Cant remember for SURE, but I think the two dome nuts that secure the rail are 6mm.I have 2 huge Mountain Bikes which I take away with me Mitch so I need the space. I'll leave the rack as is and cross the puncture problem as and when. I just have to make sure I have a suitable spanner on board at all times so I can remove the rail.
A socket would be best.
Mitch.