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I am in the process of replacing the cranks, chainrings, and BB for my 2005 Trek 3900. I have heard of 68mm and 73mm cages, and I am confused if either the BB's are bike-specific or not. What I want to know is: are all BB's compatible with all bikes? Are the cage sizes important for cranks to fit or to fit the bike? Anything known about BB's, cranks, and how they all work together would be appreciated. Thanks.
 

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Former Bike Wrench
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There are two widths of mountain bike BB's...68mm (most common) and 73mm. Both use the same English threading that is left hand thread on the driveside of the bike. Some BB's can be used for both sizes...some are width specific...varies from manufacturer to manufacturer. With the external bearing BB's...its one size fits all and they come with the two piece cranksets. With three piece cranksets (two crankarms and a internal bearing BB) the BB is sold separate and as mentioned before, may or may not be specific to 68mm and 73mm sizes. You also have to match the spindle length to the crankset.
 
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