A piston is free to slide in an engine cylinder, this piston comprising a piston head of circular cross-section, a skirt formed such that it is continuous with the lower part of said piston head, a piston pin passing through the skirt, and a connecting rod connected via this piston pin. This skirt comprises an upper part and a lower part which respectively have an elliptical horizontal cross-section with a short axis parallel to the piston pin, the horizontal cross-section of the upper part of the skirt having a higher eccentricity than that of the lower part of the skirt. The skirt further comprises a taper surface which smoothly connects its upper and lower parts. The contact surface between the skirt and the cylinder therefore has an inverted T shape so that the contact surface of the lower part of the skirt can be increased. It is therefore possible to set the distance from the center of the piston pin to the lower edge of the skirt such that it is no greater than 1.8 times the distance from the center of the piston pin to the upper edge of the skirt, so making the piston more compact while maintaining operational stability.