Luthier Tips du Jour - Truss Rods - O'Brien Guitars

Uploaded by OBrienGuitars on 06.03.2009

Truss rods are installed in the necks of instruments to help resist the large amount of tension or force
pulling on the neck from the strings.
This critical design element also allows one to dial in the correct amount of tension on the neck
to get the desired neck relief which in turn makes the instrument much more playable.
There are basically two truss rod designs, single action and double action.
Single action rods only apply tension in one direction
while the double action rods can apply tension in two directions.
For the purposes of this video I will be demonstrating
how to install the LMI patented double action truss rod.
This is the same truss rod that comes with their steel string guitar kits.
LMI also offers special order custom length truss rods at no extra charge.
The correct installation of any truss rod is critical to it functioning properly
as well as the playability and structural integrity of the instrument.
Some things to consider when designing your guitar is whether to install the truss rod
so that it is accessible from the neck end or body end of the guitar.
Another consideration is how deep to install it in the neck.
I will show two ways of installing this truss rod
and both allow you to have access to adjust the rod from either end of the neck.
The first thing I do is check to make sure that the truss rod is working properly.
I do this by clamping one end in a vise and using the truss rod wrench
to tighten the nut in a clockwise and counterclockwise direction.
This will tension the two way rod in both directions,
allowing you to verify that it is indeed functioning properly.
If it’s not, it is better to find this out before it is installed.
Next I remove the burr left from welding the nut to the truss rod.
This is easily done with a few strokes from a file.
Do this on both sides of the nut and both ends of the rod.
There are several ways to cut the truss rod slot in the neck.
Here I am going to be using a router with a 1/4 inch straight cut bit.
You could also use a table saw or even do it by hand.
First I make sure that the edges of the neck are square.
I also make sure the heel block is flush with the neck blank if it has already be glued to the neck.
The first way I am going to show how to install the truss rod
will be so that adjustment can be made on the body end of the guitar.
I mark the centerline on the neck blank.
I then line this up with the middle of the router bit and place a straightedge next to the neck blank.
This gets clamps to the router table.
I now layout the scale length on the neck blank.
In my case here I am using a 25.4 inch or 645.16mm scale.
I want the truss rod nut on the adjustment end to be sitting on the tenon of my neck joint.
You don’t want it hanging over so that the threads are resting on the tenon.
This usually means that the other end stops at the middle of the first fret.
Mark this location and transfer the line around to the back of the neck blank.
Use a square to mark on the table where the edge of the router bit is.
You will see why I do this in a minute.
When cutting the channel you want to take several passes.
Don’t try and cut it in a single pass.
When the line on your neck blank in the middle of the first fret meets the line your drew on the table,
carefully lift the neck off the table and router bit.
You can now raise the router bit a skoshe and repeat the operation.
Do this several times until you get close to the actual depth you need
to make the truss rod flush with the neck blank.
Don’t try and cut right up to the final depth. When you are getting close,

do some test cuts on the end of the neck blank and then make the final pass.
The truss rod should fit snuggly into the channel and the top should be flush with the face of the neck.
Use glue sparingly while gluing on your fretboard so it doesn’t run into the channel.
Another way to install the truss rod is to recess it slightly more than the depth of the rod
and then place a wooden shim over it.
Some builders think this helps protect against vibrations
and keeps any glue from seeping into the channel when gluing the fretboard on.
I will install the rod this time so that it is accessible from the peghead.
You want to layout the position of the truss rod on the neck
so that the adjustment nut is almost flush with the veneers you will apply to the peghead.
Position the veneers and rod on the neck so that you can visualize this
and then mark the end of the rod on the neck blank.
Next mark the end of the rod on the heel block end of the neck and square it down the sides.
Since the truss rod channel will be cut starting at the peghead end of the neck
it is hard to see the centerline while setting the fence.
So, I line it up with the center of the router bit on the heel block end,
and then clamp the fence to the router table.
Just like before you will need to mark the position of the bit on the table or fence for reference.
Here is another way of doing that using just a block of wood.
Just like before, begin by making a shallow pass with the router.
When the line on your heel block indicating the end of the truss rod lines up with the line on your fence
indicating the router bit position, carefully lift the neck off the table.
Raise the router bit and continue making passes until you are about 1/16th of an inch
or about 1.5mm deeper than the rod itself.
The middle of the truss rod is flexible and therefore can be pushed down farther into the channel.
Next, make a shim that can be placed into the channel. You want this shim to be snug.
I am going to be gluing in the shim with epoxy.
It works best if you scuff sand all surfaces with some 80 grit sandpaper before gluing.
I sand the shim, the sides of the channel and the top of the truss rod.
Place the truss rod in the slot making sure it is snug and mix up a small batch of 2 part epoxy.
Place a small amount on the shim and then place it on top of the rod in the channel.
Place a few spring clamps on it to hold it in place while the epoxy dries.
Notice how the middle of the shim flexes and actually sits deeper in the channel than the edges.
Clean up any squeeze out of the epoxy and allow it to cure.
Once it does, you can then remove the clamps and plane the shim flush with the surface of the neck.
Use a scraper for the final passes.
A chisel can be used to cut off the excess at the peghead.
If you have followed these instructions carefully then your truss rod should be installed correctly
and will work properly. However, if you are still worried about vibrations coming from the truss rod
when playing certain notes, one more quick tip is to run a small bead of caulk in the bottom of the channel before inserting the truss rod.
To learn how to adjust the truss rod after your guitar is built,
my good friend Kent Everett has an instructional DVD
that will teach you everything you need to know about acoustic guitar set up.