68mm BB in a 73mm shell ?

RedDev21
RedDev21 Posts: 7
edited April 2008 in MTB workshop & tech
Hi all, is it ok to fit a 68mm bottom bracket (Octalink splined) into a 73mm BB shell? I originally had a 73mm BB in the shell but the spline was too short which made the small chainring touch up against the side of the frame. I had a spare 68mm BB with a slighly longer spline so I fitted this and it cured the problem of the chainring rubbing on the shell. There doesn't seem to be any obvious problems, nothing is loose. However, as Im new to this bike building, just wondered if it will cause any problems?
Also, when I swapped BB's over, I just removed the drive side and replaced that, leaving the old cup in the non-drive side as the cup is threaded and couldnt get it out. Both BB's were Shimano sealed cartridge units, just slightly different sizes. (68mm x 118) & (73mm x 113).
Hope this isnt too confusing! :?

Comments

  • nicklouse
    nicklouse Posts: 50,675
    not really as the bearing on the non drive side will not be properly supported. also you may find that the non drive arm catches on the cup.

    BUT as the axel is over length and it works then the only consideration is the lack of support of the BB.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • RedDev21
    RedDev21 Posts: 7
    So is there any way I can put the correct BB size back in the shell and stop the small chainring rubbing against the frame?
  • hoathy
    hoathy Posts: 776
    The lack of support for the non-drive side is really bad though, as all of the stress is nowing to be going through the driveside thread with more leverage than it shoud ever take. I think running it like this for any length of time will lead to a messed up drive side thread. If it were me, i'd go and buy a new bottom bracket. asap.
    - Kona Hot '96 - Marin Rift Zone '09 - Cannondale Synapse Carbon '06 - Kona Caldera '98 - Kona AA '94 - Dawes Kickback II - Cannondale BadBoy '11 - Genesis iOiD SS -
  • nicklouse
    nicklouse Posts: 50,675
    yes get the correct 73mm bb with the correct axle length.

    Frame? cranks? year?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • RedDev21
    RedDev21 Posts: 7
    edited April 2008
    Im building up an old Trek 4100 frame (2003) for a commuting bike for work as I dont want to do 20 miles a day on the road on the full susser. Beginning to think it would've been alot less hassle and cheaper to have just bought a new bike but Im determined to finish it now! The cranks are hollowtech1 octalink deore + deore front mech.
    Thanx for your help !
  • nicklouse
    nicklouse Posts: 50,675
    what year deore? got a picture.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • RedDev21
    RedDev21 Posts: 7
    Originally taken off 2008 Specialized FSR expert. Have upgraded the Spesh and am using its parts on the new build.
  • nicklouse
    nicklouse Posts: 50,675
    are you sure the trek is a 73mm bb? i thought they were normally 68?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • hoathy
    hoathy Posts: 776
    I have a 4500 for 2006, thats a 73mm, so its definately possible.
    - Kona Hot '96 - Marin Rift Zone '09 - Cannondale Synapse Carbon '06 - Kona Caldera '98 - Kona AA '94 - Dawes Kickback II - Cannondale BadBoy '11 - Genesis iOiD SS -
  • RedDev21
    RedDev21 Posts: 7
    Just measured the width of the shell with ruler, it is 73mm
  • nicklouse
    nicklouse Posts: 50,675
    just that i have to say i have never noticed that the treks i have built were 73mm. But i was not looking :wink:

    there reason for the ask about the year was that some year cranks on the HT1 had some really odd axle lengths... upto 127 axles...

    if you measure what the current chain line is at the front with the 73mm bb then that will give you an idea as to the axle length needed on a 68mm bb.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • skyliner
    skyliner Posts: 613
    I don't think there'll be a support issue on the cartridge, as it has the non drive cup from the 73mm version fitted. The cart is the same length on both BB's and the non drive cups are made longer on 73mm variants to accomodate this.
    My concern is that the chainline will be out by about 2.5mm to the drive side because of the axle position on a 68mm BB. and as Nick said, you may get rub on the crank/BB cup, or heel rub on the chainstay on the non driveside.
    If it's an OEM Spesh Octalink chainset, it should run a 118mm axle with the correct shell width (73mm on the Trek) to get perfect alignment.
    It's nice to be important, but it's more important to be nice.

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