Shimano BB-UN300 Bottom Bracket - English | 68 x 113mm Spindle - Square Taper JIS

Shimano BB-UN300 Bottom Bracket - English | 68 x 113mm Spindle - Square Taper JIS Shimano

Shimano BB-UN300 Bottom Bracket - English | 68 x 113mm Spindle - Square Taper JIS

Shimano BB-UN300 Bottom Bracket - English | 68 x 113mm Spindle - Square Taper JIS

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Which Bikes Use the Shimano BB-UN300 68x113mm Bottom Bracket?

The Shimano BB-UN300 is the benchmark square taper bottom bracket, a cartridge bearing design that delivers Shimano quality and smooth, consistent rotation in a sealed unit compatible with square taper JIS cranks. The integrated chain line stabilizer reduces lateral chain movement for cleaner shifting, an often-overlooked feature that matters most on multi-speed bikes with wider cassette or freewheel ranges. The 68mm English threaded shell and 113mm spindle length cover the most common road and hybrid bike configuration -- if your bike has a 68mm threaded BB shell and square taper JIS cranks, the UN300 in this spindle length is typically the correct replacement choice.

The BB-UN300 uses aluminum adapters and ships in three structure variants: standard, E-type, and chain case -- DCL stocks the standard structure. Cartridge bearing construction means the bearing assembly is sealed and not user-serviceable, replaced as a complete unit when worn rather than repacked like cup-and-cone designs. Compatible with all square taper JIS cranks including Shimano's own FC-series square taper cranksets and many third-party cranksets using the JIS taper standard. Tools required: Shimano TL-UN74-S or equivalent splined BB wrench. A straightforward, proven upgrade or replacement for any square taper threaded bike needing fresh bearings.

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Specifications

  • Model: BB-UN300
  • Interface: Square Taper JIS
  • Shell: English threaded, 68mm
  • Spindle Length: 113mm
  • Bearing Type: Cartridge (sealed, non-serviceable)
  • Structure: Standard
  • Adapter Material: Aluminum
  • Installation Tool: Shimano TL-UN74-S or equivalent splined BB wrench

FAQ

How do I know if my bike uses a 113mm or 110mm or 122.5mm spindle?

Spindle length determines chainline. Most road and hybrid bikes with a single or double chainring use 110mm or 113mm. Triple chainring setups often use 113mm. Mountain bikes with wider Q-factor cranks may use 122.5mm or longer. Check your existing BB spindle length or consult your bike's spec sheet before ordering.

Is the BB-UN300 compatible with ISO square taper cranks?

The BB-UN300 uses JIS square taper, not ISO. JIS and ISO tapers have different angles and are not interchangeable. Most modern square taper cranks, including Shimano FC-series square taper cranksets, use JIS. Older French or Campy-spec cranks may use ISO. Confirm your crank's taper spec before ordering.

Can I service the cartridge bearing, or do I need to replace the whole BB?

The BB-UN300 is a cartridge bearing unit and is not user-serviceable. When bearings become rough or noisy, replace the entire unit. Shimano designs these for straightforward full replacement rather than repacking.

Best For

Road, hybrid, and commuter cyclists with English threaded 68mm BB shells running square taper JIS cranks in a standard chainring configuration, needing a reliable and proven cartridge bearing replacement.

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