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Over-expansion is one of the promising strategies for improving Internal combustion engine (ICE) efficiency and emissions. It can be implemented by using an unconventional crankshaft. These mechanisms have mass production potential and acceptable friction losses, but also a complex kinematic and dynamic behaviour. This paper proposes such a crank design, a small planetary hypotrochoid over-expanded single cylinder engine - UMotor - and compares it to equivalent conventional ICEs with similar intake or expansion strokes. A suitable crankshaft counter weight geometry was determined in order to minimize the overall reaction forces. The mass balancing studies were conducted via a dynamic motion analysis of the crank drive, including a Fast Fourier Transform (FFT) frequency analysis. For the tested engine speeds, the UMotor average and peak magnitude reaction forces proved to be similar to those generated in the conventional engines for intermediate and high engine speeds.