Abstract: A signal indicative of a distance from a first element to a second element, such as the end face of a piston from the end wall of its cylinder, is generated by providing an optical imager carried on a housing that is fixed on the end wall with the optical imager pointed to the end face. An image processor is arranged to identify image components, such as the end nut on the piston, from the acquired images of the first element. From the location of the nut the processor can identify the location of the outer edge of the circular end face and can determine the area of the end face in the image by counting pixels within the area. From the measured area as it appears in the image the distance can be calculated. The end face is illuminated by a light source shining through a light pipe passing through a drilled hole in the end wall. Characteristic markings can be applied on the end face for measuring distance when the nut is too close to be visible.