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.