Shimano BB-UN300-K Bottom Bracket - English | 68 x 122.5mm| Square Taper JIS

Shimano BB-UN300-K Bottom Bracket - English | 68 x 122.5mm| Square Taper JIS Shimano

Shimano BB-UN300-K Bottom Bracket - English | 68 x 122.5mm| Square Taper JIS

Shimano BB-UN300-K Bottom Bracket - English | 68 x 122.5mm| Square Taper JIS

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What Bikes Need the Shimano BB-UN300-K 68x122.5mm Bottom Bracket?

The Shimano BB-UN300-K is the long-spindle variant of the BB-UN300, pairing a 68mm English threaded shell with a 122.5mm spindle for applications where additional spindle length is needed to achieve correct chainline. The cartridge bearing construction delivers sealed, smooth rotation in a non-serviceable unit with Shimano's standard sealing quality, and the integrated chain line stabilizer reduces lateral chain movement for consistent shifting performance. The longer 122.5mm spindle is most commonly called for on MTB triple chainring setups or specific road frames where frame geometry pushes the chainrings outboard to clear wide chainstays.

English threaded 68mm shell, JIS square taper interface, aluminum adapters, standard structure. Compatible with all square taper JIS cranks -- confirm that 122.5mm is the correct spindle length for your specific crank model and chainline requirement before ordering, as spindle length selection is determined by the crankset spec sheet and not by shell threading alone. Installation requires a Shimano TL-UN74-S or equivalent splined BB wrench. If you are replacing a worn UN300 or similar cartridge BB and the old unit was 122.5mm, this is the correct OEM-quality replacement.

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Specifications

  • Model: BB-UN300-K
  • Interface: Square Taper JIS
  • Shell: English threaded, 68mm
  • Spindle Length: 122.5mm
  • Bearing Type: Cartridge (sealed, non-serviceable)
  • Structure: Standard
  • Adapter Material: Aluminum
  • Installation Tool: Shimano TL-UN74-S or equivalent splined BB wrench

FAQ

What is the most common use case for a 122.5mm spindle?

The 122.5mm spindle is most commonly used with MTB triple chainring cranksets where the wider Q-factor and triple ring configuration requires the spindle to push the crank arms outboard for proper chainline. Some road triple setups also use this length. Confirm your crank spec sheet before ordering.

How do I confirm what spindle length I need?

The easiest way is to measure your existing bottom bracket spindle or look up the spindle length recommendation in your crankset's tech documentation. Shimano's product compatibility charts (productinfo.shimano.com) list recommended spindle lengths for each crank model.

Is the BB-UN300-K different from the BB-UN300 in any other way?

The BB-UN300-K is functionally identical to the BB-UN300 -- same cartridge bearing construction, same adapter material, same chain line stabilizer -- the only difference is the 122.5mm spindle length.

Best For

MTB and touring cyclists with English threaded 68mm BB shells running triple chainring square taper JIS cranksets or any square taper crank requiring a 122.5mm spindle for proper chainline.

Shimano BB-UN300-K Bottom Bracket - English | 68 x 122.5mm| Square Taper JIS Reviews:

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