One of several choices.

External BBs are subject to front wheel spew contamination of the grease , but they are fast to replace.

Big socket grips the outer splines (you wont bring it with you).


UN 51 is for a different, square Taper , crankset , so You have to buy a New Crankset too.

Or just replace the UN one with a New One .. maybe your chainrings are worn on your crank .

SKF BB are a premium square taper sold thru Compass Cycles
if you have a desire to seek "Best"

https://janheine.wordpress.com/2011/...rld-exclusive/



OEM tend to use current parts, so Trek's 520 uses the external type.