Use your finger to spin the lock nut counterclockwise until there is a about a one inch gap between it and the frame.

Wrap a piece of tape around the belt-tensioner bolt where it meets the frame.

During reassembly, tighten the belt-tensioner bolt until the tape is flush against the frame (or until the belt is sufficiently tensioned) and then tighten down the belt-tensioner lock nut to the frame.