On the senders I've seen, that nut is on a stud that is insulated as it goes thru the flange and is connected to a stiff, insulated, conductive strap. Maybe you ae twisting that strap inside. Grounding that stud/wire should read empty, not full. Full should mean you are breaking connection somehow. Torque the nut so it is reading correctly while wire terminal is ON TOP of the nut. Then add another nut to sandwich the wire terminal without moving the lower nut. If there are already two nuts just be sure bottom nut does not turn when tightening top nut.
