Abstract: Mechanism for counterbalancing a crankshaft, connecting rod and piston assembly. Carried by the shaft are the typical or conventional counterbalancing mass means. The invention provides additional counterbalancing mass means which are rotatable relative to the shaft and relative to the regular counterbalancing means in a manner to counterbalance through 360.degree.. The additional counterbalancing means are driven by a gear means coaxial with the shaft and additional driving planetary gear means mounted on a shaft which planetates, the planetary gear means not rotating about their axes. The additional counterbalancing mass means are driven at a speed twice that of the regular counterbalancing masses. The conventional counterbalancing masses and the additional counterbalancing masses take relative positions throughout the 360.degree. to counterbalance at all positions.