A computer program capable of predicting cyclic engine bearing forces, and dynamic unbalance in a single cylinder engine is used to evaluate and optimize counterweight size and angular location.The program requires inertial, geometric and locational input for each of the moving parts in the slider-crank mechanism. This information together with a cylinder pressure history allows calculation of dynamic bearing forces. The vector addition of these bearing forces with cylinder pressure force and cylinder side wall force determines the net unbalanced force produced by a single cylinder engine.The program evaluates means of reducing unbalance forces by simulating the addition of eccentric masses attached to the crankshaft and/or other shafts rotating at the same speed. Rotating mass can be added at any location along the crankshaft or any other parallel shaft rotating in the same (forward) or opposite (counter) direction. Working within counterweight locations specified at run time, the program selects-mass-eccentricity values at each location so as to optimally balance the engine. For the purposes of this presentation, optimum balance is obtained when the maximum magnitude of the rotating unbalance force vector is minimized, while net torque or couple about all three axes is simultaneously minimized.The program selects optimum (as defined above) counterweight masses within the constraints of counterweight locations specified at run time. Whenever possible, this choice includes the cancellation of unbalanced torque or couple. Because the simultaneous addition of mass to both a counter-rotating shaft and a forward-rotating shaft allows greater rotating unbalance force reduction, the state of balance possible for an engine with counterweight masses located on both forward-and counter-rotating shafts is superior to that of an engine balanced without counter-rotating shaft counterweight locations.Because the program allows computation of the state of balance utilizing both counter- and forward-rotating shaft mass, the balance benefits obtained from these additional shafts can be calculated and evaluated against their costs.