The crankshaft is an eccentric shaft that transform the piston's alternate linear motion to perpetual circular motion, which is then transmitted to the gearbox via the flywheel and clutch.

The crankshaft is typically cast, and the areas where the connecting rod will be used are precisely moulded by milling. The crankshaft is made from low alloy steel. The crankshaft is also cast, and it is made of globular garnet cast iron. To make the material resistant to abrasion, warping, and torsion, the parts in the centre of its cross-section, such as in other shafts, are made of a softer alloy. To prevent micro-boiling, the plain bearings (crankshaft bearings) that support the crankshaft are made of grey cast iron.

The crankshaft is lubed through the lubrication channels on it or piston rods that strike and splash the oil in the crankcase .