"How To" Make a Dual Acting Trussrod
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Most necks get thinned for comfort to a point where they can no longer support the tension applied by a set of strings, so we need to insert something into the neck to restore some of its strength, we call this item a truss rod.

Truss rods come in many many forms, you can have flat or hollow square steel bars, these are non adjustable truss rods, Martin used these in the early days, modern versions of these are Carbon fibre - super strong with no weight.

You can have single acting trussords such as those in some Gibsons and Fenders which are just a single bar attached to a nut, they allow tension to be applied against the pull of the strings, you can also have single action truss rods which are a single bar fitted into a U channel.

Then you have more modern versions of truss rods which are Dual action, this allows you to strengthen the neck but also be able to apply tension in any direction to straighten the neck should it also start bending

This walk through is about making a Low Profile Dual acting Truss rod.  

Time to Manufacture, approx 1hr
You can buy dual acting truss rods, here is an example from Stewmac, the only issue I have with them is they are a little tall, approx 11.5mm
That flat bar is 5.8mm wide
Some round mild steel, a little under the width of my flat bar.
The thread of the nut is way smaller than my round bar, so it needs to be necked down and thread cut.

You can see the nut sitting above the round bar in this photo.
The arrow is pointing to a round nut, these are available from place like LMII, they are broached on one side for an allen key and the other side is pre-threaded, I am checking in this photo the thread to make sure I cut the same on my round bar.
Using a lathe, I machined this down and cut a thread, the thickness in this area is now 4.7mm.
With some stainless steel, I need to make a sleeve to fit the nut into.
Drill it until the nut fits internally.
I always start by laying a scaled fretboard over the top of the neck and marking the centre position between the nut and first fret
I start with some flat bar, 3.5mm thick.
And to a decent depth, the nut is inside on this picture, when we part this piece of we leave a good 1mm thick shoulder on the front edge