How to Rebuild a 50cc Motorcycle Engine

Updated February 21, 2017

One way to become familiar with single-cylinder motorcycle engines is to rebuild a 50cc engine. In some cases, you can do a simple top-end rebuild with the engine still mounted on the motorcycle. A complete rebuild needs to be performed on a workbench, though. Any engine rebuild requires knowledge of bolted connections, engine tools and your motorcycle's repair manual. Compile the necessary tools and obtain the kits necessary to rebuild your 50cc motorcycle engine.

Drain the engine oil into an oil pan. Locate the master link on the drive chain. Remove the link and take the chain off of the motorcycle.

Examine the engine thoroughly. Identify and detach the external components, cables and lines that connect to the engine. These include the fuel line, carburettor, gearshift pedal, spark plug wires, throttle cable and clutch cable. With some bikes, removing the gas tank and rider's seat makes it easier to access certain components.

Loosen and remove the motor-mount bolts. Ask an assistant to help lift the engine out of the motorcycle frame and place it onto a workbench. Be careful with your motorcycle engine. Forcing it out of the frame or dropping it can break fins on the cylinder casing or damage the engine cases.

Remove the spark plugs from the cylinder head. Remove the cylinder head bolts. Tap the sides of the head with a rubber mallet to unseat the head gasket. If necessary, free the head from the cylinder by positioning the tip of a cold chisel along the seam of the head gasket, while tapping the end of the chisel with the mallet. Lift the head off of the cylinders.

Remove the nuts and washers at the top of the cylinder. Free the cylinder from the top of the crankcase using the cold chisel and mallet along the seam at the lower edges of the cylinder. Lift the single-cylinder up and off the threaded studs.

Enlist a motorcycle machine shop to service the cylinder head, the valves and hone the cylinder sleeve. Ask the shop to measure the cylinder. Obtain the new piston, piston rings, wrist pins and clips. Include a new head gasket and cylinder gasket.

Refer to your repair manual for procedures and information regarding the lower end tear-down. Obtain the tools and replacement parts necessary for a lower-end rebuild. Typical parts include seals, washers, crank bearings, gaskets and replacements for any damaged parts. The tools you need depend upon the make and year of your motorcycle engine.

Clean the engine casings and all bolts in a utility pan. Brush on generous amounts of engine cleaner with a utility brush. Clean the inside and outside for a thorough job. Clean the threads on all bolts and nuts. Wipe everything down with clean rags.

Refer to your repair manual or consult a motorcycle mechanic when installing crank bearings, seals and new parts. Use gasket sealer to hold the larger gaskets in place as you reattach the covers to the primary case. Put the new cylinder gasket on the engine crankcase.

Attach the new piston to the piston rod using a new wrist pin and wrist pin clips. Put the new rings on the piston. Slide the cylinder to the threaded studs and install it on the engine crankcase.

Place the new head gasket on the cylinder. Attach the cylinder head with the saved bolts. Tighten the head bolts to the specified torque with a torque wrench. This will vary depending upon the make and manufacturer of the engine.

Ask an assistant to help you position the rebuilt engine in the motorcycle frame. Tighten the motor-mount bolts. Fill the engine crankcase with fresh motor oil. Reconnect the removed components, cables and lines. Reinstall the drive chain on the motorcycle.


Always refer to the repair manual for torque specifications when tightening any bolts. Coat new seals with motor oil before you install them. Coat the piston and rings with motor oil before installing the cylinder on the crankcase. Replace any internal oil or transmission fluid filters. Consider replacing the clutch plates during the engine rebuild.

Things You'll Need

  • Repair manual (year and model of your motorcycle)
  • 50cc engine rebuild kit
  • Oil pan
  • Metric tools (sockets, ratchet and wrenches)
  • Torque wrench
  • Rubber mallet
  • Cold chisel
  • Utility pan
  • Engine cleaner
  • Utility brushes
  • Motor oil
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About the Author

William Machin began work in construction at the age of 15, while still in high school. In 35 years, he's gained expertise in all phases of residential construction, retrofit and remodeling. His hobbies include horses, motorcycles, road racing and sport fishing. He studied architecture at Taft Junior College.