A radiotracer technique has been developed to monitor the rotational movement of piston rings during engine operation. Each ring to be monitored is modified by incorporating two different radiotracers at an angular separation of ∼120°. After engine assembly, a high-resolution detector is mounted outside the engine block to measure the γ-ray emission rates of each of the isotopes. Gamma-ray intensities monitored during engine operation provide information to determine the angle of ring-gap orientation in real time. The radiotracer method for ring-rotation measurement can be applied to any ring in the engine and may be modified to monitor the simultaneous movement of two rings in a given cylinder. Because the radioactive sources are confined and are of low intensity, radiation safety precautions are minimal.