Introduction
Some bottom brackets can be adjusted to tighten or loosen the bearing. Others are fit-and-forget, when they wear out the whole unit must be replaced. Some have cartridge bearings that can be replaced. Identify an adjustable bearing by the lock-ring on the left-side cup
Step 2

Unlock
If the bearing is adjustable 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.
Step 3

Tools
Once the lock-ring has moved back the cup can move. If it has hexagonal flats turn it with a spanner. If it has holes use a peg spanner.
Step 4

Adjust
If the bearing is too loose screw the cup inwards clockwise. If it's too tight screw the cup outwards anti-clockwise. Make small adjustments and check between them.
Step 5

Re-lock
Once the bearing is in the target zone hold the cup still with a tool and screw the lock-ring down against the frame to jam the cup in place.
Step 6

Finish
Make a final check and replace the crank.