On many small bikes the flywheel is the outer rotor on the end of the crankshaft. It holds strong magnets and turns around the stator coils. It also adds weight to smooth the engine, and may carry the timing marks, a trigger magnet, a starter-clutch gear or the cooling fan.
What it does
As the magnets sweep past the stator coils, they make the electricity for ignition, lights and charging. In a CDI system a small trigger magnet on the rim tells the ignition when to fire. On a points magneto the flywheel also hides the points, which are timed to marks on its rim.
Signs it is worn or failing
- No spark or weak charge with a good stator: damaged or weakened magnets. See Check an alternator rotor and No spark at the spark plug.
- Metal chips or magnetic debris stuck inside the rotor.
- Rub marks on the magnets or stator from a worn crankshaft bearing.
- Loose or damaged key and keyway: timing that jumps and a bike that loses spark after a kickback. See Kickstarter kicks back against my foot.
- Cracked rotor or damaged taper.
When to replace it
Replace a flywheel with a cracked rim, damaged taper, damaged keyway or weak magnets. Clean off metal debris first and check the stator for damage. The flywheel is normally replaced as a complete part.
What to match when buying
- The exact model, since flywheels differ in magnet layout, trigger magnet position, weight and taper.
- Whether it has a starter clutch or a cooling fan.
- Use a pre-owned part only if its magnets and keyway are in good condition.
Fitting notes
- Remove it only with the correct puller, never with a hammer on the rim. See Remove and refit a flywheel.
- Keep the key in place and tighten the centre nut to the value in your bike's manual.
- Keep magnets and metal bits away from the stator and trigger.
- Reset the timing after refitting. See Set points timing on a flywheel magneto.