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First off I'm thinking 2 x 10 speed XTR drivetrain but using XX crankset. Will this work with the XTR front mech?
Also, what's the difference between 156 164 166 XX cranks?
yup
Also, what's the difference between 156 164 166 XX cranks?
q-factor
1) should do
2) different Q factors (widths), be aware that some frames may not like the narrower ones, but IMO narrow is good,
Q factor? explain please
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Generally speaking, narrower Q-factor is better but as mentioned above, you need to check it very carefully with your frame, certain frames won't work with the narrower cranks due to flared chainstays, weird suspension linkages etc.
And yes, the XTR mech will work fine with the SRAM chainset.