Is this a nut used to hold the feeler where you want to be able to ease the feeler arm around? If so then the feeler moving will move the nut eventually. The only good way to handle such a thing so you can adjust the retention pressure is with a D washer and flat on the shaft or with a keyed washer in a groove in the shaft such as found in pedals. A torque isolation washer of this sort along with a couple of spring lock washers will let you set the retention torque but allow you to still move the arm around without loosening the nut.