The steel arm which connects the piston rod with the crank on the driving wheel or driving axle. It is used to convert the forward and aft motion of the piston into the rotating motion of the axle. It is designed in a tapered form and has a 'little end', where it is connected to the crosshead on the piston rod, and a 'big end' where it is connected to the crank arm. The tapering is to allow for the greater stresses experienced at the crank end.