Going from a triple to a double on a MTB. Square taper UN53 BB and Bulletproof crank. There are 2 choices - 107 and 110 - how do I determine which is right for this application?

55/Rad

The one that gives the best chainline. But that 3mm is likely not an big issue either way.Is there some kind of spindle length spec for the crank you mention?

My guess would be 110mm. Your mountain bike is likely to have a 135mm rear dropout spread. The wider bottom bracket will give you a slightly better chainline.

Not that I know of. The Bulletproof crank is not in hand as yet and the site I ordered it off - bikeparts.com - didn't offer any help.

Rear is an 8 speed freehub. Shifters and ders are Sora.

55/Rad

Then what makes you think either a 107 or 110 would be right?

The cheap BB I pulled with the triple crank is a 118. I replaced it with an Ultegra 109.5 and a Coda crank - a 175. But my kid is only 4'6" and I want shorter cranks. The Bulletproof crank, a double, is a 155 but uses a square taper BB. The UN53 comes in 4 sizes - 107, 110, 115 and 118.

I'm assuming the 2 smaller ones are for 2x's and the larger ones are for 3x's.

55/Rad

Cranks have specific spindle length specs.I have double cranks that use spindle lengths from 102 to 122 and that may not cover the complete range. The un53 118 was used with many shimano road triples. There were also shimano mtb triples that used both 107 and 110 square taper.

The bikes that I have with 135 rear dropout seem to have ~45mm chainline to the middle cog of a 9sp cassette. Do you have one of the bb so that you can see where the midline between the two chainwheels will sit? I agree with Sidney though, 3mm is not much diff. That being said, I tried a 107 on a 9sp double chainwheel with 135mm rear (Kogswell P) and decided that 110 shifted better, I may have just adjusted better on the second setup.....(square taper un73) YMMV

When I did mine I got the one of the larger sizes available, did the chainline measurements with that BB installed, and then figured what size I'd really need and ordered one of those. Heck, it's just $25.

This is a link to Sheldon Brown's page which gives some chainline specs for the bulletproof cranks w/ a 107 BB. Maybe it can help you.

https://sheldonbrown.com/harris/fixed.html#cranks