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Bottom bracket advice please

Hi everyone I’m new to the group and my son (12yo) and I are building him a jump bike, it is based on his old Boardman mtb, it’s an aluminium frame with 26” wheels, it had a 3x7 set up but he is putting new wheels on it and he has decided to put a 1x9 set up on the bike, now to the question the BB is a shimano 73x122 sealed unit that sits within the frame I’m having some trouble getting hold of it and I was wondering if I could replace it with a Shimano HollowTech II bottom bracket assembly complete with left/ right-hand cups as he is going to install new cranks too, I wasn’t sure if it would change the angle that the chain would sit at. Thanks in advance for your help it would really help us , regards john


  • whyamiherewhyamihere Posts: 7,629
    You'll be fine fitting a Hollowtech II BB and cranks. Make sure you fit the spacers according to the instructions (if I remember correctly it will be one BB spacer on the driveside with a 73mm shell, but check that) and the chainline will be fine.
  • Thanks that’s great news how are you on gearing ? Lol
    My son only really uses the middle front gear (34t) of the 3x and on his old bike he was usually in the smallest gear (14t). The new cassette we are looking at is a 11-34t so I’m hoping this will allow him to climb the hills as he did but will give him a bit more top end when he needs it, do you think he should stick with the 1x 34t?
  • whyamiherewhyamihere Posts: 7,629
    Chainrings are (relatively) cheap, so can be good to experiment with. 32t is a little more common and will make the hills slightly easier, while the smaller cogs in the new cassette will give higher gears, so I'd start there and see how he gets on.
  • Thanks that really helps just one last thing I’ve been offered a set of wheels with a 20mm thru axle but the bike has a 9mm QR can the wheels be converted to a 9mm ? Thanks after this I’ll leave you alone I promise
  • whyamiherewhyamihere Posts: 7,629
    That depends on the hub. Some can be converted (like certain Hope hubs for example), some can't. Do you have more details on the wheels being offered?
  • 20mm Thru axle hub adapter to 9mm quick release or install new hub.
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