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  1. #1
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    XTC29er carbon frame BB size

    Hi,
    I bought a brand new carbon XTC29er frame.
    I'm totally confused on the BB size...maybe I got a special modified frame?

    My frame measures 88.5mm wide at the bottom bracket and the pockets are 13.5mm deep for the press cups? is that correct? I cant find info anywhere. I got this funny feeling this is something not OEM?

    I bought Shimano SM-BB91-41A pressfit bottom bracket. It will work however do I have to run shims? what size? probably depends if 88.5mm is the correct frame size?

    I'm going to run M970 XTC cranks as a 1x9 setup.

    Please advise if my BB width is correct or do I have something special?

    Please advise what BB should be used with this frame and with/with out spacer?

    Thank you,

  2. #2
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    If this is the regular composite model:
    http://dfccyclesandfitness.com/image...ctfrm_11_z.jpg

    Just put those BB91 cups in with one spacer, you don't need to put the plastic inside the BB since the BB shell is fully sealed-aluminum insert.

    This will take the Shimano HT2 crank in.

    EDIT: edited post, saw the BB width of 89.5
    Last edited by Goran_injo; 01-23-2013 at 04:19 AM.

  3. #3
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    mine is 89mm wide thereabouts measured with calipers, same as you pretty much. drive side appears to have a black spacer on it. It has xt cranks. spacer measures about 2.5mm. Im not really sure if thats a spacer or not (maybe itts just the cup), but the drive side cup isnt as flush with the frame as the non drive side anyhow. dont know if that helps. as good as i can do.

  4. #4
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    Check this doc out, it will explain it:
    http://techdocs.shimano.com/media/te...9830625426.pdf
    Last edited by Goran_injo; 01-23-2013 at 04:20 AM.

  5. #5
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    It appears as if my frame is 1mm narrower than Giant sells them at. maybe somebody had a q-factor difference in what they ran. I bought this frame off a racer as his spare..maybe he had something special. Anyways...I will mount the bb91 in it with the 2,5mm shim on the RH side just like the instructions for frames that are 89.5mm wide. Then I will run two .5mm shims over each side of the crank spindle to make the right width for the XTR cranks.

  6. #6
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    Update - I ended up putting on the BB91-41A as if the frame was 89.5mm wide.

    RH Side: I then made a .020" shim to put between the crank and the BB so the offset was correct. Based on both BB cups being 13.5mm I assumed that Giant (for some reason) made it 1mm narrower equally spaced per side. I then used a smear of 3-bond 1211 (best silicone on earth) on the shim to help seal it. The Cranks have an oring to seal against the BB so I needed something between the shim/BB

    LH side: Nothing used the Shimano adjustment thrust system to take up the gap.

    Result Cranks feel good and are centered as OEM.

  7. #7
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    The shell is 89.5mm. I used a BB92 with one 2.5mm spacer on the drive side. I run XTR m985 cranks, and it's a perfect fit. The 2.5mm spacer is to get the shell to 92mm. It was VERY simple. I'm not sure of the differences of BB91 and BB92, if any. I'm not sure why you guys are have any trouble.

  8. #8
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    Quote Originally Posted by mucky View Post
    The shell is 89.5mm. I used a BB92 with one 2.5mm spacer on the drive side. I run XTR m985 cranks, and it's a perfect fit. The 2.5mm spacer is to get the shell to 92mm. It was VERY simple. I'm not sure of the differences of BB91 and BB92, if any. I'm not sure why you guys are have any trouble.
    You must of not read my post good enough. My BB was 88.5mm and I was confused since the BB paper work said 89.5mm . Thus I first wanted to find out why my frame was different...then I wanted to make it correct which I did. Life is fine now!

  9. #9
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    I read it just fine. I can't imagine why your shell would be 1mm off. Maybe the previous owner had the shell faced and had too much taken off. Any glad you got it worked out. Still a PITA though.

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