I was going to originally use a 3/8" threaded rod but I had a real hard time finding one, so I welded a 3/8" bolt on a piece of 3/8" rod to extend it to the needed length.

The idea is to use the aluminum bar as a type of bow and the steel rod is like the string of the bow that when tightened with a nut on one end of the rod, it will bow the bar which is inside the neck of the guitar.

In order for the rod to grip the ends of the aluminum U bar correctly I made some ends out of aluminum stock. I used aluminum simply because it was a lot easier to work with then steel. In the end that is shown in the picture I put a groove on the far end so that the steel bar could be anchored so in would not be able to turn when the nut is tightened on the other end. I welded a ball on the end of the rod then filed it flat so it could rest in the groove of the end piece when assembled. I also beveled the 2 top edges so that the U bar can be crimped around it. The hole in the center is 3/8".

Here is one of the end pieces in place (un crimped) inside the U bar.

Here is the end that has been crimped . You can see the ball that has been wielded on the end of the rod.

Above is the rod assembled inside the aluminum U bar. It's difficult to see but there is a nut and a washer on one end. You will notice that I wrapped about 3 or 4 layers of masking tape on 3 places of the steel rod. This is to avoid any possible vibrations in case the rod happens to touch the walls of the U bar.

I also slightly bent in the walls of the U bar in order to assist the truss rod in bending the desired direction when tightened. On the left is a rough idea of bending the sides in looking end on at the U bar.

After the truss rod was assembled I laid out a strip of masking tape. I stuck on a 1/4" strip of paper in the middle of the masking tape, then pasted it over the open end of the truss rod. This is to assist in preventing any rattle or vibrations once it's installed in the neck. The paper is stuck on the masking tape so that there is no chance that the making tape will stick to the steel rod.

Here it is with the tape on. This side will go face down in the groove that is cut out in the neck of the guitar. This truss rod that I am using for an electric guitar is a total of 17 1/2 " long. For an acoustic guitar it would be a little shorter.

Here is the board that I am making the neck out of. Although it is acceptable and practical to sometimes laminate pieces together for a neck, this is to be all one piece. It is 3/12" X 2" by 26". (quarter sawn). It's a piece of Mahogany that I found laying around from an old table that is probably older than me. (and that's pretty old). It's probably pretty stable wood as when I checked it with a straight edge, it was as straight as an arrow.

After making sure the top and side was flat and square I routed out the 1/2" groove for the truss rod. It's important to make the groove at this stage of the game while you have a good straight surface to work with before carving the neck. You can use a router or a circular saw works good too. When using a circular saw you can set the depth you need on the saw and then make multiple cuts to the correct width of the channel using a guide. I usually tape the guard up when I do this but you really have to be careful or you can have a serious accident and lose your fingers and you'll be playing slide guitar for the rest of your life. (No joke intended).

It is my belief that the truss rod should fit in such a way as it is not too lose or not too tight. If it's too loose it could rattle around later. If it is too tight it will not be able to freely move and bow correctly when it is adjusted.

In cases when the groove was cut too big and the truss rod is too loose, you can always put a strip of felt in it to keep it from rattling around.