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  1. #1
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    newbie question about threaded open-bearing bottom bracket axles

    Hi, I'm new here and somewhat out of my depth, so please don't slate me.

    I am attempting to transfer the parts of my cheapo full-suspension mountain bike to a new, lighter frame.
    When I removed the bottom bracket I found that it was an old open bearing type, consisting of just an axle and two bearing cups.
    Also at the end of the square taper it has a threaded section which apparently is only found on childrens or cheap bikes (like mine ). The only reference I could find to this sort of axle was here

    http://www.utahmountainbiking.com/fix/crank2.htm

    I want to fit a new cartridge bottom bracket to my new bike but I dont know what dimensions I need to work with my chainset.
    The old axle was 146.5 mm end to end, but obviously this is of no use. The distance between the ends of the two square tapers is 125.5 but the only likely replacements I can find are 122.5 or 127.5.

    Would either of these replacements work or do I need to buy a new chainset aswell?

    Thanks to anyone who can help!

  2. #2
    Senior Member mechBgon's Avatar
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    First measure the width of the two frames' bottom-bracket shells. Are they both the same? If so, install your old bottom bracket, install your driveside crank, and see how it looks. Is the inner chainring roughly lined up with the outer face of the frame's BB shell? Is there a bit of clearance between the chainrings and the frame's chainstays?

    Once you've done that, you have at least a little basis for judging whether to pick a 122.5mm or a 127.5mm cartridge-bearing BB (yes I am glossing over tons of nitpicky details, before anyone yells about it).

  3. #3
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    Ok, I've done that and I see where you're coming from. The chainstays on the new frame definately flare out a bit more than they did on the old one, so I guess I'm looking at getting a longer BB, thanks.

    If anyone could let me know about any other details to look out for (the more nitpicky the better!) it would be much appreciated.

  4. #4
    Senior Member mechBgon's Avatar
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    The end result is what counts. So pick up a 127.5mm cartridge BB and slap it in there, if the 122.5mm wouldn't be long enough.

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