't even think to check so its a good thing these guys know what to look for!

    I'm going to try and tighten both bolts on the crankset after a week so that this doesn't happen again.

  9. #9
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    Quote Originally Posted by polskon View Post
    I'm going to try and tighten both bolts on the crankset after a week so that this doesn't happen again.
    You need to be wary about that. If the bolt is actually backing out, that needs to be addressed, but repeatedly re-torqueing the crank bolts is rumored to eventually lead to cranks splitting. The theory is that the metal will deform a little over time, allowing a repeated tightening to push the crank further down the taper until they split.

  10. #10
    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by polskon View Post
    I'm going to try and tighten both bolts on the crankset after a week so that this doesn't happen again.
    You just need to check the torque on the bolt just once or twice and that's it. Bolts are like really strong rubber-bands. You need to tighten them enough so that they're stretched. Then they try to shrink back to their original length and it's this pulling-back action that clamps everything together. Also generates sufficient friction to prevent self-loosening. The ONLY way to know if you've got the crankarm-bolt to the proper tension is with an automotive-style troque wrench with at least 10-12" handle. Itty bitty L-shaped allen-keys will not work. That's why I recommend people use hex-head bolts rather than allen-key bolts on their crankarms. Prevents you from using the wrong tool. Just do a search on here for "loose crankarm" and see how prevalent and common this issue is. And most of the problem comes from not using the proper tool correctly.

    Here's a list of recommeded torques: Park Tool - torque specifications. Note that crankarm-bolts are some of the tightest fasteners on a bike.

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