Abstract: The present invention discloses an internal combustion piston engine having a cylinder and a piston, the cylinder having an internal cylindrical bore and an inner wall, the piston being slidable within the cylinder, the piston having a top portion and a skirt portion below the top portion, the skirt having a wide portion with a diameter greater than the diameter of the cylinder bore, the piston being made of a resiliently deformable material and, upon insertion into the cylinder, the piston deforms such that there is no clearance between the wide portion of the piston and the inner wall of the cylinder. The skirt portion of the piston may also have an elliptical cross-sectional profile, the elliptical cross-sectional profile having a long axis longer than the inside diameter of the cylinder and a short axis shorter than the inside diameter of the cylinder.