Rebuilding a scooter engine is one way to learn the basics of automotive repair. Scooter engines are small enough to be carried, easy to work on, and can be assembled on in a home garage. The engine should first be dismantled and the parts cleaned before it is rebuilt.
Buy a workshop manual for your scooter. Purchase new bearings, gaskets, oil seals, and rubber gaskets for your model engine from a scooter dealer.
Place your new engine scooter bearings in the freezer for at least 30 minutes. This causes the metal to shrink slightly. Turn on the propane torch flame with a lighter and set it so the flame is blue in colour. Heat the bearing socket in the engine case to be worked on, moving around the socket until the case aluminium is hot. Turn the torch off and quickly use a hammer and socket drift to drive the cold bearing from the freezer into the engine case socket. Be careful not to hit the engine case. Do this for all the bearings to be installed.
Place an oil seal over the inside of the installed crankshaft bearing and seat it in the engine case by hand. Insert your crankshaft assembly through the oil seal and into the main crankshaft bearing. Tap the crankshaft in place with a punch and hammer. Install your main transmission drive through its own bearing and do the same with the rear axle and gear set. Use a hammer and punch or socket drift to drive the axles home into the bearings.
Insert the flywheel bearing into the opposite engine case half the same way as the crankshaft bearing with a hammer and torch. Wipe the edge of the engine case halves with a thin layer of grease and apply the engine case gasket to the edge (the grease holds it in place). Install by hand the kick start gear on the opposite half of the case, if it has one. Press closed the engine case halves and tighten them with bolts and nuts, using a socket wrench and sockets.
Turn the half-built engine case around, if it uses a transmission belt drive instead of rotary gears. Place the rear clutch on the rear axle spindle sticking out of the engine case. Loop the drive belt around the rear axle and the crankshaft axle towards the front. Place the variator and rollers assembly onto the crankshaft arm by hand, and push them in place. Tighten the variator with its securing nut while holding the variator in place with a variator holding tool.
Place the outer cover over the transmission area. Secure it with cover bolts and nuts using a socket wrench. Insert the clutch holding tool in the rear of the case. Tighten the clutch nut to secure the clutch assembly. Replace the rear clutch nut access cover.
Pull the gudgeon pin through the piston into the crankshaft arm, using a gudgeon pin tool. Insert circlips into the side of the piston with a circlip tool. Carefully install the piston rings by hand onto the piston. Grease the cylinder barrel insides with engine oil using your fingers. Slide the cylinder onto the piston after placing a cylinder gasket on the engine case where it will be installed. Push the barrel into place and secure the cylinder cap. Tighten it with a socket wrench and insert the spark plug into the top.
Place the stator and flywheel on the other crankshaft arm sticking out of the engine case side. Push both in by hand. Secure the stator with a screwdriver and screws. Tighten the flywheel next with a flywheel nut using a socket wrench and a flywheel holding tool.
Use mechanic's gloves to avoid cuts.
Never force any part to fit. If the part does not install easily then something is being done wrong. Take the part off and reinstall it correctly.