To ease working accuracy for parts, to improve an assembling property, and to stabilize a clearance in an overlapped face of the parts each other.

A semicylindrical bolster 3 is brought into contact with the flat underface central part of a piston crown part 2a. A pair of left and right arms 44 having cicular arc-shaped cross-sections extended to a crank shaft direction is provided in the small end part of a connecting rod 6. The arm 44 is nipped between a cylindrical recess 5a in the upperface of a seat cylinder 5 supported in an inner space of a skirt part 11 and a cylindrical protrusion in the underface of the bolster 3. A conical face 5b is formed in the lower end of the seat cylinder 5, and plural projected pieces 8 are formed in the inner wall of the skirt 11 with a space in a cicumferential direction. A notched locking ring 33 with a conical face 37 formed on its upperface is engaged between the conical face 5b in the lower end of the seat cylinder 5 and the upperface of the projected pieces 8 of the skirt 11.