Abstract: Piston for internal combustion engine, such as for a free piston engine, of the type comprising one assembly of two co-axial pistons, the inner cylinder having an external diameter which is substantially less than the inner diameter of the outer cylinder, thus defining a closed annular chamber designed to receive a cooling fluid. Longitudinal ribs are provided in the closed annular chamber which ribs extend over most of the height of said chamber and are inclined in alternate directions with respect to the piston axis, thus defining in said chamber, a succession of adjacent longitudinal channels whose section increases from one end of the piston to the other.