The torsional vibration of systems involving engines exhibits some phenomena not typical of vibration in general. These effects result from the geometry of the reciprocating mechanism and are revealed by detailed kinematic and dynamic analyses. Apart from the system's inertia varying with crank rotation, recent work by the authors has shown that it is also affected by piston-to-cylinder friction. In previous published work, that omitted friction, differences were observed between the measured and the predicted natural frequency function. This short paper revisits those differences and includes friction effects. (c) 2007 Elsevier Ltd. All rights reserved.