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Cranksets · Shimano

Shimano BB-UN300 Square Taper Bottom Bracket

The UN300 is the sealed-cartridge square-taper bottom bracket Shimano sells as the successor to the long-running UN55 and UN26, and it is the right replacement for the creaking, gritty bottom bracket on most hub-motor e-bikes. It comes in several shell widths (68 and 73 mm are common) and spindle lengths, so you need to measure or read the old unit. Installation needs a splined bottom-bracket tool. Once in, it is a fit-and-forget part — which is exactly what you want on a commuter that gets ridden in the rain.

Fixing a creaking or loose bottom bracket on a square-taper bikePairing with a new square-taper cranksetHub-motor e-bikes with standard threaded shells

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Key considerations

  • Measure shell width (68/73 mm) and spindle length (often 110–127.5 mm) from the old unit
  • English (BSA) threading; left cup is reverse-threaded
  • Needs a splined BB tool
  • Not used with Bafang mid-drives, which replace the bottom bracket entirely

Pros

  • Reliable sealed cartridge
  • Shimano quality at a low price
  • Many sizes

Considerations

  • Size must be matched
  • Tool required

Compatibility notes

  • BSA threaded bottom bracket shells with square-taper cranks; match shell width and spindle length to the original

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Mechanic working on bike components
Cranksets

170 mm Square Taper Single Crankset, 44T with Chainguard (generic)

The typical hub-motor e-bike has a single front chainring on a square-taper crankset, and when a crank arm strips or the chainring wears hooked, the replacement is a generic 170 mm alloy crankset with a 44T or 46T ring and a plastic chainguard. These are sold by many sellers and are functionally interchangeable. The important checks are square-taper bottom bracket (not Octalink or Hollowtech), crank length, and chainring size; keep the same tooth count to preserve your gearing. A square-taper crank puller is needed for removal.

Best for: Replacing a stripped crank arm or worn chainring on a hub-motor e-bike · Bikes with a square-taper bottom bracket · Budget repairs

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E-Bike MotorsFeatured

Bafang BBS02B 48V 750W Mid-Drive Conversion Kit

The BBS02B is the mid-drive kit that turned thousands of ordinary bikes into e-bikes. It replaces the bottom bracket with a motor unit that drives the chainring, so it uses your gears and climbs far better than a hub motor of the same wattage. Kits bundle the motor, a chainring, cranks, display, throttle, brake sensors or cutoff levers, and a speed sensor; the battery is sold separately. Installation is a weekend job with a few special tools. It is not quiet under hard load and it is hard on chains, but it is the most proven budget mid-drive there is.

Best for: Converting a rigid or hardtail bike with a standard 68–73 mm threaded bottom bracket · Hilly commutes where a hub motor struggles · Riders who want gear-assisted climbing rather than raw throttle power

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