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  1. #1
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    Frankenbike Project: BB, Square Taper, Octalink, etc. questions

    Okay, so I bought a 2008 Rockhopper frame because it was a great deal ($30) and figured I would carry over what I could from my 1993 Yokoma Yosemite MTB (Deore parts). The hardest part is this BB/Crank issue.

    I got the BB off the Yokoma and tried to put it on the Rockhopper, but part of the threading was hanging out the Rockhopper tube. The BB seemed to be the same total length (126mm long) as the Rockhopper BB is listed (68 x 126mm) and the threading fits perfectly, but the problem is this; the Rockhopper frame tube that the BB goes in is 2 10/16" (~67-68mm) and the Yokoma tube is 2 13/16" (~81mm) long. So a good chunk of the threading (3/16") is sticking out of the Rockhopper tube.


    So I can get the recommended Shimano BB for the Rockhopper new for really cheap ($13 shipped), but it is Octalink whereas my old Deore crank is square taper. My whole goal in this project was to make a new bike for as cheap as I can (with decent parts) and buying a whole new crankset to match the BB is my last option.

    Any thoughts?

    And yes, I can only assume that some of my explanation demonstrates that I am not a skilled MTB mechanic. I am simply a novice that is enjoying a fun project. Thanks for any help!

  2. #2
    just pokin' along desertdork's Avatar
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    If you want to use the Deore crank, you need a JIS square taper BB that will fit the frame. Your Rockhopper has English BB threads and a 68mm width, but you'll need to determine the correct spindle length for your particular crank. Sheldon Brown lists 118mm and 122.5mm as possibilities for the Deore square taper cranks of that era. Shimano UN-54 bottom brackets are sealed cartridge units that are available in these sizes for under $25. You may find something similar for a few dollars less.

    I suspect you're referring to the Specialized component list for your Rockhopper, and that's how you came to think an Octalink BB is needed for your Rockhopper; that's because the '08 Rockhopper was spec'd with an Octalink crank for that year. The BB interface must always match the crank, and the width of the BB spindle is also dependent on the particular crank. Also, check your measurements; I believe the 2-13/16" width you give for the Yokota is very close to 73mm which is the common "oversized" BB shell width for mountain bikes.

  3. #3
    coprolite fietsbob's Avatar
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    +1 MTBs have had both 68 and 73 mm wide BB shells as standard over time.

    Spline BBs are made in few lengths, square taper a large variety of lengths
    Last edited by fietsbob; 12-26-10 at 11:29 AM.

  4. #4
    we be rollin' hybridbkrdr's Avatar
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    Just some random notes:
    Shimano BB-UN54 > BB-UN26

    From what I read, Hollotech II is stronger than square taper but Hollotech II bbs aren't as well sealed as square tapers. The Hollowtech IIs are easier to remove/replace.

  5. #5
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    Thanks

    Thanks all. I think that UN54 will work. Now I just have to decide whether to drop $20 on a BB that I am going to have a hard time finding a crank rings to match later on when the current, fairly worn, rings give out. Anyone know if you can find rings at a decent price to fit the old Shimano SG system? If not, I may just need to bite the bullet and buy a new crankset/BB system.

  6. #6
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    Quote Originally Posted by hybridbkrdr View Post
    Just some random notes:
    Shimano BB-UN54 > BB-UN26

    From what I read, Hollotech II is stronger than square taper but Hollotech II bbs aren't as well sealed as square tapers. The Hollowtech IIs are easier to remove/replace.
    Hollowtech II is the external cup bottom bracket that has the spindle integral with the crank arms. The OP would require a completely new crank and bottom bracket and none of these are particularly cheap.

  7. #7
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by ktandchris View Post
    My whole goal in this project was to make a new bike for as cheap as I can (with decent parts)

    Any thoughts?
    Yup. Put a 2mm freewheel spacer on each side of the bottom bracket.

  8. #8
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    Spacers

    Quote Originally Posted by Retro Grouch View Post
    Yup. Put a 2mm freewheel spacer on each side of the bottom bracket.
    Now that is a great idea! Don't I need 2.5mm spacers?

  9. #9
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by ktandchris View Post
    Now that is a great idea! Don't I need 2.5mm spacers?
    I guess that would be ideal but I know for sure that 2 mm spacers are get-able and I don't know for sure about 2.5 mm spacers.

  10. #10
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    Quote Originally Posted by Retro Grouch View Post
    I guess that would be ideal but I know for sure that 2 mm spacers are get-able and I don't know for sure about 2.5 mm spacers.
    2.5mm bb spacers are widely available due to the massive amounts of external bb cranks out in the market.

  11. #11
    Senior Member BCRider's Avatar
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    If you decide to just replace the whole works then go for a cheap version of one of the new external bearing setups. You can get them in off brands or "house" brands made by major names but with someone elses name for well under $100 on Ebay, through the online bike shops or perhaps locally depending on your city.
    Model airplanes are cool too!.....

  12. #12
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    Quote Originally Posted by reptilezs View Post
    2.5mm bb spacers are widely available due to the massive amounts of external bb cranks out in the market.
    Well the spacers worked and my local bike shop gave them to me for free! Thanks

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