The crankshaft is the heart of a piston engine. It converts the up-and-down (or 'reciprocal') motion of the pistons into a rotational motion which can be used to drive the wheels.
The crankshaft or 'crank' runs in two or more main bearings housed in the cylinder block. It carries big end bearings on which are mounted the connecting rods which connect to the pistons.
At one end the crankshaft carries a flywheel which transmits power through the clutch into the gearbox.