Originally Posted by
Simon Barden
If it is single action, then loosening it (turning left) will put as much concave bow in the neck as it has naturally under string tension, but won't add any more. But on a double-action truss rod there is a loose/dead patch when everything goes slack and then it starts to bite again and will then start add more concave curve to the neck. If you loosen it so much that the adjustment nut comes off without that second 'bite', then it's single action and you just re-thread the nut on the rod and tighten it until it just bites, so it's not loose and rattly.