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I'm looking for an upgrade to my crankset and I would also like to replace bottom bracket. My current setup is Shimano M522 (3 x 10) which I would like to replace with SLX M677 (2 x 10) and current bottom bracket is First R30. Are there any compatible bottom brackets from Shimano that can fit in my bike frame (73 mm wide) and which will be compatible with SLX M677 crankset? Shimano recommends BB-MT800-PA as compatible press fit bottom bracket but according to specifications it is available in 89.5 - 93 mm.

I'm confused about options and alternatives - please help.

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  • What bike do you have?
    – DWGKNZ
    Commented Sep 20, 2016 at 4:54

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Sounds like you have a BB30a bottom bracket (30mm spindle and 46x73mm shell). In order to run any Shimano Hollowtech II cranks you will need some form of adapter to reduce to a 24mm spindle. There are a few available (obviously your existing one) as well as some such as WheelsMfg.

The Shimano bb-mt800-pa is too small a diameter for your BB (41mm vs 46mm), you need an adaptor. The 89.5-93 is the full length to the outer shell. On a 68mm bb you need spacers between the bb and crank.

Both the M522 and M667 are Hollowtech II so the new cranks will work with the old BB.

Other considerations for shifting from 3x10 to 2x10 are chain line, front derailleur and shifter.

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