Unlock the cup and cone bottom bracket
If the bearing is serviceable the left-hand cup will have a lock-ring around the outside. Turn this anti-clockwise to unlock the cup. This is easiest with a lock-ring spanner that fits the notches on the ring exactly, you can also use a universal tool with one tooth or a tap it round with a hammer and an old screwdriver. Remove the lock-ring.
Once the lock-ring as moved back the cup can move. If the cup and cone bottom bracket has hexagonal flats turn it with a spanner. If it has holes use a peg spanner.
Loosen The Left Side
Turn the left cup – the adjustable cup – anti-clockwise.
Dismantle The Bearing
Remove the left cup, collect the balls from the left side, they may be loose or in a clip. Pull out the axle and note which way it fits it may not be symmetrical, one side may be longer than the other. Remove the balls from the other side.
Use a light to inspect the inside of the right cup. If the bearing surfaces are in good condition there’s no need to remove the right cup, the fixed cup.
Remove The Right Cup If Necessary.
On most bikes the right cup – the fixed cup – is screwed in on a left-hand thread. The exceptions are some old Italian and French bikes. The fixed cup may have two shallow flats or be hexagonal. There’s a bike-shop tool that locks a socket onto the fixed cup to turn it, and various spanners that fit fixed cups. As a last resort you may need to partially strip the frame, clamp the cup in a vice and twist the whole frame.
Replace The Balls and clean parts
Remove the old balls and replace with a set of exactly the same size, they are usually 1/4 inch. It’s better to use loose balls as you can fit more in, than if they’re in a clip. Usually loose is 11 each side, there are usually 9 in a clip. More balls means each one carries a lighter load. Clean the axle and cups with degreaser.
Reassemble the cup and cone bottom bracket
Pack the left cup with grease and press the balls into the grease.
Grease The Cup Threads
Grease the threads on the frame with anti-seize assembly grease, unless the right cup is on a standard – right-hand – thread in which case clean its frame threads, and the outside of the right cup with acetone, let it dry and apply thread-lock adhesive. Right cups on a standard thread tend to unscrew in use.
If the fixed cup is out pack it with grease and press the balls into the grease then screw it in on the right-side, it usually goes in on a left-hand thread.
If the fixed cup is in the frame put a sausage of grease around the right race on the axle and press the balls into the grease. Feed the axle into the frame taking care not to disturb the balls. Make sure the axle goes in the correct way round. The longer arm – it there is one – usually goes on the right, any writing on the axle usually reads from left to right.
Screw the left cup into the frame.
Fit the lockring loosely.
Adjust accordingly and then tighten the lockring to fix into position. Replace the left hand crank.