Guide 01
Bottom bracket
Easy install
~15 minutes
Mind the reverse thread
The bearings that let your pedals spin. On your frame it screws in like a jar lid —
genuinely one of the friendliest jobs on the bike, with two catches worth knowing.
ELI5
The two crank arms (your pedal levers) join through an axle, and that axle spins on bearings.
The bottom bracket is those bearings + their housing. It lives in the BB shell —
the chunky tube at the bottom of the frame where everything meets.
Your shell: 68 mm BSA threaded
- 68 mm = how wide the shell is.
- BSA / “threaded” = the BB screws in on cut threads. This is the easiest, most
reliable type — far friendlier than press‑fit (which needs a hydraulic press and famously
creaks). You lucked into the nice one.
How it goes
- Clean threads, check the faces. Ideally the shell is “chased” (threads cleaned) and
“faced” (end surfaces made flat & parallel) once — a 10‑minute shop job, or a good frame
arrives ready. Like wiping a jar rim clean so the lid seats square.
- Grease the threads. Always. Stops seizing and creaks. Never install dry.
- Screw in the two cups with the correct BB tool (a splined wrench matched to your BB).
- Torque firm — roughly 35–50 Nm. A good firm pull with the tool gets you there; a
torque wrench is nice.
- Fit the cranks — the axle (usually built onto the drive‑side crank) slides through,
the far crank attaches, snug to spec.
The gotcha that catches everyone
On a BSA BB the drive (chain) side is REVERSE‑threaded — you tighten it
anticlockwise, backwards from normal. The non‑drive side is normal. It’s deliberate
(pedalling would otherwise slowly unscrew it). If a cup “won’t tighten,” you’re almost certainly
turning it the wrong way.
Tools
BB tool / splined wrench (~$20, matched to your BB type) · grease or anti‑seize ·
torque wrench (nice‑to‑have).
The BB is also part of your chainline
The BB’s axle (spindle) length sets how far out the chainring sits. With a
square‑taper BB you choose the spindle length to push the chainring in/out until it lines up
dead‑straight with the Alfine sprocket. So picking the BB isn’t just a bearing decision — it’s how
you dial the chain straight. Full method in
Chainline & chain tension.
Buy
A square‑taper, 68 mm BSA BB in the spindle length your chainline calc lands on (see the
chainline guide before ordering) · matching crankset · single chainring.