The crankshaft of the motorcycle engine adopts a combined type, which is formed by pressing the left half crankshaft, the right half crankshaft and the crank pin. Ball bearings are installed on the main journals of the left and right half shafts to support the crankshaft on the crankcase. Both ends of the crankshaft are respectively equipped with a flywheel, a magneto and a clutch driving gear. The connecting rod is an integral structure, the big end is a torus, and a needle roller bearing and a crank pin are installed inside to form a crank connecting rod group. When installing the piston ring in a two-stroke engine, pay attention to align the opening of the piston ring with the positioning pin in the piston ring groove to prevent the piston ring from rotating in the ring groove, causing air leakage and scratching the inlet and exhaust on the cylinder liner. Breath.