Shimano BB-UN300

The Standard of Reliability. Sealed for Life.

The Shimano BB-UN300 is a high-quality sealed cartridge unit designed for square-taper cranksets. Unlike older cup-and-cone systems that require constant adjustment, the UN300 features precision industrial bearings housed in a rock-solid steel body. It is engineered to provide a smooth, creak-free rotation for thousands of kilometers, making it the most cost-effective performance upgrade you can give to a hybrid, city, or entry-level mountain bike.

Why the BB-UN300 is the Industry Benchmark:

• Superior Sealed Cartridge: The internal bearings are protected by a robust multi-lip sealing system that acts as a fortress against water and road salt, ensuring your pedaling stays smooth through all four seasons.

• Exceptional Spindle Stability: Forged from high-tensile steel, the square-taper spindle is remarkably stiff, preventing the "crank flex" that can cause ghost-shifting or power loss during heavy climbing.

• Anti-Creak Design: The precision-machined threads and internal sleeve are designed to provide a perfect, concentric fit within your frame, eliminating the annoying "click" or "creak" common with budget alternatives.

• Massive Versatility: Available in a vast array of spindle lengths (from 107mm to 127mm) and shell widths (68mm BSA or 73mm), ensuring a perfect chainline for almost any classic 3x, 2x, or 1x drivetrain.

• Easy Installation: Designed for the standard 20-tooth splined bottom bracket tool, making it a simple, standardized swap for any home mechanic.

• Lightweight Aluminum/Steel Construction: Features a lightweight aluminum left-hand cup and a durable steel drive-side body for the optimal balance of weight and structural longevity.

Technical Specifications:

• Series: Shimano Non-Series (Replacement for UN26/UN55)

• Type: Square Taper (JIS Standard)

• Thread: BSA (English) / 68mm or 73mm Shell

• Spindle Material: High-Tensile Steel

• Cup Material: Aluminum (Left) / Steel (Right)

• Weight: Performance-focused for the category (approx. 280g-320g)

The Alegro Perspective: In the shop, we call the UN300 the "bulletproof option." If you have a trusty old steel frame or a daily commuter that lives outside, this is the bottom bracket you want. It’s not flashy, and it’s not made of carbon, but it is one of the few parts on a bike that you can truly install and then completely ignore for five years. It’s the definition of "Shimano Quality". Simple, effective, and practically indestructible.

Indestructible Rotation. Proven Performance. Power your Commute with Shimano.