SHIMANO BB-UN300 Bottom Bracket — JIS Square-Taper, 68mm BSA, 122.5mm Hollow Spindle, 50mm Chainline

This BB-UN300 bottom bracket from SHIMANO pairs classic square-taper cranks with modern materials: a hollow alloy spindle for lighter weight, precision bearings for steady rotation, and a durable black finish for corrosion resistance. It’s engineered to fit 68mm BSA shells and produce a 50mm chainline, making it suitable for road, commuter, and touring setups that use JIS square-taper cranksets. The unit ships without crank fixing bolts, so those are ordered separately.

How does the BB-UN300 improve pedaling feel?

The hollow alloy spindle reduces rotating mass while preserving stiffness, so each pedal stroke transfers power efficiently to the drivetrain. High-precision bearing interfaces reduce friction for smoother rotation under load and over long miles.

• Hollow spindle for lower weight without sacrificing strength
• Precision bearings for consistent rolling performance
• 50mm chainline that maintains drivetrain alignment

Which cranksets and frames is this compatible with?

This model is made specifically for JIS square-taper cranksets and requires a 68mm BSA threaded bottom bracket shell. It will not fit ISO or non-threaded press-fit frames without adapters; confirm your crank taper standard before purchase.

• Compatible: JIS square-taper cranksets only
• Frame requirement: 68mm BSA threaded shell
• Accessories: crank fixing bolts sold separately

What are the installation and maintenance considerations?

Installation follows standard BSA bottom bracket procedures: thread and torque to specification, ensure proper bearing preload, and fit crank fixing bolts to the crank’s recommended torque. Routine checks and light lubrication at service intervals will prolong bearing life and maintain smooth rotation.

• Threaded BSA installation—no special tools beyond typical BB and crank tools
• Use correct crank fixing bolts for secure fit
• Periodic inspection and cleaning recommended for longevity

What does the BB-UN300 offer compared with other bottom brackets?

Compared with heavier solid spindles, the hollow spindle balances weight savings with sufficient strength for everyday riding. Its straightforward square-taper interface and proven SHIMANO engineering make it a durable, serviceable choice for riders who prioritize reliability over the latest integrated spindle standards.

• Weight-efficient hollow spindle versus older solid designs
• Classic, serviceable square-taper interface for ease of repair
• Durable finish and quality bearings from a trusted component maker

Who is this for? The SHIMANO BB-UN300 suits riders who use JIS square-taper cranksets on bikes with 68mm BSA shells—commuters, touring cyclists, and those updating or maintaining classic crank systems. If you want a dependable, low-maintenance bottom bracket with a lighter hollow spindle and proven bearing performance, this BB-UN300 is a practical upgrade. Note that crank fixing bolts are not included, so add the correct bolts when ordering.