PURPOSE: To lengthen the stroke of a piston by slidably engaging the recessed part of a receiving plate of semi-circular cross section, formed at the end part of a connecting rod, to the protruding part of semi-circular cross section provided at the crown rear face of the piston, and bringing a keep, supported at a skirt part, into contact with the rear face of the receiving plate.

CONSTITUTION: A protruding part 4 provided with the spherical face 5 is formed at the crown rear face of a piston 2. The periphery of the protruding part 4 is partitioned by an annular space part 6, and cooling lubricating oil is filled therein. A bowl like receiving plate 12 is integrally formed at the tip part of a connecting rod 13, and its spherical recessed part 12a is slidably engaged with the spherical face 5. In order to hold this engagement, an annular keep 8 provided with a spherical recessed part 8a is brought into contact with the rear face of the receiving plate 12 and supported by a cylindrical nut 9 screwed with a skirt part 7. The measure (p) between an oscillating center (a) and a piston crown face 2a is reduced, and the arm (length r) of a crankshaft 16 is lengthened to that extent, so that the stroke of a piston 2 is enlarged, and therefore the air exhaust quantity is enlarged to a great extent without changing the height (h) of a cylinder body.