To meet a need identified in earlier valve train friction testing, a tappet rotation monitor was developed and implemented on a cam/tappet tribometer. The system was based on reflecting a light beam from a rotating and translating segmented surface onto a phototransistor. The resulting output was then processed by linear frequency to voltage conversion and multiple cycle integration. The experimental apparatus, signal processing circuit, and the testing circuit are presented. The multiple cycle integration was found to yield average tappet rotation as a function of cam shaft position. These results agreed well with earlier attempts to measure tappet rotation with high speed photography, were repeatable, and correlated with cam shaft speed. Some selected data is also presented which indicates that tappet rotation reduces cam/tappet friction.