How to find zero? – the clock face is rotary! A highly precise sensor which adjusts electric ignition.The sensor determines the position of piston and thanks to it adjusting the ignition can be very precise. Durable adapter is 17,5 cm long and it’s made from one piece of aluminium (none plastic parts inside!)The measuring rod is screwed into dial indicator and it doesn’t make a friction forces in the adapter, so this tool works even “upside down” (it’s important in boxer type engines, where tdc tool works almost horizontal) The small hole on the spark plug thread side works as a stabilization sleeve for the measuring rod so the measurement is more accurate.The thread size: M14x1,25.This device is very useful In adjusting ignitions of a variety of engines.The sensor is brand new, in good working order. It has been never used before.