Heretofore others have suggested the use of a single cylinder vertical crankshaft engine in a tractor such as illustrated in U.S. Pat. Nos. 3,311,186, 3,613,815 and 3,003,574. The axis of the single cylinder is horizontal and is longitudinally disposed in relation to the tractor. Such engines have primary unbalanced inertia forces acting at engine speed which are compensated for by appropriate counterweights rotating at engine speed. Such single cylinder engines, however, do not have a secondary moment of unbalance acting at twice engine speed which is created by the axial spacing of the connecting rods on the crankshaft as exists in a typical two-cylinder opposed piston engine. Because of the expense and space requirement for rotating means for balancing the secondary moment of unbalance, almost all, if not all, of the small horsepower engines presently manufactured for lawn and garden equipment do not include means for balancing the secondary moment of unbalance.