The enhancement of efficiency will play a more and more important role in the development of future (small) internal combustion engines. In recent years, the Atkinson (or Extended Expansion) cycle, realized over the crank drive, attracted increasing attention. Several OEMs have investigated this efficiency-increasing principle in the whole range from small engines up to automotive engines until now.In prior publications, the authors outlined the remarkable efficiency potentials of an Extended Expansion (EE) cycle. However, for an internal combustion engine, a smooth running performance as well as low vibrations and noise emissions are relevant aspects. This is especially true for an Extended Expansion engine realized over the crank drive. Therefore, design measures concerning friction and NVH need to be taken to enable possible series production status. Basically, these measures strongly depend on the reduction of the free mass forces and moments. Hence, the focus of this publication is laid on mass balancing measures of an Extended Expansion engine based on a linkage system.This paper first gives a brief overview of the specific 2-cylinder engine layout of the designed EE prototype engine. The second part deals with the determination of occurring free mass forces and moments. In the following, possible as well as performed mass balancing arrangements are presented and evaluated. Finally, a comparison between the EE prototype engine and other conventional 2-cylinder engines is performed.The determination of free mass forces and moments is based on numerical calculations. They include a kinematic simulation of the crank drive and, derived from that, accelerations of each relevant engine part. The geometry data are determined via CAD and the appropriate masses are calculated according to the corresponding density of the applied materials. In addition, results based on a Fourier transformation are presented, whereas free mass forces and moments are mathematically split into harmonic orders, which are helpful to evaluate balancing measures.