This Z-axis adjustment knob has a M8 thread and can be screwed on the end of the lead screw of a i3-style printer with metric M8 threaded rods for the Z-axis. The main purpose is it to make adjusting the dual lead screw easier, so that you can easily have them both on the same height.

I am using M3 square nuts with M3 8mm screws to prevent the knob from unscrewing.