Service a cup-and-cone bottom bracket by removing the cranks, then the lockring and adjustable cup with a special wrench, catching the 11 balls of about 1/4 inch on each side, cleaning and inspecting everything, repacking with grease and adjusting until the spindle turns freely without play. Standing on one pedal puts about 1.7 kN on the nearer bearing, so a pitted cup or spindle will never run smoothly however you adjust it, and the thread directions must be checked before force is used.
Part of Bicycle Repair & Fabrication
The bottom bracket is the bearing assembly in the frame’s bottom-bracket shell through which the pedal-crank spindle turns.
Why It Wears
The bottom bracket is harder to reach than the hubs but works harder. Take an assumed 80 kg rider standing wholly on one pedal: P = 80 × 9.81 = 784 N. The pedal sits outboard of the nearer bearing, so the spindle acts as a lever across the two bearings. With the pedal load 90 mm outboard of the nearer bearing and the bearings 80 mm apart (both assumed distances):
- Far bearing reaction = P × 90 / 80 = 784 × 1.125 = 882 N, in the opposite direction
- Near bearing reaction = P × (1 + 90 / 80) = 784 × 2.125 = 1,666 N, about 1.7 kN
The nearer bearing carries more than twice the rider’s weight, with the 11 balls of that side sharing it, and the load changes direction every pedal turn. So the bottom bracket wears, and a pitted track becomes obvious as roughness every revolution.
What You Need
- A crank puller, or the appropriate extractor, to remove the cranks. (Cranks are held by a cotter pin or by a bolt on a tapered square; they cannot simply be pulled off, and hammering them off damages them. Recalled bicycle practice; the carrier does not cover it.)
- A special wrench, or a large adjustable wrench, for the lockring and bearing cup
- A cup, solvent, rags and grease
- Replacement balls (11 per side, 1/4 inch)
Method: Remove, Clean, Repack, Adjust
Step 1: Remove the cranks. Take off the pedals if needed and use the puller; do not strike them.
Step 2: Check the thread directions before applying force. On a conventional English-threaded bracket the adjustable cup (non-drive side) and its lockring unscrew in the usual way, but the fixed cup on the drive side has a left-hand thread and is loosened by turning clockwise. Other standards exist, so test with gentle effort, and stop if it feels like it is tightening (recalled practice; verify on your own bicycle). Forcing the wrong direction damages the shell threads, which cannot be repaired by hand.
Step 3: Loosen the lockring and the adjustable cup with the special wrench. The assembly sits inside the frame’s bracket shell, and a large adjustable wrench will do if you do not have the special one.
Step 4: Catch the balls, usually 11 per side, 1/4 inch each. Count them.
Step 5: Clean and inspect. Wash out the shell, clean the cups, balls and spindle cones. Pits on the cups or the spindle make the crankset feel rough whatever the adjustment.
Step 6: Repack and reassemble. Pack with grease, set the balls in, refit the spindle, thread the cup in until the spindle turns freely with no side play, and lock the ring while holding the cup still.
Step 7: Refit the cranks, tighten them firmly, and check them again after the first ride.
Common Mistakes
- Using the wrong thread direction — strips the shell.
- Hammering cranks off — bends the arm or damages the taper.
- Skipping the inspection — a pitted track cannot be adjusted out.
- Not rechecking the cranks — a loose crank can fall off under load.
Safety
Slipping wrenches and loose cranks
A large wrench slipping on a stuck lockring can injure hands, and a crank that loosens in use can drop the pedal under the rider’s foot at speed. Brace the frame, pull the wrench toward a stable hold, tighten the crank fixings firmly, check them after the first ride, and keep flames away from the solvent (Bearing Maintenance).
What’s Next
- Learn next: Headset Bearings — the steering bearings
- Combine with: Chain Repair — the chain that the crank drives