Removing a flywheel: with a puller and what happens without

Workshop guideUpdated 2026-09-13ELDI / Racing Planet editorial team

The flywheel sits on the taper of the crankshaft. It is not held there by the nut, but by the friction between two cleanly pressed conical faces. That is exactly why you will not get it off undamaged with a screwdriver, a lever or a hammer. The flywheel puller is the only tool that applies the force where it belongs: centrally on the end of the shaft.

Here you can read how the taper works, how to identify the right thread, how the job runs step by step and what happens if you try it without a puller.

Comparison: drag the slider. On the left what hammer and levers do, on the right the puller.

Why the flywheel sticks on the taper

The crankshaft stub and the flywheel hub are both ground to a taper. When you tighten the flywheel nut, it presses the flywheel onto the taper. The conical faces wedge together, and the friction joint transmits the torque. The nut only holds the flywheel in position, it transmits no force itself in the direction of rotation.

The small half moon Woodruff key in the groove of the shaft stub has a different job. It fixes the angular position of the flywheel relative to the crankshaft so that the ignition timing is correct. It is not built to transmit power. If the taper is not cleanly pressed together, the Woodruff key shears off and the ignition timing wanders.

After years in service the taper often sits tighter than on the first day. Moisture, corrosion and changes in temperature bond the two faces together. You then need a high axial force, built up evenly, to break the seat loose. That is exactly what the puller delivers.

Why levering and hammering do damage

If you lever behind the flywheel with two screwdrivers, you apply the force at the rim, not in the centre. The flywheel tilts on the taper, cants over and ends up running out of true. A distorted flywheel runs untrue, the air gap to the coils is no longer correct, and in the worst case it rubs on the pole shoes. The magnets are brittle. A blow on the rim can loosen or break them.

A hammer blow on the end of the shaft hits the crankshaft directly. The crankshaft of a two-stroke is pressed together, not made in one piece. The crank pin and the webs can twist out of line. The shaft then no longer runs true, the bearings suffer and the engine vibrates. On top of that the blow upsets the thread on the end of the shaft. An upset thread can neither be undone nor tightened cleanly afterwards.

Identifying the thread: M17, M22, M26 or M27

The puller engages a thread on the flywheel. On most ignition systems this is a male thread on the hub, and the puller then carries the female thread. Before you buy a tool, measure the outside diameter with a vernier caliper and check the pitch with a thread gauge. Without a gauge, the thread finder, and the thread table lists the dimensions.

Thread	Ignition system	Typical vehicles	Tool
M17x1	Piaggio	Vespa Ciao, Bravo, SI	Flywheel Puller M17x1
M22x1.5 and M26x1.5	Bosch, Ducati	Zündapp, Kreidler, Hercules, Puch	Flywheel Puller M22 / M26
M27x1.25 right hand	Ducati	Hercules, Garelli, Malaguti	Flywheel Puller M27x1.25

M22 and M26 differ clearly in diameter, the pitch is the same. M26x1.5 and M27x1.25, on the other hand, are only one millimetre apart and have different pitches. An M26 puller on an M27 thread only engages by force and destroys both threads. So always measure instead of guessing. If you know the vehicle, the tool finder will also lead you to the right puller.

Removing the flywheel step by step

1. Remove the spark plug

   Unscrew the plug with the spark plug spanner. Without compression the crankshaft turns freely, and you notice at once if anything binds while you are undoing it.

2. Undo the flywheel nut

   Hold the flywheel with a strap wrench or a suitable holding tool. Alternatively, engage a gear and have the rear brake applied. Undo the nut, take off the nut and washer and clean the thread on the flywheel with a brass brush.

3. Screw on the puller

   Wind the pressure spindle right back. Screw the puller onto the flywheel thread by hand until it seats. It has to take the full length of the thread. A puller that only engages two turns will strip the thread out of the hub.

4. Set the spindle

   Wind the pressure spindle in by hand until it stands centrally on the end of the shaft. Check that it sits cleanly in the centring bore and is not resting at an angle on the edge.

5. Build up tension

   Hold the puller with one spanner and tighten the spindle with a second one. Increase the force evenly. The taper usually lets go with an audible crack. If it does not let go, leave the tension on and give the spindle head a short tap with a hammer. The impulse travels axially through the spindle into the shaft and breaks the seat loose.

6. Take off the flywheel

   Unscrew the puller, take off the flywheel and secure the Woodruff key straight away. It likes to drop into the casing. Lay the flywheel down with the magnets facing up on a clean surface so that no swarf sticks to them.

Removing a flywheel without a puller: what really happens

Two methods do the rounds online. Both work occasionally, both carry a real risk. You should know what you are doing.

Heat. The flywheel is heated at the hub with a hot air gun. Aluminium and steel expand at different rates, and the hub is meant to loosen off the taper. The problem: the heat also goes into the magnets, the coils and the shaft seal behind them. Permanent magnets lose magnetic force at too high a temperature, the insulating varnish on coils goes soft, and the shaft seal hardens. Besides, heat alone does not release the taper, you still need axial force.

A blow on the nut. The flywheel nut is screwed on flush, one person pulls the flywheel outwards, a second one strikes the nut with a plastic mallet. That does sometimes release the taper. But the blow goes unchecked into the thread, the crankshaft and the bearings. On a flywheel that has been stuck for years the force is not enough anyway, and you hit harder and harder. That is exactly when the damage described in the section above occurs.

To be honest: on a loosely seated flywheel that was fitted only recently, the nut method can work without consequences. On a flywheel that has not been off for twenty years, it will not work without a puller. Weigh the cost of a crankshaft against the cost of the tool, and the decision is easy.

After removal: taper, key, refitting

Clean the taper and the hub with a lint free cloth and brake cleaner. Both faces have to be dry and free of grease, otherwise the friction joint slips. Check the taper for scoring and rust pitting. Light marks can be smoothed out with fine abrasive fleece, deep scoring is a job for a specialist workshop.

Check the Woodruff key. It must show no signs of shearing and must sit in the groove without play. A damaged key is replaced, never filed to fit. Put the flywheel on so that the slot in the hub lines up exactly over the key, and press it onto the taper by hand.

The flywheel nut is tightened to the torque given in the workshop manual for your vehicle. Too little torque and the flywheel works on the taper until the key shears off. Too much and the thread suffers. The torque calculator helps with the conversion, the tightening torque table gives general reference values for thread sizes.