Short version – trying to figure out if I can run a left XTR M985 crank arm on an M980 chainset, and if so, what the effect is.

Long version – Merlin cancelled my M980/Stages 180mm order as they had their stock levels wrong. They have the 180mm M985 Stages left arm available. They and Stages say it is only for the M985 chainset not the M980. The difference between them is 6mm of Q factor (and a different BCD, that’s irrelevant here), but a) would the left arm of the M985 physically fit on the M980 chainset, are there any mechanical differences in the interface, and b) would it mean the left Q factor is 3mm less than the right?

Given I ride roadbikes through to fat bikes with 100mm BBs, have had adjustable chainline and asymmetric BBs before, and generally am not very princess and pea about bike setup, if it fits/works am I likely to notice this?

Thanks!

I don’t know for sure, having not tried it, but the Tech docs look identical so I don’t see why it wouldn’t fit. You could always adjust chain line using bottom bracket spacers to suit the Q factor.

buy it, try it, return it if it doesn’t fit?

road certainly doesn’t fit on MTB (well a cube HT) i tried a 7900 on a SLX

SLX Left, 7900 Stages right

however it depends on your frame/chainstay clearance, there is no reason it won’t bolt on

Thanks for the answers! I’m running singlespeed and can adjust chainline on the hub so chainline doesn’t matter, which should make things even easier…

Being a luddite and not having used hollowtech2 BB/cranks yet (just about to install the M980 but), would that road crank arm fit onto an MTB spindle, it’s just the frame/chainstay clearance that is the issue? So if the M985 crank interface is 3mm narrower then the M980, as suggested by the difference between the road and mtb arms above, it doesn’t matter that it i.e. goes 3mm further onto the spindle?

Well, Stages claim it will work, so let’s see! Merlin have them on offer for £450 at the moment, bargain!

Q factor is achieved by different bendiness of arms – as photographed. They don’t have shorter axles or owt, the axle interface is identical on all. So yes, the issue is with clearance, not with the actual engagement on the axle.

Thanks! The thickness of the BB/Spline interface end of the crank looked different between the road and mtb cranks in that pic, so I wondered if that was an issue too.. So you think the XTR cranks have identical interfaces but the bendiness/angle of crank away from that is slightly less in the M985? Guess I will see when they arrive!