Rebuilding a 50cc scooter engine provides a chance to replace all the parts that wear down and could result in engine failure, if you don't take care of them regularly. This includes bearings, oil seals, and gaskets. By reassembling the unit periodically, you can replace and inspect durable parts as needed. The process takes only basic automotive tools available at your local hardware store, with a few speciality tools. The process below assumes the engine has been completely dismantled and the parts cleaned with solvent to remove old oil and grit.
Buy new engine bearings, oil seals, and engine case gaskets for your 50-cc engine from a scooter dealer or parts shop. Put the bearings in the freezer of your refrigerator and wait at least 30 minutes. Light a hand-held propane torch and heat up a specific engine case bearing socket.
Grab an engine bearing from the freezer (now very cold) and quickly position it in the heated up bearing socket. Use a wrench socket of matching size to the bearing diameter to carefully hammer the cold bearing into the hot engine case socket. Do this same procedure for all the bearings needing installation.
When the engine case is cool again, insert a bearing-holding circlip with a circlip tool into the bearing socket (its a tension snap-ring). Insert the oil seals by hand next to each of the bearings in the same socket (this keeps the oil from escaping the engine cases). Insert the crankshaft assembly into the main bearing that is now installed. Use a hammer and metal punch pin to tap it the crankshaft arm into the centre of the bearing.
Check that the spin on the crankshaft rotates correctly by moving the crankshaft around its bearing (look for resistance or odd angle friction). Attach the clutch or front drive sprocket (depending on engine design) to the crankshaft arm sticking out of the main bearing's opposite side. Tighten it down with a securing nut and socket wrench. Insert the rear axle and gear assembly on the axle into the rear wheel bearing. Tap it in as well.
Attach the rear sprocket and transmission to the rear axle. Tighten it as well with a socket wrench and bolt, screwing it into the rear axle end. Route the chain drive over the rear and front sprockets, if you are using a chain drive engine. Position the chain adjusters inside the engine case on their anchor bolts (they screw into the case where the chain goes around). Position them to take up slack and tighten the anchor bolts with a socket wrench and studs screwing into the engine case.
Place the engine case gasket on the engine block and close up the case with the engine cover after inserting the kickstart pedal mechanism in the cover. Secure it with the pedal and bolt on the outside using a crescent wrench. Tighten the engine cover with nuts and washers on each of the engine case studs using a socket wrench.
Attach the piston to the crankshaft using a gudgeon pin tool (it forces the connecting pin through the piston and crankshaft end), new piston bearing, and circlip tool. Place the cylinder base gasket on the engine block. Grease the inside of the engine cylinder and position it onto the engine, carefully slipping the piston into the cylinder by squeezing the piston rings. Push the cylinder to the engine block. Insert the cylinder cap and tighten it with nuts and washers, using a socket wrench.
Place the stator plate on the engine where it sits, surrounding the crankshaft arm sticking out of the engine case on the flywheel side. Secure it with screws. Route the wiring up through the engine channel at the top of the engine case. Place the flywheel onto the crankshaft once positioning the woodruff key on the crankshaft arm (this forces the flywheel to spin with the crankshaft), enclosing the stator plate. Tighten the flywheel down with a socket wrench and securing nut.
Insert the spark plug into the cylinder cap and tighten it with the socket wrench. Use a screwdriver to connect the ignition coil bracket and CDI unit to the back of the engine. Route the spark plug wire from the CDI to the cylinder and place the cap on the spark plug. Prepare to insert the rebuilt engine into the scooter.
While both the flywheel and clutch can be installed with basic automotive tools, the job can be easier when you use speciality flywheel and clutch tools specific to your engine.
Never force engine parts to fit together when rebuilding. If they are not connecting fairly easily, then something is in the wrong position or not installed correctly.