The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation.
There are many performance crankshafts available for mopeds. These can generally take more stress at higher RPMs than a stock crank, which is perfect for kitted engines. Manufacturers of performance cranks include Top Racing, Rito, Parmakit, and DMP. For a breakdown of crankshaft options on Puch mopeds, see Puch E50 Crankshafts.
Installing a performance crank alone does not increase your speed. A performance crank can however influence the carter flow and the combustion chamber size.
A performance crank with full "cheeks" (otherwise known as journals or webs) can improve the carter flow. Sometimes there are holes in the cheeks, filled out with nylon or cork. This is done to reduce the weight of the crank, increasing gas reaction but reducing low end torque. This is mostly done on high-rpm machines, since they don't need a lot of low end torque.
Some performance cranks have a shorter shaft that allows for greater or less displacement. Shorter or longer shafts are also used to fit cylinders that are not meant to go onto a specific engine. Although it does not fix the timing-problems, it does fix the piston coming above or too much below the cylinder.
A common mistake is that a shorter or longer shaft would influence the length of the stroke. This is not true. The stroke is determined by where the shaft is connected to the crank, how far it is off center. The further off center, the longer the stroke.
Some performance cranks come "stuffed". This means that some of the space which is empty on normal, unstuffed cranks, where the con rod is connected to the crank is filled in with a light weight material (ie plastic or cork.) This helps compensate for added volume resulting from casematching, and thus raises compression.
On mopeds, usually one or both ends of the shaft have a taper on them. On a Puch E50, the flywheel has a taper matching one side of the crankshaft taper, and the clutch has a taper matching the other side of the crankshaft. The taper is what holds these parts in a stationary position on the crankshaft as the moped is running. To measure the crankshaft taper, please see Crankshaft ignition tapers.