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  1. #1
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    New BB puts chainline out

    I have a 2006 Specialized FSRxc Pro - the BB was getting noisy so I decided to replace it.

    The spec for the bike shows it was supplied with an Octalink ES30, 118mm, 68mm shell. Unfortunately this seems to be discontinued, so looking at what was available I chose an ES51, Octalink, same spindle size, same shell size. As I understand it 118 is pretty normal for a full suss triple? The cranks are labelled as M470.

    Before I started I measured the chainline as 47.5 and after removing the cranks noted that the spindle projected 25mm on the drive side and 20mm on the non-drive.

    I fitted the new BB by screwing in the fixed cup on the drive side until it had about 1 thread to go, then the plastic cup on the non-drive, which I screwed all the way in. Then I torqued the drive side, and it went all the way in. The new spindle only projected 23mm on the drive side, but the same 20mm on the non-drive.

    Because the drive side spindle is shorter, the chainline is now 45.5 and the dérailleur won't go in far enough to change to the granny.

    Trying to figure out what is wrong, I've measured the old BB, and it's about 120mm. Looking carefully at the badly scuffed label, I think it's 121mm. Which explains why the 118 is too short. I haven't had the bike from new, but even if the previous owner has put in a non-standard BB, why might the spec size for the bike not work?

    I guess I need to go and buy a 121mm BB - any other suggestions, or any recommendations for what I should buy?

    Thanks, Chris

  2. #2
    Former Bike Wrench
    Reputation: mtnbiker72's Avatar
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    The Shimano M470 requires a 121mm spindle for a 47.5mm chainline and a 126mm spindle for a 50mm chainline.

    You'll need to buy a different BB

  3. #3
    mtbr member
    Reputation: sanjuro's Avatar
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    I do not trust the specs, since they will change during production runs. I always measure.

  4. #4
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    Thanks, both.

    mtnbiker72, I've put back my old BB and ordered a 121

    sanjuro, agreed, I would measure if I could, but didn't have callipers available to measure it in place, wasn't sure what state the old one was in and didn't want to risk taking it out until I had something to replace it with.

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