I don't think I've ever built a boost bike from scratch before.
BB measured at 73mm, dropouts at 148 - it's boost right?
Hope BB comes with 3 spacers. On a 73mm shell you normally fit one spacer on drive side.
XT 8120 crankset*.
When I fit the crankset and bottom out the non drive crank theres a gap between the face of the crank and the BB (so you can move it left and right).
What's the obvious thing I've missed?
Second question -
I've just noticed this is a 55mm chainline which I assumed was boost (and 52mm was non-boost) but it seems this is a new boost standard. I've actually got a 52mm crankset as well (but at 175mm length) - is using the 55 going to be an issue or does the frame need to be designed differently for a 55?
The XT 8120 12 speed chainset comes with two plastic spacers that fit between the bottom bracket and the cranks. That’s what you’ve missed.
Check out the forth pic
https://www.bike-components.de/en/Shimano/XT-FC-M8120-B2-Hollowtech-II-Crankset-p71491/
So looking at Shimano's diagram here
what are parts numbered 5 and 6? are there some extra spacers I don't have or are those just parts of the the shimano BB and a Hope doesn't have them?
Thats them. The parts labelled 6 are two plastic spacers that come with the cranks. They are not part of the bottom bracket, hope or Shimano, it doesn’t matter.
Ah yes. Buried in the bottom of the box.
Right, so now just have to decide whether to use the 170mm 55mm chainline and get shitty performance in lowest gears or the 175mm 52mm chainline and get more pedal strikes. I really should have paid more attention when I was buying these....
How are people finding 55mm chainline? I was never very happy with non-boost 11speed and this sounds like it will be worse.
Although I suppose you can just run both spacers on the non-drive side and it will still be 52mm?
I’m running the 55mm chainline with these cranks on 2 bikes and shifting is perfect. I have the 52mm chainline cranks on another bike and haven’t noticed the difference.
You’ll find the 52mm chainline cranks won’t fit some frames.
I had to buy boost cranks for the first time last year and wanted 165 so ended up with slx 55mm chain line ones because they were available and the right price. I can’t remember where I put the spacers. So maybe I’m running 52mm chain line. Shifts fine anyway. I’m not sure that helps much other than to say it will probably be fine. I suppose try 52mm first but if it’s too close to the chainstay you’ll have to swap a spacer round to make it 55.
I've just posted a similar thread earlier in the week as I wasn't sure about the spacers either. I bought the SLX 7120 cranks which have the same 55mm chainline. I ended up using one of the 3mm spacers on the non driveside. I also have one 2.5mm BB spacer on the driveside as you normally would with a 73mm bb.
No spacers meant I got the gap like you mentioned and trying to use both spacers (one either side as shimano doc says) meant the non drive side crank arm wouldn't fit correctly.
