I am having trouble getting a crankset to fit on a Shimano UN55 BB Square Taper Bottom Bracket (68x127.5mm).

Neither a Shinano Alivio nor a Vuelta Pista crankset seem to correctly sync with the square taper bb. There is a gap - I've mounted both and posted the photos below.

I've been converting an old 70's Nishiki to a fixed gear and updating the parts. I feel ignorant and don't know what question to ask to find the part that will fit. Any help is greatly appreciated.

The bottom bracket is way to wide. Look up the cranks and see what the manufacturers suggest for width but I am guessing it is closer to 115 than 127.5. The BB must fit what the cranks need. Roger

Thanks Roger. I appreciate it. I did not know that so thank you.

Quite often, the older cranks used a noticeably longer spindle than current cranks.

In a pinch, you can flip the spindle.
Notice how the circular part (before the taper starts) protrudes much more from the cup on the DS

you need a 107 or 110 probably something a lot shorter than what you have

It's a Shimano UN55 cartridge and English threaded so it can't be reversed. I agree with Roger that it's way too long. A 127.5 mm is about the longest cartridge spindle Shimano ever made and it was mostly used on triple cranks that needed a wide chain line. The OP want to use a single chainring on a fixed gear and he needs a much shorter spindle.

Guess maybe I should have read the first line!

I'll pile on- a 127.5mm spindle would be correct for an early-'80's triple crank, such as a Sugino Aero Tour, on a mountain bike where the cranks would need to be further apart to clear the chainstays. For a Shimano Alivio triple, the spindle length would be 115mm since the crankarms are angled from the spindle to the pedal end, while a Vuelta Pista is a single ring crank needing only a 107mm or 110mm spindle.

The great thing about square-taper bottom brackets and spindles is that the large number of available lengths gave bike builders a lot of flexibility in spec'ing the configuration of the crankset. Current "two-piece" cranks lock the builder into one set of dimensions, ignoring the different requirements of riders and applications. Humph...

Very informative, thanks to everyone.

One more thing, I think that you can probably flip the mounting of the chain ring on the crank. Put the chainring behind the spider and that will move the chainring quite a bit closer to the frame.

It looks like from the pictures that the chainring is mounted on the outside of the spider.