Πρόκειται για μικρές οπές στα έμβολα (πιστόνια).

http://www.dpengineparts.com/el/pistons/sxedia-pistonion.htm...

Ορισμός: a gas port is a hole drilled from the piston deck (a vertical gas port) or from the top ring land (a horizontal gas port) to the rear of the top ring groove. The purpose of a gas port is to apply combustion pressure directly to the top ring, forcing the ring face firmly against the cylinder wall.