Straightening a bowed neck: Correction and Reassembly
We started off looking at a Gibson ES-335 where the truss rod had burst through the back of the neck (around the nut-to-first-fret position). I felt that the rod access cavity seemed a little deep but wasn’t sure of what I’d find as I dug deeper. And dig, I did. I removed the fingerboard to access the neck face beneath and then removed the wood fillet keeping the truss rod in place.
What I found was what appeared to be a straight truss rod channel that had been improperly cut from the factory. The channel had an odd kink and was routed to less than 1mm from the rear surface of the neck.
Now what?
First order of business is to repair the damage caused when the truss rod broke through the rear of the neck. With easy access to the ‘internals’, this is pretty straightforward. A little glue and clamping does the trick. I can worry about finish on it later on because I’ll be touching up around the fingerboard too. Right; easy part, done.
What about the rest of the neck?
I’m not happy with that truss rod channel. It’s too damn deep and, as is, it’s just not going to allow the truss rod to work properly. So, let’s fix the channel.
Fixing the depth problem means installing a fillet of wood that will sit under the truss rod. This will raise the rod up and provide some much-needed mass behind it.
I thicknessed a piece of maple to fit the truss rod channel. Then I shaped the bottom of the fillet to match the odd profile at the bottom of the channel.
I left this insert fillet purposely high because I have a fiendish plan. I plan to cut an arc along the length of the fillet to establish a curved ‘bed’ for the reinstalled truss rod. Let me illustrate what I mean here.
With one fell swoop, I can correct the depth and shape of the existing channel and can make the truss rod more effective (which, in this case means 'actually works without bursting through the neck’).
The bottom section of the fillet will be glued into the channel. Then the truss rod can be reinstalled and the second section of fillet glued in over it. It’s a simple matter to trim that top fillet flush with the face of the neck.
But not so fast…
Extra straightening and strengthening
Don’t forget that our neck’s taken on quite a bow and I need to keep that in mind as I proceed.
While I’m confident that the newly curved truss rod will operate well, I want to give this neck every chance of remaining straight against string-tension. The fact that the neck’s already settled into a bow means I’m considering some approaches to straighten it out before I start reassembling things.
And, since I’m about to glue in the new bottom section of that maple fillet, let’s look at a little trick that might improve my chances of straightening this neck
Straightening before working
What if I straightened the neck before I glued in these new fillets?
I strapped the guitar into the Erlewine Neck Jig and used it to pull and hold the neck in a straight position. This means I can glue in my new maple strips while the neck is ‘artificially’ held straight. When I remove the guitar from the jig, this should give me a head-start towards ‘natural’ straightness.
I double-checked my fillet-bottom shape and glued it into the jigged straight neck. I checked straightness again and found that, even just adding this bottom section to the artificially straightened neck, the relief was definitely improved. There’s a fair chance that adding that second maple fillet would give me an even closer result but I’m all about playing things safe on this repair. Given the bow — and what this guitar’s been through — I’m going to add some additional reinforcement while I have the chance to do so easily.
Taking no chances here. Some additional strength, I think.
Carbon fibre neck reinforcement
Carbon fibre rods are a common method to add strength to guitar and bass necks. They’re light and strong and work well. As you’d probably expect, Stew Mac sells carbon fibre neck rods in a few useful sizes. The 3.2 x 8.2mm (.125” x .325”) size is good for adding a reinforcing strip along either side of the truss rod.
Since I haven’t filled the truss rod channel completely, I decide to make use of it to guide my router (Dremel tool mounted in a router base).
I’ve roughly cut a piece of scrap pick guard material to the same footprint as the router base and double-stick taped it to the base. I’ve used and off-cut of maple fillet (slightly thinned) and stuck that to the router base a particular distance from the router bit.
When this wood fillet is inserted into the truss rod channel, I can slide it along the channel and guide the router. The router will cut a nice new channel parallel to the truss rod channel. This way, it’s easy to get a clean, straight, and consistent cut. When one side’s done, I rotate the router 180º around and cut a similar channel on the other side of the truss rod.
Then, I epoxied the carbon fibre rods into place. I should note that all of this routing and glueing was done while the neck was held straight in the neck jig. This meant the routed channels were straight and the newly glued carbon fibre rods went in 'straight'. This all contributes to everything staying that way after the guitar's removed from the jig.
Carbon fibre installed, it’s back to the truss rod. I reinstalled the truss rod and glued in the maple fillet over it.
After all of this, I’m left with a perfectly straight neck with no tension on the truss rod. That’ll do me. I left the guitar aside for a couple of days to see how it settled, just in case I needed to do anything additional before putting the fingerboard back on. All seemed well so, back on it went (those Gibson fretboard ‘locating pins’ are quite helpful at this stage).
Another couple of days settling time before stringing up and we’re still looking good. Nothing much moving. Phew!
At this point, we’re just looking at some finish touch-up, a light level to ensure the frets are even after the fretboard removal trauma, and some setup. And, even though I knew the work I’d done, you’d better believe I was nervous tightening that truss rod for the first time. 😉
And there you go. Like I said weeks ago, this neck-straightening thing can get pretty involved. I didn’t need to do much in the way of levelling the neck itself while the fingerboard was removed but the carbon fibre reinforcement is certainly an option for certain neck deformations. The truss rod channel work here was a little unusual, to be sure, but it helped straighten the neck and provided a means to make the truss rod, you know… work.
That’s it for this one. It was weird, challenging, and interesting. And, let’s face it, what more can we ask for? 🙂
This article written by Gerry Hayes and first published at hazeguitars.com
