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  1. #1
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    Bottom Bracket size

    I bought an octalink bb and crankset to replace my existing square. The size of the square was 68x121 so i bought the same size octalink. Though it seems with the octalink the cranks are far out of what they were used to be. Do you have any idea how I can fix this? Thanks

  2. #2
    Fat-tired Roadie
    Reputation: AndrwSwitch's Avatar
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    Get the correct-sized bottom bracket for your crank.

    Most cranksets have a specification for spindle length. Two, a lot of the time. You're only interested in the spec for a 68 mm bottom bracket shell.

    If you don't have the documentation for your crank and can't find it on Shimano's web site (assuming this is a Shimano crank?) you can also just measure the clearance you have between your crank arms and chain stays and get a bottom bracket that's got a shorter spindle by almost twice that distance.

    A shop would probably be able to tell you which bottom bracket to use, and there's a good chance they'd even have it in stock. While not the current type on high-end bikes, Octalink was pretty common for a long time and not so long ago. And they're small. So, easy and sensible item to stock.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

  3. #3
    Tool
    Reputation: StageHand's Avatar
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    Square taper and Octalink are sized a little differently. Determine which crank you have, and then find the correct bottom bracket for it.

  4. #4
    R.I.P. DogFriend
    Reputation: jeffj's Avatar
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    If you can put your old BB and drive side carank arm back on, you can measure the distance from one of the chainrings to the side of the seat tube. Then install your new setup and then take the same measurement. Note the difference. If there is no offset to your bottom bracket (I don't know if any Octalink bottom brackets were even made with an offset, but some square tapers were), then you would take that difference and double it (because there will be an equal amount added to each end of the spindle) to determine proper BB spindle length. In other words, if you have a 113 BB and you need an additional 3mm to match the old chainline, then you would get a 119 bottom bracket.

    There you go, clear as mud

  5. #5
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    To be perfectly clear: For a proper fit, you must consider the diameter and the width when changing bottom brackets. Usually, the width of your frame will determine this. Sometimes you will use spacers on one side or the other, or both, to make sure your chain line in more or less straight.

  6. #6
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    I'm wondering why you wouldn't upgrade to a 2-piece system if you were going to replace both the BB and crank anyway.

    Is it a cost thing? I haven't looked at prices on cranks or BB's any time recently, so I have no idea how much more expensive it would be to go to a 2-piece crank over an Octalink.

    Is it a compatibility thing?

    Both?

    Sorry I'm not offering any actual help here.

  7. #7
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    From my experience you need a 3mm longer octalink to get the same chainline you had with a square BB. My bike went from 110mm square to 113mm Octalink.

    Tim

  8. #8
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    IF you do not have any issues with the shifting of your front gears, few mm wider should not be a problem. If you FD can not reach the biggest chainring, then I guess you'll need to find shorther BB axle.

  9. #9
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    The front der. is designed to work with a chainline of 47.5mm (old standard) to 50mm (current standard). This is the distance from the middle chain ring to the center of the seat post.

    Measure your current chainline. Subtract 50. Multiply by two. Subtract from 121.

    Ex. Your current chainline is 54. You need a 113 spindle length.

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