I found some interesting text on www.alliedluthier.com; http://www.alliedlutherie.com/truss_rods.htm Quote : Truss rod installation: These truss rods should be installed with a filler strip over them. Installing a filler greatly reduces the possibility of a truss rod rattle and reduces the amount of backlash in the rod. My installation procedure goes like this: Mill a 1/4" wide, flat bottom slot in the neck that is at least .400" deep. Make a filler strip approximately 3/16" thick that snuggly fits the truss rod slot. Rough up the flat backbone of the rod with very course sand paper or a file. This creates a good gluing surface. Put the rod in the slot with no glue. Apply epoxy to three sides of the filler and press it into place over the rod. Don't overdo it with the glue. You don't want it to squeeze down and foul up the threads. Apply LIGHT clamping pressure along the length of the rod (see photo). You want enough pressure to eliminate any space between the flat bar and the round bar, but not so much that it makes the rod hard to turn. Check the resistance by turning the rod a quarter turn in each direction. You will notice that the filler strip sink in a little more in the middle than at the ends. After the glue dries, level off the protruding filler strip. Avoid turning the truss rod more than 1/2 turn until after the fingerboard is glued on and fully dry. Is it really necessary to use a filler strip? Isn't it enough to make a really snug fit for the truss rod and glue the fingerboard on it?