the engine where the input fuel air charge is compressed in the usual manner of operation of a two cycle engine referring to fig6 the piston 50 of the invention may have annular grooves 51 for piston rings of the usual type which ride in fixed position therein in the illustrated embodiment the top face 52 of the piston is domed slightly to conform to combustion chamber design however such doming is not required to realize the advantages of the invention one particularly novel aspect of the invention is rounding of the piston side wall 53 into the piston face 52 the rounding need not be of a particular radius but should smoothly join the side wall 53 and the face 52 with a generous curve tongent to both surfaces 52 and 53 at its points of intersection therewith in one configuration of the preferred embodiment a piston of 1 55 in diameter very successfully performed with such curvature commencing at the top edge 54 of the upper ring groove 51 and extending upwardly through a vertical use of approximately 0 110 in where it joined the slightly domed face 52 of the piston although the inventor has not run studies to determine if the exact nature of this curvature can be correlated with the curvature of the outer walls 31 of the input passages to determine an optimum configuration it is possible that such a relationship may be determined it is speculated that even a simple beveling of the piston edge would show some improvement in the flow characteristic of the cylinder but not so.