Abstract: Apparatus for balancing or equalizing the mass forces of an engine having reciprocating pistons turning a crankshaft rotates a drive gear twice as fast as the crankshaft. A planetary gear is eccentrically carried on the drive gear and meshes with internal teeth on a stationary sun gear at a gear ratio of 1:2. The planetary gear has an eccentrically placed counterweight of a mass such that it balances the mass forces of the engine of the same order as the ratio of drive gear to crankshaft speed, Order II mass forces for this example. The drive gear also has an eccentrically placed counterweight which balances the planetary gear as they rotate with the drive gear. The counterweight on the planetary gear is preferably placed as near as possible to the pitch diameter of the planetary gear so as to make its path of travel as nearly linear as possible.