Shimano BB-UN300 Bottom Bracket - English | 73 x 118mm Spindle - Square Taper JIS

Shimano BB-UN300 Bottom Bracket English 73 x 118mm Spindle Square Taper JIS

Shimano BB-UN300 Bottom Bracket - English | 73 x 118mm Spindle - Square Taper JIS

Shimano BB-UN300 Bottom Bracket - English | 73 x 118mm Spindle - Square Taper JIS

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Which bikes use the Shimano BB-UN300 73 x 118mm Bottom Bracket?

The Shimano BB-UN300 73 x 118mm is the benchmark square taper bottom bracket for mountain and wide-shell bikes, a cartridge bearing design that delivers Shimano quality and smooth, consistent rotation in a sealed unit compatible with square taper JIS cranks. The 73mm English threaded shell and 118mm hollow spindle cover the common mountain bike and wider Q-factor configuration, so if your frame has a 73mm threaded BB shell and square taper JIS cranks, the UN300 in this spindle length is typically the correct replacement. A hollow chromoly spindle keeps weight down while handling real off-road power without complaint.

This bottom bracket ships without crankarm fixing bolts, which are sold separately, so add a pair if you do not already have them. 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 FC-series square taper cranksets and many third-party cranks using the JIS taper standard. Installation requires a Shimano TL-UN74-S or equivalent splined BB wrench. A straightforward, proven replacement for any 73mm square taper threaded bike needing fresh bearings.

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Specifications

  • Model: BB-UN300
  • Interface: Square Taper JIS
  • Shell: English threaded, 73mm (1.37 x 24t)
  • Spindle Length: 118mm
  • Spindle: Hollow chromoly
  • Bearing Type: Cartridge (sealed, non-serviceable)
  • Crankarm Fixing Bolts: Not included, sold separately
  • Installation Tool: Shimano TL-UN74-S or equivalent splined BB wrench
  • Product Code: EBBUN300C18X

FAQ

Do I need a 68mm or 73mm shell version?

Measure your frame's threaded BB shell width. Mountain bikes commonly use 73mm, while many road and hybrid frames use 68mm. This is the 73mm version. If your shell is 68mm, choose a 68mm BB-UN300 instead.

How do I know if I need the 118mm spindle?

Spindle length sets your chainline and crank clearance. The BB-UN300 comes in several lengths. Check the spindle length stamped on your existing bottom bracket or consult your crank and frame spec before ordering this 118mm version.

Are crank fixing bolts included?

No. This version ships without crankarm fixing bolts. Reuse your existing bolts or add a pair to your order.

Can I service the cartridge bearing?

No. The BB-UN300 is a sealed cartridge unit and is not user-serviceable. When the bearings become rough or noisy, replace the entire unit rather than repacking it.

Best For

Mountain bike, commuter, and touring riders with English threaded 73mm BB shells running square taper JIS cranks who need a reliable, proven cartridge bearing replacement in the 118mm spindle length.

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