M12-1.25 Class 04 DIN 439B Hex Jam Nut Extra Fine Zinc Plated, 50 ct

Jam nuts are often used when a nut needs to be locked in place without clamping against another object. The jam nut essentially acts as the "other object", as the two nuts are tightened against each other. They can also be used to secure an item on a fastener without applying force to that object.