the 10 mm nut is around the adjustment stud. it must be loosened so you may adjust the valve. a deep socket 10 and a wrench will make it work. the problem lies in tightening it back up.

once the 10 mm nut is loose, the adjustment stud (which is surrounded by the 10 mm nut) will move when you try to tighten the 10mm nut back up. you are usually suppose to hold the adjustment stud down with a screw driver, and use a wrench to tightend the 10 mm nut up. to lock you adjustment in place.

on a d series you can get a way with an angled 10mm box wrench.

on a gsr motor it's difficult without the snap on tool.