Abstract: This invention is concerned with an improved system of dynamic balancing of the primary and secondary vibrations associated with combustion and inertial momentum effects, in particular, in single cylinder engines. By means of the primary and secondary balancing shafts, the invention achieves maximum balancing effects in small spaces, by combining the power transmission and dynamic balancing functions of the shafts; by mechanical arrangement of the primary and secondary balancing shafts in the vicinity of and parallel with the crank shaft; and by a judicious choice of the inter-shaft distances to permit placing the largest possible momentum balancing weights to the balancing sections.