Creanoso Guitar Truss Rod Allen Wrench Adjustment Tool for Martin Acoustic Guitars To adjust the truss rod on Martin Guitars and most acoustic guitar 5mm ball end guitar truss rod. Dimension: 2.95" x 4.92" Bonus: Creanoso Guitar Pick Counter-clockwise on the nut always gives you some relief. Clockwise removes relief. Do your adjustment a little at a time with the guitar tuned up to pitch.