Replacing the Truss Rod in a Bass Guitar

After not playing my bass for a few years I took it out of its case to find the neck had a severe bow in it. I tried adjusting the truss rod but it wouldn't adjust far enough to fix the bow. At first I thought if I made some washers to put behind the existing one I could get the rod to adjust a bit further; but my washers had to be so small (to fit around the rod but still inside the cavity) that they were crushed when I tightened the nut and they jammed up the threads on the rod. Next I tried to clean the threads so I could put a longer nut on; but that didn't work out either and ended up stripping the threads on the rod. At this point I should have given up and bought a new bass but instead I decided to replace the truss rod and add 2 carbon fiber reinforcing rods for stability.

To get the fretboard off I heated the neck with a heat gun and separated the fretboard from the neck wood with a drywall putty knife. Something sharper would have been faster but I thought the dull edge of the putty knife was less likely to damage the neck wood. Since I was going to glue the pieces back together I wanted to keep the existing edges as smooth as possible.

Once the fretboard was off I was able to pull the old truss rod out.

NOTE: Later in the process I drill some holes and use a finish nail to realign the fretboard while gluing. If I were doing this again I would drill 2 holes through the fretboard in opposite corners before separating it. This would be more accurate than the way I did it and relatively easy to hide with some wood filler.

The cut the slots for the carbon fiber rods I made a jig that held the neck in place and had pre-cut slots to use as a template. The exact placement isn't too important, basically I just wanted to had as much wood surrounding each rod as possible. The important thing is getting the slots to the right width and depth and making sure they're straight so the rods fit in snug and have good contact for gluing.

Here's how I made the jig:

- Trace the neck onto a sheet of plywood and draw in a center line.

- Lay the rods out on the neck when they slots should go and measure the distance between them.

- Using the measurement, draw the slots for the reinforcing rods into the neck tracing and cut them out with a router.

- Put the neck back on the tracing and screw some blocks to the plywood beside the neck to hold it in place.

- Secure the neck into the jig (I used beadstrap and 1x2)

Once the neck is in the jig you can use the jig slots as a template for the neck slots. Set your router bit depth to the thickness of the jig top plus the depth you need for the rod.

The router cut slots with round edges so after cutting them I used a chisel to square out the corners.

I used a quick setting epoxy to glue the rods into the slots.

If I were to do it again I would try to find something that sets up slower and possibly has a thinner consistency. It took several minutes for each slot to spread the epoxy and when I pushed the rods into the slots they sat ~.5 mm proud of the neck wood. If I had seen it coming I might have been able to use clamps to push the rods in flush but the epoxy sets up so quickly that I decided to just sand them flush instead.

When I was test fitting the new truss rod I noticed that the slot was much longer than it should have been. I cut a piece of scrap down to shim the slot so it wouldn't slide down into the neck. The old slot was cut down past where the neck bolts to the body so I don't think that part of the neck would have been adjustable anyway.

Before I took off the fretboard I cut a few scratches into the side with a knife so I could use them to line it up when I put it back on. When I was test fitting the fretboard over the new rods I realized that fretboard would probably slide when I was gluing it so I put indexing pins in the neck to hold the fretboard in place.

I made the indexing pins by drilling 2 small holes in the neck wood and tapping a finish nail part way into each one. Then I cut the top off the nail with side cutters leaving just a small bit sticking out. Next I lined up the fretboard using the scratches and tapped it down onto the pins to create marks on the underside of the fretboard. Finally I drilled holes where the marks were for the tops of the pins to sit in.

If I were doing this again I would have drilled holes through the fretboard before removing it instead.

Next I put the new truss rod in the slot and covered it with painters tape. Then I spread a thin but thorough layer of carpenters glue onto the surface of the neck wood. Next I peeled up the tape to leave the middle of the neck glue free (this made sure that when I clamped the fretboard down the squeeze out wouldn't get into the truss rods threads). Finally I lined the fretboard up using the pins and wrapped it tight using a long rope.

I didn't get as many pictures as I should have during this process because once the glue was down I didn't want to lose any time.

I used epoxy to glue the but back on. If I had been thinking I would have put a few strings back on the guitar to hold the nut down instead of using clamps. The shape of the nut and the roundness of the beck made it hard to get the spring clamps situated.

In the time that the fretboard was off the neck wood expanded; so after I glued it back together there was a slight edge where the fretboard meets the neck. I sanded this off using 150, then 220, then 2000 to hide the scratches.

I expected to go through the old finish and expose the neck wood so I was planning to remove the finish completely and rub in some danish oil. It turned out that the existing finish was thick enough that I got the edge smooth without going all the way through so I didn't bother.

I didn't actually expect this repair to work so when it did I was pleasantly surprised.

I put a new set of strings and starting doing a set up then only to realize that the B string's saddle wasn't wide enough for a .125 and that the same saddle didn't adjust far enough back to get the string intonated.

I guess the next step is to modify that saddle.

This is tool I used at the beginning to get the crushed washers off of the old truss rod.

I bent a piece of a brake line and cut some nasty teeth into it so I could reach it into the truss rod cavity and dig away at the washers. The truss rod went inside the line so the teeth could wrap around and file away the washer pieces.