Ports are openings found in the cylinder wall of a two stroke engine. The piston at particular times during its up/down cycle covers up a port to close it off; this action replaces the valve train found in a four stroke engine. The positioning of these ports determines when they are opened or closed, and so an opportunity is found to fine tune an engine by the altering their size and shape. "Port" can be used as a verb (porting, ported), describing the process of enlarging or modifying ports in a cylinder. It is important to note that the term "port" is also used to refer to openings in the cylinder head or block of a four stroke engine through which exhaust and fuel/air mixture pass.