A 107mm BB gives the miche primato advanced a 42mm chainline. So to get a 47.5mm chainline I’d need a 118mm BB such as a UN55 – right?

With regard specifically to the UN55, the axles are not all symmetrical. There have been a number of threads about this on the CyclingUK/CTC forum. According to the thread below, assuming the 107mm BB is likewise a UN55, the 118mm UN55 might give a chainline just under 1mm short of your required 47.5mm, but there is some scope to use a spacer to fine tune.

https://forum.cyclinguk.org/viewtopic.php?t=105385

Note also the comments about some rather loose left hand cups, and I have also seen comments elsewhere suggesting that the plastic cups used on the cheaper square taper BBs from the likes of Shimano and Stronglight are just not up to the stresses of fixed gear use (I don’t know whether that would also apply to any extent to singlespeed use).

The bike (genesis day one) originally comes with a 116mm BB maybe that would be better. It’s fixed I was hoping to try out… if not the UN55 are there any other options?

I was not suggesting that you do not use a UN55 (it would be my first choice), merely noting that a 118mm UN55 might not give you exactly 47.5mm chainline, and warning that plastic cups (found I think on the cheaper UN26 and some Stronglight BBs) might give problems, as might very loose metal cups found occasionally on some UN55s according to the the CyclingUK forum thread.

If you were using a 3/32″ chain (which is what I use on my fixed with an old Shimano road chainset), then getting the chainline spot on is less critical. If you plan to use a much less flexible 1/8″ fully bushed chain, then I expect you will need to get very close to your 47.5mm, so you might need to consider using a spacer to fine tune the chainset’s chainline with your chosen BB.

I would be inclined to install everything using your 116mm BB, and then measure the actual chainline of the sprocket in situ (i.e. check that it is 47.5mm with your chosen hub) and likewise measure the actual chainline of the chainset, to see how far short of 47.5mm it is, and use that measurement to confirm if a 118mm UN55 is the best choice.

Makes sense – cheers

I should have added that another option would be a bottom bracket with cups and lockrings which allow fine tuning of the chainline, e.g. Phil Wood and Royce. That is a more expensive (but arguably nicer) solution. You would still need to establish which of their BBs would be the right size, given that the range of movement they offer is limited. In the hopefully unlikely event that a UN55 is not available in a close enough size, Phil Wood or Royce might be your fall back options (note that they require specific tools to fit, and their use of lockrings would likely necessitate the BB shell being faced).

You might also want to post a thread on the LFGSS or CyclingUK forums, since posters on those forums are far more likely to be able to better advise you on getting the chainline right for a fixed drivetrain with 1/8″ chain, chainring and sprockets.