Piston-connecting rod spherical coupling structure

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.