Abstract: Apparatus for determining time of occurrence of the maximum excursion, or top dead center or bottom dead center, of an engine piston or similar structure. The piston excursion is continuously measured, and a signal, corresponding to the portion of the cycle having the closest proximity, is selected. The signal is compared to a threshold, and the duration wherein the signal exceeds the threshold is measured by a first digital counter. The measured duration is divided by two, the resulting number counted by a second counter during the subsequent piston cycle, beginning simultaneously with the first counter. The second counter completes the count of one-half of the duration at the midpoint of the measured duration. The apparatus produces an output signal at the midpoint of the measured duration, which corresponds to the maximum excursion, or top dead center or bottom dead center, of the piston motion.