Spring supported crankshaft coupling structure for engine

A spring-supported crankshaft coupling structure (2) used in an engine and coupled between a piston (51) and a crankshaft to enhance the output torque of the engine is disclosed to include a connector (53) axially movably coupled to the piston (51) by a slip joint, a stop device (55) fixedly fastened to the piston to limit the moving distance of the connector relative to the piston, a spring member (52) mounted in the piston and adapted to impart a downward pressure to the connector, and a connecting rod (56) adapted to couple the connector to the crankshaft.