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  1. #1
    over researcher
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    68 to 73 BB adapter - Shimano P/N 1T498020 -or- just use UN73 with old UN72 keeper

    I used a UN72 68x113 bottom bracket for my current bike. The frame I will be buying in the next week or two takes a 73x113, and since I'm building on a budget, I plan on moving over my old Turbine LC cranks until I get around to buying the Deus XC.

    The replacement to the UN72 is the UN73, but it has the plastic, well, the plastic other side jobby. I've seen an adapter on eBay that allows you to run a 68mm BB in a frame that requires a 73mm BB, but I'm wondering if the spindle would no longer be centered in the shell. Could I just buy a UN73 and use my UN72 keeper so I don't have that garbage plastic?

    Thank you.

    Roy

  2. #2
    mtbr member
    Reputation: GearHead's Avatar
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    Quote Originally Posted by royta
    I've seen an adapter on eBay that allows you to run a 68mm BB in a frame that requires a 73mm BB, but I'm wondering if the spindle would no longer be centered in the shell.
    If you use a BB designed for a 68 mm BB shell width on a bike with a 73 mm BB shell width, it will shift the crank over 2.5 mm. This will cause your chainline to be 2.5 mm further out and a difference of 5 mm between the distance of the left vs the right pedal to the center of the frame. As long as your chainline stays around 47-48 mm shifting will still be fine, if it gets upwards of 50 mm you may experience shifting problems like all of the new external BB cranksets. A lot of people are offsetting the new external BB cranksets 2.5 mm towards the non-drive side with no noticeable difference in the different pedal offsets of the left vs right side.

    What is your current chainline?

    You may be able to use your current 68 mm BB with your new frame as is. A lot of frames have the BB shell threaded pretty far into the BB shell. If the threads go in far enough you can just thread the non-driveside BB adapter cup in 5 mm further than normal until it reaches the BB.

    Quote Originally Posted by royta
    Could I just buy a UN73 and use my UN72 keeper so I don't have that garbage plastic?
    The plastic BB adapter cup seems like junk compared to the metal one, but they work just as well. I have never broke one even when tightening them to the recommended 50 ft-lbs of torque.

    Your old metal BB adapter cup should work with the new BB unless they have added the new nylon seal on the BB shaft/bearing like they did on the newer Octalink BB's.
    There is no added value to my participation - in fact, just more confusion.

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