Interesting one this…..
My bike came with a crappy sq taper crank, so I bought a cheap SORA crankset from Merlin to fit. Didn’t even think about axle lengths etc. and sold the sq taper crankset on ebay.
The Sora crankset has never shifted as well as the previous one did. The chainring clearance is right, it is parallel to the rings etc. but it is like the rings are slightly further inboard than they were before so the mech is almost at it’s inner limit of movement. It shifts, just not as well as it used to. This is when dropping from big ring to little ring….
To see if it was just me having issues with set-up or some kind of geometry/spacing issue I put a 2.5mm spacer on the DS BB shell. And once re-set the mech shifting was improved massively.
I haven’t left it like that as it meant that the splines on the NDS crank weren’t fully engaged, but it was a useful test.
I suspect the axles on the sq taper BB were a bit longer than the equivalent length on the SORA crankset, resulting in the rings sitting further outboard than they do now with the SORA crank.
But, all the literature says you don’t need any spacers with a HT-II crank on a 68mm shell & I can’t find the original spec of the sq taper BB to find the axle length and confirm or deny my suspicion.
I am 90% sure that my 75kg weight and distinct lack of power would mean that the cranks are OK with a bit less spline engagement on the NDS with the 2.5mm spacer in, but I am not sure I want to risk it.