The BBUN26 is the basic model, BBUN55 offers higher quality at what is still a reasonable price All are for BSA / ISO threads ( 137 inch diameter, 24 tpi ) and fit a 68 mm or 73 mm wide bottom brackets The different lengths (113 123 mm) are to preserve a good chainline with different cranksets and different numbers of chainringsShimano's BBUN55 Square Spindle Bottom Brackets are the unofficial standard for Englishthreaded cartridge bottom brackets Quality, value, and durability come together to keep you cranking for many, many miles Fits JIS SquareTaper Cranksets Hollow spindle Crankarm fixing bolts not included
