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I have a new bike with 105 r7000 but crankarm is 170mm. On my old Ultegra 6800 172.5 crankarm, I have power meter on left arm. What will be better - put my complete Ultegra crankset onto my new bike with FD105 r7000 or take only from 6800 right arm and install it with new r7000 big and small chainrings?

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    This question could be improved by formatting and more clearly explaining what the issue is. Jun 21, 2023 at 12:43

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From what I can read from related posts such as Crank Arm Compatibility - Shimano Ultegra 6800 and R8000, you can mix and match within reason, Hollowtech II cranks should be compatible, but assuming that you always want the powermeter and you shouldn't mix crank lengths, you have two options

  • Go full R6800 crankset
  • Go R6800 with R7000 chainrings (correct me if they changed the bolts), but I'm not sure if that is an actual improvement in terms of material and weight (Ultegra is one tier up and improvements between generations are usually small, so my gut feeling says that "full" old Ultegra isn't worse than 105)

Worth noting is that they obviously changed the chainring spacing between those iterations, only by 0.4 mm narrower on the newer gens with adaptions to the matching FD, but still. I don't think it is a dealbreaker but you might not be able to get the perfect FD adjustment in every extreme chainline because the R6800 crank needs more space and the new derailleur doesn't accommodate it by design, even if it is just 0.4mm

See also: http://forums.roadbikereview.com/components-wrenching/new-ultegra-8000-compatibility-6800-a-360975.html

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