Crank / BB compatibility issues
 

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.

http://bikeforums.net/attachment.php...hmentid=265086http://bikeforums.net/attachment.php...hmentid=265087

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.