Patent ID: 7290518

Claim:
A spherical coupling structure for a piston and a connecting rod, used in an internal-combustion engine, comprising: a protrusion formed on the reverse side of a crown of the piston and having a bottom surface on which a concave first spherical surface is formed; a spherical small end formed on the connecting rod and fitted in the first spherical surface so that the distal end is capable of sliding; a holding member having a concave second spherical surface and holding the small end by slidably fitting the portion of the small end that faces the large end in the second spherical surface; a first screw formed on the protrusion; and a second screw formed on a holding/fastening part for holding the holding member, wherein the small end is held by the first spherical surface and the second spherical surface by screwing in the first screw and the second screw; and the piston is composed of an upper piston that includes the protrusion and a lower piston that includes the holding/fastening part, and the masses of the upper piston and the lower piston are distributed so as to substantially match each other.