I’m building up a fixed gear on a 1970s Claude Butler frame. Never done it before, lots to learn etc.

I’ve got a Miche Primato 165mm chainset, but what bottom bracket do I need? The Primato BB widths seem to range from 107mm to 119mm. Amongst other things (eg. compatibility), I want to be sure of chainline... the rear hub will be a Sturmey Archer S3X 3 speed fixed hub, 120mm. I checked with uncle Sheldon and he says stuff about halving the OLD and subtracting the dropout-to-sproket distance, but I’m still not sure where this leaves me with making a decision on a bottom bracket. Halp!

Bottom bracket spindle lengths are generally specified by the crank design/mfr. That doesn't mean this will work for all frames however. Remember the chain ring must clear the chain stay.

If you have an old bottom bracket around install it and the crank with the chain ring attached. Check the chainline. If it's wide or narrow by a certain amount, select the bottom bracket that will get you there.

It seems that 107mm is the specified BB length for that crank, but who knows if it fits your application?

Build your wheel first, with the chosen hub. Put the frame in a stand,
dropouts pointing toward the sky. Put your wheel in your chosen frame.

Run a string parallel to the rear sprocket (if you have a long enough straight
edge, use that) up to where the chainwheel ought to be.Tie the string off
so you can measure your total distance between string and BB shell edge.

This gives you a rough idea of how much space your assembly ought to
take up. Take your crankset to someplace that has a lot of different BB's
and find the one that seems to work best when assembled to take up
the same space.

Like reddog has stated, I don't think there's away to find the specs for
your application of this particular crank. The Phil Wood BB's (and some
others) allow for a 4-5mm adjustment for chain line because of the design
of the cups (no shoulder to bottom against the shell).

Something like that might be your best bet for this.