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Excavating and rethreading a
Broken Truss Rod
© Frank Ford, 7/22/02; Photos by FF

Truss rods break mainly because of overtightening and excessive friction on the screw threads brought on by rust and lack of lubrication. When a nut is turned against that kind of excessive friction, it can twist the rod in addition to tightening it, so breakage becomes a real possibility. So, whenever possible on an older instrument, I remove the nut and lubricate the threads before trying to tighten it at all.

I must admit I've never actually seen a broken Taylor truss rod. I think it's because the rods are set very deeply into the neck, so they don't require a lot of torque to have a strong action, and because the population of Taylor guitars is not old enough to see much rusting on the truss rod threads. I am using Taylor as an example here even though I'd prefer to use an old Gibson simply because I don't have a Gibson in the shop with a broken rod, and I do have this Taylor neck. In fact, I have a pair of Taylor necks, kindly supplied by Zack and the folks in the service department at Taylor Guitars. I cut one of the necks down the middle so you could see an internal view. I "broke" the truss rod on the other one at the place where so many of those old rods break -- right at the end of the nut against the washer. That, of course, would be the place most likely to get a bit of a twist if the rod were rusty, so it's the most likely break point.

You can see that there is no hope of screwing anything on the bit of rod, even if there is a threaded portion left down in there. Clearly, I'd want to fix this broken rod rather than go through all the difficulty of replacing it!
From the side you can see that Taylor was kind enough to thread the rod about an inch farther down than necessary. Many of those other truss rods are not threaded very far in at all, and many of them break right at the point where the threads stop.
Unlike Gibson, Taylor leaves the truss rod pocket very small, so I'll open it up just a bit to allow access for my tools. The truss rod cover will still hide the hole entirely.
This piece is really about a pair of tools I just made up to accomplish this particular type of repair. One is a sort of "core drill" to excavate around the truss rod, allowing access for new adjusting hardware. As I made up these tools, I learned more about operating my new milling machine (how did I live before I got my Rusnok?) and lathe, not to mention heat treating and tempering steel. Hey, metal work is almost as much fun as lutherie!
The other is a die for rethreading the rod. Most American single acting truss rods have a 10-32 thread, so this tool will work on a variety of species. Clearly, I'll have to make a new die for each thread pitch and rod diameter I intend to rethread. Click here to see a bit of the tool making process.
Both tools have "through holes" so they can go in as deep as necessary, more or less.
I milled 5/16" hex drive shanks on the back ends of the tools so they would fit my long T-handle adjusting wrench. It's the same wrench I'd normally need to use on this size truss rod.
The maximum outside diameter of my tools is the same as the wrench, 7/16." Most truss rod pockets measure at least 1/2" in diameter, so I figure I have just a bit of safety as I drill down toward the shaft of the neck.
The tool fits right in there with its handle attached.
If you look at the cutaway view again, you'll see I've marked the approximate track my cutting tool will make. I'll have a ways to go before danger of breaking through the outside of the back of the neck. Taylor's neck is about as thin as any, so I figure there'll be a greater margin of safety on most other brands.
I can align the tool and the long shaft handle with the axis of the truss rod, so I'll be sure of going in straight. And, the rod will guide the tool as well, so it can't really go off track.
At this point, I've drilled in a bit more than 1/2" and you can see how my new hole is a bit smaller in diameter than the original, and nicely centered.
In this case, I happen to know there are enough threads left on the rod so I don't have much of a problem. But the initial threads are distorted from when the rod was broken, so I'll use my die to "chase" the original threads. If the rod had been broken right at the end of the threads, I could use the die to create new threads down into this blind hole!
If I had made new threads, they would not have run all the way to the bottom of the hole. They'd be close, but a turn on the nut would stretch the rod and lengthen it a bit. So, to allow for some adjusting room, I turned a brass spacer to allow for compression of the wood as well. This is a thin spacer. I figure I'd want to use 1/4" or more on some of those old timers.
Now the nut is back in place (lubricated, of course).
And the truss rod works normally again. Even my little short handle rod fits nicely.
Update 4/12/07 Here's the real deal - a 1964 Fender Stratocaster with a broken truss rod:
This is a valuable old guitar, so it's well worth the extra effort to do what needs to be done. It has only a 3/8" diameter access hole, so I'll have to cut some smaller diameter tools to reach in there.
The truss rod had been broken off flush at the bottom of the hole.
On the lathe, I turned down a pair of tools - both the drill and the die. Reducing the outer diameter of the die makes the side walls of the tool so precariously thin, I don't think the tool could acutally stand the strain of producing new thread. I figured I'd be lucky enough to get it to chase the existing threads.
First the little core drill to dig the wood back about 1/2" to expose the remaining threads on the rod.
Then the die, to chase and clean up the threads.
The original truss rod nut was nickel plated.  To avoid the hassle of making a nut and then having to send it out for plating, I simply made the entire nut of solid nickel silver.
The new nut needed to be 1-7/8" long.  Here it is on the mill, getting the cross slots in the end.
A trip on the buffer, and the nut looked just like a new original.
Here's the nut, along with a little brass spacer washer.
A quick view from the end, showing the exposed rod and washer in place.

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