Hold the engine still to undo a crankshaft, flywheel or clutch nut

The nut on the flywheel, the clutch centre or the front sprocket is tight, and the engine turns when you put a spanner on it. You need a safe way to lock the crankshaft. The method you choose depends on whether the engine is in the frame and on which nut you are undoing. The same methods are used to hold the engine at top dead centre for a leak-down test.

Safety

• Key off and battery negative lead disconnected on electric start bikes.
• A breaker bar that slips can hit your hand or face. Pull, do not push, and keep your body out of the swing.
• Never jam a screwdriver, a bolt or a punch through the flywheel or between gear teeth. It breaks the flywheel, the teeth or the case, and these parts are costly.
• When you tighten the nut again, use the torque in the manual. A flywheel turns very fast close to your leg; it must not come loose.

You need

• The holding tool or strap wrench made for flywheels, or a holding tool for the clutch centre if the manual lists one.
• A socket that fits well, and a long bar for the loosening torque.
• Soft cotton rope (not nylon, not wire) if you use the rope method.

Steps

1. Engine in the frame, front sprocket or clutch nut: select top gear and have a helper hold the rear brake hard. This locks the gearbox and the crankshaft through the chain. It is the easiest method for the front sprocket nut, and it works best before the chain is removed.
2. Flywheel nut: fit a flywheel holding tool into the holes in the flywheel, or a strap wrench around its rim. Hold the tool still and undo the nut with the socket. Do not hold the flywheel by its magnets or by the pickup lug.
3. Rope method (four-stroke, plug out): turn the engine until the piston is rising on the compression stroke with both valves closed. Feed a length of soft cotton rope into the plug hole, leaving a long tail outside. Turn the engine gently forward until the piston presses on the rope and stops. Now undo the nut. Afterwards, turn back and pull every bit of rope out. Count that the full length came out.
4. Leak-down or TDC hold on the bench: fit a long bar on the crankshaft nut and hold it firmly, or clamp it so it cannot swing. Air pressure in the cylinder can turn the crank hard and suddenly.
5. Undo the nut a little, then check: did the crankshaft stay still? If it moved, stop and choose a firmer method before you go on.
6. To refit, use the same holding method and tighten to the manual torque with a torque wrench. Use thread locker only if the manual calls for it.

Check your work

• The nut came off with no marks on the flywheel, the gears or the case.
• No rope is left inside the cylinder.
• The nut is back at the manual torque.

If it did not fix the problem

• The nut is seized or the corners are rounding: Remove a stuck or seized fastener.
• Next steps for the flywheel: Remove and refit a flywheel (magneto rotor).
• Finding the right position first: Find top dead centre on the compression stroke.