When your mechanical clock needs to be wound, you need a clock key. The easiest way to determine what size key you need is to measure it. This guide will show you how to measure your clock for a clock key.
On the back of your clock is the winding arbor (the steel shaft the key fits on to). With your digital gauge, measure the square shaft from flat side to flat side to get the millimeter size. Most winding arbors taper out towards the the clock and the end section is most likely the smallest. This helps to make the selection of the key size a little less critical, as the key will slide onto the shaft until it reaches the ideal point.
You can either order from that size or use this chart to find the clock key # that corresponds to your size.