Patent ID: 8474430

Claim:
A crankshaft mechanism for an engine comprising: a piston; a crankshaft having a left part and a right part, each of the left and right parts being provided with a circular-shaped crank arm having a crescent-shaped counterweight part integrally formed on an inner surface thereof; a crank pin bridging between the circular-shaped crank arm of each of the left and right parts; and a connecting rod connecting said piston and the crank pin to each other with one of said counterweight parts being disposed on each side of the connecting rod; wherein each of the crescent-shaped counterweight parts is formed only on a portion of the inner surface of the crank arm opposite to where the crank pin is located, and an inner side of a circumferential end ( 44 e ) of each of the crescent-shaped counterweight parts is cut out along a curve at an equidistance (D 1 ) from a center (Q) of said crank pin to obtain a shape on the inner side of the circumferential end of each of the crescent-shaped counterweight parts to avoid a projected part, projecting in a direction of a piston pin, from each side at a lower end of a small end part of said connecting rod when said piston reaches a bottom dead center.