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What size square taper BB do i need?

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What size square taper BB do i need?

Old 05-04-18, 06:10 AM
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Abu Mahendra
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What size square taper BB do i need?

68 BB shell - what square taper spindle length do i need for the proper chainline with Shimano MTB cogset?
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Old 05-04-18, 06:24 AM
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Originally Posted by Abu Mahendra View Post
68 BB shell - what square taper spindle length do i need for the proper chainline with Shimano MTB cogset?
Which crank model in particular? Shimano has made a lot of different ones over the years.
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Old 05-04-18, 06:38 AM
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Originally Posted by ThermionicScott View Post

Which crank model in particular? Shimano has made a lot of different ones over the years.
it's not a Shimano crank. it's a rather generic one.
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Old 05-04-18, 06:55 AM
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Did the crank come off another bike? If so do you know that bike's BB axle length and the resulting chain line? Does the crank purchase info suggest a length? Do you have any BB on hand to test fit and measure chain line? Maybe on another bike?

You're kind of asking what shoe size you need without giving any real data.This is why if there is a LBS near you would be best to go there. Andy

Last edited by Andrew R Stewart; 05-04-18 at 06:56 AM. Reason: more content
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Old 05-04-18, 07:10 AM
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Originally Posted by Abu Mahendra View Post
it's not a Shimano crank. it's a rather generic one.
Why are you refusing to provide information that could help us answer your question?
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Old 05-04-18, 08:55 AM
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Each crankset is designed to use a spindle of particular length. The spindle length you need in this case will depend on what your specific crankset was designed for. Knowing the brand and model of the crankset, you (or we) could probably look it up online. Without that info, it's a trial-and-error process. Try a spindle, measure your chainline, then adjust by using a longer or shorter spindle as needed.
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Old 05-04-18, 09:05 AM
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Get both at the same time in the same box crank .. designs differ a lot in profile ..


Example; such as .. 80s's XT 730 used a longer, where XTR 900 came in to meet a shorter BB, by design..



UN26 BB are cheap... get one of each length...





....

Last edited by fietsbob; 05-04-18 at 09:10 AM.
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Old 05-04-18, 11:46 AM
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A proper chainline makes it so that the midpoint between the outer and inner chainring lines up with the center of your chosen cassette (for x2 cranks). For single cranks, the front chainring should align with the center of your casette. Pop your equipment onto your bike with an old BB and simply measure where the line from the midpoint falls on your casette. Order a size based off the size of the old BB you used as a refrence. To get a better grasp of the deflections involved, install a chain. Where does the chain land on the casette when the chain is a straight as possible etc.
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Old 05-06-18, 05:25 PM
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Originally Posted by Andrew R Stewart View Post
Did the crank come off another bike? If so do you know that bike's BB axle length and the resulting chain line? Does the crank purchase info suggest a length? Do you have any BB on hand to test fit and measure chain line? Maybe on another bike?

You're kind of asking what shoe size you need without giving any real data.This is why if there is a LBS near you would be best to go there. Andy
Never mind that 68mm shells are also used with French and Swiss-threaded BBs.
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