Shimano BB-UN300 bottom bracket, 115mm square spindle - EBBUN300B15B

Shimano ’s BB-UN300 EBBUN300B15B bottom bracket is a square-axle bottom bracket, designed for cranksets compatible with this standard. With its 115 mm axle, it offers a reliable solution for replacing a worn bottom bracket or refurbishing a drivetrain fitted with a square-mount crankset.

Part of the Shimano range of components, designed specifically for city bikes, trekking bikes, mountain bikes and all-rounders, the BB-UN300 prioritises simplicity, reliability and ease of replacement.

Its sealed cartridge system protects the bearings from external contamination and minimises the need for routine maintenance.

A simple and reliable square-axle bottom bracket

The Shimano BB-UN300 uses the traditional square-axle standard, also known as Square Taper. This system is fitted to many cranksets Shimano, and other compatible cranksets.

The square taper offers the advantage of a simple design: the cranks are secured directly to the tapered ends of the axle using fixing bolts.

Before ordering, however, it is essential to check that your crankset is compatible with a square taper axle and that the 115 mm axle length matches your drivetrain configuration.

Sealed bearing cartridge

The BB-UN300 features a cartridge design, with the bearing assembly protected inside the bottom bracket shell.

This design results in a compact, low-maintenance system. When the bearings develop play, become noisy or rotate unevenly, replacing the entire bottom bracket is generally simpler than servicing the bearings themselves.

For a bike used on a daily basis, this approach offers an excellent balance between simplicity, reliability and maintenance costs.

115 mm axle: check before assembly

The 115 mm axle length is one of the most important details to check before purchase.

The bottom bracket length affects the position of the crankset relative to the frame and, consequently, the chainline in particular. Two square-axle bottom brackets may have the same mounting type whilst requiring different axle lengths.

Replacing it with an identical model is generally the safest option: if your old bottom bracket is also a 115 mm Shimano BB-UN300, this part number will allow you to maintain a similar configuration.

Easy installation and low maintenance

The BB-UN300 is a cartridge bottom bracket designed to be fitted into a frame with the corresponding threading.

Once correctly fitted, the system does not require regular bearing adjustment, unlike some older cup-and-cone bottom brackets. However, you should ensure you use the recommended tightening torque and check that the frame threads are clean before installation.

Workshop tip: before ordering, note down the part number of your old bottom bracket or measure its axle length precisely. Also check the thread standard of the frame’s bottom bracket shell.

The benefits of the Shimano BB-UN300

• Shimano quality.
• Tried-and-tested square-axle design.
• 115 mm axle suitable for compatible set-ups.
• Low-maintenance bearing cartridge.
• A practical solution for replacing a worn bottom bracket.
• Simple and reliable design.
• Suitable for many bikes fitted with a square-axle crankset.
• A good balance between reliability, simplicity and price.

Conclusion

The Shimano BB-UN300 115 mm square-axle bottom bracket – EBBUN300B15B is a reliable and practical bottom bracket for replacing a compatible square-axle model.

Its cartridge design, sealed bearings and 115 mm axle make it a particularly suitable solution for bikes where simplicity, reliability and ease of maintenance are the top priorities.

Before placing an order, it is essential to check three things: the square-axle standard, the 115 mm axle length and the compatibility of the frame’s threading.

Technical specifications

• Brand: Shimano
• Model: BB-UN300
• Part number: EBBUN300B15B
• Type: cartridge bottom bracket
• Axle: square taper
• Axle length: 115 mm
• Bearing type: sealed cartridge
• Application: compatible with square taper cranksets
• Installation: threaded bottom bracket
• Maintenance: low maintenance
• Suitable for: city bikes, trekking bikes, leisure mountain bikes, all-round bikes and other compatible set-ups