Crankshaft

On small single-cylinder engines the crank is usually a pressed-up assembly: two flywheels, the crankpin and the connecting rod are sold as one unit. It turns on main bearings and is sealed with oil seals at both ends.

Symptoms of a problem

• A knock from the bottom of the engine that gets worse under load (big-end play)
• Increased vibration (crank run-out)
• Rumbling (worn main bearings)

When replacing a crankshaft, renewing the main bearings and seals is recommended.