Materials and Tools:
Hod Rod Adjustable Truss Rod with 4 mm hex nut (Available from Steward MacDonald)
Table Saw: -or-
Purchase A Adjustable Truss Rod:
I have settled in on using the Hot Rod Anchor-less Neck Adjusting System that is available from StewMac. This is a two-way adjustment rod so you can correct both up bow and down bow situations, which is a real blessing when you need it. The installation of their system is extremely simple and adjusting the neck is much easier.
Truss Rod Selection:
For Acoustic Guitars, these adjustment rods (especially the hex head variety, can be installed at either end of the fretboard. The first location, at the peghead side of the guitar, will require a truss rod cover to be installed on the peghead to cover the access slot for the hexhead nut.
If you decide to install the adjustment nut at the head block of the guitar, you will need to plan on routing the slot not only through the neck, but continue it completely through the head block. You will need easy access to the nut from the inside of the guitar, so this usually calls for an access hole to be drilled in the transverse brace that runs in front of the sound hole. I prefer this location because it keeps the peghead neat and clean without that extra cover plate screwed to the peghead.
These truss rods come in a variety of lengths. If you are using one for a 14 fret to the body guitar, you should choose the 14 1/4″ length. If you are using the 12 fret to the body guitar, then you should choose the 12 1/4″ length. Also note that the truss rod is not designed to actually run into the guitar body at all. This permits removal of the neck without conflict from the truss rod.
Cut The Neck Slot For The Truss Rod:
Rout or Dado a 7/32″ (5.5 mm) wide x 7/16″ (11 mm) deep slot down the center of the neck. If you have shaped your neck already, you will have to use a router to do this. It is preferable to cut this slot with the table saw right after the neck scarf joint has been completed and the neck is still formless and before the headpiece veneer in installed.
You will also need to provide access for the adjustment nut as it is a bit larger than the trussrod itself. For the 4mm Allen nut, use a 1/4″ recess, about 1/2″ deep. The recess can be made with a piloted drill bit, or by clamping a hardwood filler strip into the slot, and drilling as though there is no slot.
Note all of our Acoustic Guitar Plans show details of installation of the truss rod with the adjustment within the head block.
GLS Tip: Now that you have cut the truss rod slots and made provisions for the nut access, feel free to go ahead and finish shaping your neck and doing your neck prep work, such as fitting the neck, final peghead shaping,etc., before you actually set the truss rod. Plan on installing truss rod immediately prior to the installation of the fretboard on the guitar neck.