@WillAdams, running the wrench gcode through CutViewer Mill and using CVM’s measuring tool, the wrench flats are 12.5mm apart. I’m using a 1/2" wrench.
@MrHume, all ER collets and collet chucks are metric on the outside. And it’s not like you’re installing a head bolt on a car engine–it does not take a whole lot of torque to retain the Nomad tools adequately. The wrench Will mentions is machined out of a piece of aluminum-plastic laminate, which says something about the torque level required…