Piston

A piston is used to transfer force from expanding gas in the Cylinder . The expanding gas is in the case of model Steam engines etc Steam . This force is transferred to the Crank via the piston rod.

Essentially a piston is a movable blockage in a tube, once pressurised gas (in this case Steam ) is introduced to the tube the piston must move up the tube. The tube or Cylinder is pivoted and the piston is connected via a rod to the Crank thus creating a rotating movement.