This is a pre-emptive question so when I come to fit the things I have some collective hive mind knowledge (still waiting on the stupid tool they make you buy to fit the chainring). Bought some new SLX 7120 cranks and I noticed they come with 2 smaller spacers. A quick look at the docs suggests they fit over the axle inbetween cranks and BB. Anyone got some of these fitted and how many of these spacers did you end up needing, are they just to remove any play in the system or space out the chainline?
I'm guessing as the tech doc says fit 3mm spacer between the DS crank and the BB that you have two 1.5mm spacers?
I'd say they're to take up slack in the BB system, or help seal the BB, I'm not 100% sure, but the doc says fit them, so you should do what it says.
This is worth a watch (he explains the spacers from about 5:00) :
Ah right ok so the ones I bought are for the 55mm chainline so the spacers help achieve that.