Replace cartridge bottom bracket

Skill level 3 of 4 · OverhaulReplaceAdvanced45 minCartridge bottom bracket (square taper)

Sealed bearing unit threaded into the frame's BB shell. The cranks attach to its square spindle ends. One cup threads in from each side (English/BSA threads: drive side is reverse-threaded). Spindle length (printed on the cartridge, e.g. 122.5 mm) sets chainline, which is where the chainring sits sideways.

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Tools

1/4" click torque wrench (about 2–17 Nm) + bits: Pro Bike Tool 1/4 inch drive click torque wrench set (2.2–16.9 Nm) ($$)
Bike grease: Park Tool PPL-1 PolyLube 1000 ($)
Digital calipers: Clockwise Tools DCLR-0605 IP54 Digital Caliper ($$)
Repair stand: Park Tool PCS-9.3 Home Mechanic Repair Stand ($$$$)
12" adjustable wrench: Crescent 12 inch adjustable wrench ($)
3/8" click torque wrench 10–60 Nm: Pro Bike Tool 3/8 inch Drive Click Torque Wrench Set 10-60 Nm ($$$)
3/8" ratchet + 14/15 mm sockets + 8 mm hex bit: TEKTON 3/8 inch drive socket set metric ($$)
Square-taper crank puller: Park Tool CCP-22 ($)
Splined crank puller (Octalink / ISIS): Park Tool CCP-44 ($$)
Cartridge BB tool (20-notch): Park Tool BBT-22 Bottom Bracket Tool ($$)
Pin spanner + BB lockring spanner: Park Tool SPA-1 + Park Tool HCW-5 ($$)

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Steps

Remove both crank arms (see Remove and install square-taper cranks).
Measure the old spindle length and the shell width (68 or 73 mm), and read any markings. Buy the same spindle length unless you are correcting chainline.
Non-drive side first: fit the BB tool and turn counter-clockwise to remove the cup.
Drive side is reverse threaded (English BB): turn CLOCKWISE to remove. Hold the tool in with the crank bolt loosely so it cannot slip.
Clean the shell threads. Grease them for metal cups; Shimano says do not grease plastic cups or adapters.
Thread the drive side in first by hand counter-clockwise, then the non-drive cup. Never force: cross-threading ruins the frame.
Torque both, refit cranks.

Torque

Fastener	Torque
Cartridge BB cups	50–70 Nm
Crank bolts	35–50 Nm

Diagram

Cut in half, seen from behindBottom bracket and cranks

Square-taper crank + cartridge bottom bracket (section)

Signs it needs doing

Grinding or rough feel when spinning cranks with the chain off
Side play in the cranks
Creak under load