- 1). Remove the scooter panel covering the engine by hand. Use a screwdriver to loosen and disconnect the screws holding on any engine covers, such as the fan cover and cylinder cover. Remove the covers by hand. Use a socket wrench to disconnect the rear suspension and rear wheel and hub if necessary to lower the rear of the engine (on some engines this gives more clearance to access the cylinder).
- 2). Disconnect the sparkplug wire and cap by hand. Use a socket wrench to loosen and remove the cylinder cap holding nuts. Pull the loose cap off the cylinder studs. Use a socket wrench to loosen and disconnect the exhaust from the cylinder exhaust stub. Carefully pull the cylinder off the cylinder studs and engine. Catch the piston and crankshaft arm as they appear out the backside of the cylinder. Place the old cylinder aside.
- 3). Remove the gudgeon pin circlips from both sides of the piston with circlip pliers. Insert a gudgeon pin tool through the piston, and secure the tool on the other side with the lever disc that will catch the gudgeon pin. Twist the tool handle so that it pulls the pin out of the piston. Remove the piston from the crankshaft once the pin is completely removed from the inside of the piston. Be careful not to lose the piston bearing inside the piston and crankshaft end. Remove the old cylinder gasket at the base of the cylinder studs on the engine.
- 4). Insert the bearing again into the crankshaft end, and place the new, smaller piston onto the crankshaft arm. Insert a new gudgeon pin that fits the smaller piston, and push it with your thumb through the piston, bearing, and crankshaft end until fully inserted. Secure the pin by re-installing the circlips on either side of the piston with circlip pliers. Install the piston rings on the new piston if they are not already in place.
- 5). Insert a new cylinder gasket on the cylinder studs, and push it down past the piston against the engine cases. Grease the new cylinder insides with engine oil. Insert the new, smaller cylinder onto the cylinder studs until it meets the piston. Carefully insert the piston connected to the crankshaft into the cylinder, squeezing the piston rings while fitting it in. Once seated, push the cylinder down to the engine case as fast as possible (the piston will push inwards). Tap the barrel with a rubber mallet to make sure it is as far as it will go.
- 6). Place the new cylinder cap on top of the cylinder on the cylinder studs. Use a torque wrench to tighten down the cylinder nuts. Make sure to tighten them to the factory settings for your scooter (refer to your scooter workshop manual or scooter manufacturer website). Insert a new spark plug and re-attach the sparkplug wire. Reinstall the exhaust. Reconnect the rear suspension and rear wheel and hub if removed, and tighten with a socket wrench. Test the engine to confirm it works properly by turning on the scooter.
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