Neil Bevan, St Albans
Cranks have a tapered, square hole which mates onto a matching square taper on the bottom bracket axle. When the two holes are exactly the same size and shape, tightening the bolt squeezes the crank up the axle taper until its firmly fitted. The pair become as one. If the joint is allowed to work even a little bit loose, the crank moves on the axle as you pedal. The corners of the steel axle dig into the softer (alloy) faces of the crank. As time goes on, the joint fit gets worse and the axle digs in further until the crank is only good for scrap.
Once things have gone this far, replacement is the only option the crank taper will be so deformed it’ll never fit securely on the axle again. Before things go that far, its worth trying to clean up the two surfaces.
Remove the crank arm and use a degreaser, but be careful not to get any into the bottom bracket. Rinse off the degreaser. Lightly grease the crank bolt. Refit the crank and tighten firmly Shimano recommend a torque of 40-45Nm. You won’t be able to tighten the bolt enough with a normal short 8mm Allen key, so use one that’s at least as long as the crank. If need be, slip a short piece of pipe over the Allen key to extend it.
If the crank still works loose, be sure to replace it before the damage spreads to the axle. A new crank arm will set you back upwards of £13. If you leave it until the bottom bracket axle is damaged, you’ll be looking at an additional £15 for a new bracket at least.