Clarification - I'm using the hex key to insert into the wheel... and yes, I take full responsiblity that I can dork up and damage the hex keyI'll check when I get back from work but almost 100% sure it's a 4mm hex key for Millennium truss rods. Using SA will damage the hex nut.
I stand by using metric. 3mm hex key dropped in nicely,couldn't get a 3/32" to fit properly.
Key I found too tight was 1/8th (3.175mm).