I have a Shimano M900 triple crankset (last square-taper XTR crank), which I've used on road bikes since 1995 (built my first road bike with a triple while working at a shop during high school).
I've always used it with a 103mm bottom bracket, but ideally I'd have something 95-97mm, for proper road chainline and narrower q-factor.

The problem: nobody makes a square-taper BB narrower than 103mm, except for Phil Wood's 92mm (which is too narrow).
Am I missing any options here, or should I just accept that 103mm is the best I can do?

I have never seen a square taper spindle less than 103 mm. A quick check with my Sutherland's 6th ed. confirms. A 95mm spindle (if you could find one) in a 68mm shell doesn't leave a lot of taper exposed to mount a single crank let alone a triple. If a 103mm spindle is too wide for a proper chain line, I'd suspect an alignment problem in your rear triangle.

I run a road crank on a 103mm spindle to get the right chainline for a fixed-gear, and there is no more than a millimeter total of clearance from either crank arm to the BB shell. Absolutely no way anything shorter could be used.

If you want a narrow Q-factor, MTB cranks are just the wrong place to start.

Phil Wood stated on there web site that they make the spindle in any length you need upon special request. The shell made in a few sizes. Phil Wood BB do not use a lock ring on them. You can adjust the cups to align the crank. The cups use locktite to hold them in place. I hope this helps. https://www.philwood.com/products/bbhome.php

Yup, t as Phil is in San Jose California, and makes their stuff right there ,
call them on the phone ask if they will make a non in stock spindle length..

So you're sure that is Exactly what length you need?

I can confirm that the frame's rear triangle is in perfect alignment (checked everything, including dropout faces being parallel and RD hanger alignment) before building up this bike. Additionally, chainline was the same when I'd previously used this crank on other road frames.

Secondly, the M900 crank has a small reinforcement bulge on the inner side of the crankarm interface with the taper. The outer diameter of this bulge is narrower than the inner diameter of the fixing cups, so it fits inside and therefore would allow the crankarm to be mounted on a narrower BB. See the attached pics (sorry they are tiny; they are the best I could find on the internet at the moment). Super-careful measurements would require removing the crankarm and testing fit around BB sleeve, but it looks like both the DS and NDS crankarms have about 4mm to move inward.



Other possible issue for going as narrow as I'd like, is whether the beginning of the taper would interfere with spindle/bearing interface.

Precise measurements on the M900 crank with 103mm Dura-Ace BB:
DS chainline is at 45.8mm (center of seat tube to center of middle chainring)
Q-factor: 157mm (NDS crank is 1.5mm further outboard)

I'd like a triple chainline of 43 or 44mm (measured to middle ring) because I use the 26t granny gear less often than the big ring.


Given the effort and cost involved ($200 for Phil Wood BB with cups in stock lengths, maybe $250 or more for custom length) I probably will continue using the 103mm BB, or maybe spend the money to get a newer subcompact (44/28) double crank.