Building a Deore 104 bcd sub-compact crank...
 

I'm looking to build a subcompact 48/32 crankset based on a 104 bcd Deore crank, and just looking for advice on avoiding any incompatibilities/pitfalls. Looking to start with something like a M610 or M615 crankset, and buy the 104bcd 48t & 32t chainrings separately. 1. Whether I start with a double or a triple shouldn't matter, because either way it will end up with an unused 64bcd mount - right? 2. Is there any m-series 104 bcd crank that won't fit? Going 10 speed cassette/shifters, so probably will favor 10 speed chainrings, but not sure that really matters, even. 3. How do I ensure proper spacing between the chainrings? What length chaingring bolts do I need? Anything else? Appreciate the feedback.

I would think your chain line would come out a bit screwy.

Not familiar with that crank, square taper? The chainring spacing is determined by the crank. 9sp spacing? Front deraiieur? Just looked at those chainrings. The 64 bcd is for the 24-28 T inner. Where /how are you going to put the 32? No bueno. 9sp triple crank would work, 32-36 middle, 42-48 outer.

These are 10 speed hollowtech spindles.

I'm a little smarter now, after buying (returnable) parts...

The problem I've found with starting with a double is that the biggest chain ring has to mount to the inside of the crank spider, followed by ~3mm spacers (not sure exact spacing for 10 speed), followed by the smaller chain ring. Fine for chain alignment, but requires you to get "extra long" (e.g. ~8mm long) chain ring nuts.

A triple solves the extra long chain ring nuts problem, w/ the big chain ring on the outside, and the spider acting as the spacer, but then introduces a chain line biased to the outside...which is probably not a big deal.

The problem with either of these solutions, and I haven't tested this personally yet, is that shimano puts a bunch of divots and nubs and what not on the inside of the big chain ring, and it is intended to be paired with a specific next size down, and 16T difference doesn't exist for these size rings.
In practice, this mostly likely will function fine, but my not "feel smooth" going from small to big...but I don't know yet.

I decided to change up, and go 44/28, using a square taper BB - big ring on the inside of the spider, and little ring on the 64bcd mounts. Chain alignment can be maintained with a slightly longer BB.