I'm restoring my 1971 Martin D-35. I already replaced the 1/4" braces with 5/16" scalloped (due to too much bellying) and the large RW bridgeplate with a smaller maple one. Now that the back is glued back on, I need to do a neck reset. While I'm doing that, I thought I might stiffen up the neck a little. The relief goes from dead flat unstrung to .016" when strung up. A recent newsletter article from StewMac talks about inserting a carbon fiber rod inside the square-tube, non-adjustable truss rod to help stiffen up the neck. Does anybody have any experience with this technique?
I'm most concerned with relief. They say I can dial in whatever relief I like, glue the carbon fiber rod in and it will maintain that shape and relief. But I'm wondering what happens when I string it up? Will the string tension cause significant additional relief ? If so, how much can I expect? If it's only another .001-.002", I can allow for that when I set the relief statically.

Any help would be appreciated, especially from Frank ! Thanks


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Hi Mike.
How timely your questions!
I am just about to reset the neck on my 1971 D-28, after years of just letting it go. But now my young son likes to play it, so I guess it's time has come.

I'll begin by removing the fret board, as it will have to be replaced, Martin planed it down thin like they did back in the day.
I think I may replace the entire square rod with carbon fiber.

If I were you, I would try to build it back as flat as you can, and let the strings give the relief.
Some luthiers like to add the extra mass by filing up the tube. I've heard it said that increasing the mass is good for tone, but I haven't done that job myself. I doubt I'd be able to hear a difference because resetting usually has a profound tonal effect in itself. Pouring it full of epoxy is one easy way.

Now, neck stiffness is a bit of a complex issue, and one that bears talking about. Clearly the T-bar and hollow tube rods are there to increase stiffness and rigidity, and they do just that. Other factors have a very big effect on neck stiffness. Particularly the rigidity of the fingerboard-to-neck joint, species of wood, and the tightness of frets.

I've found, for example, that the simple act of regluing a fingerboard with hide glue can increase neck stiffness so much that a truss rod is not necessary even on wartime instruments.
Thanks Frank and David for your inputs. I also asked the question on the UMGF and StewMac customer support. Sounds like there is not a lot of experience out there with this technique, and I realize that there's no way to predict with accuracy what I'll end up with due to all the variables that affect neck relief. But I feel better now about just setting a low relief value, maybe .004-.006" and take whatever additional relief I get when I string it up. Whatever I end up with, I think it will be significantly lower than the .016" relief it has now.
If you have it all torn apart why not install a adjustable truss rod? The value is gone any way.

Robro ROn
I heard some say that there is a difference in sound between the adj and non-adj truss rods, including several people that have a lot more experience than I do. I dont know if there is a difference, but the addition of the carbon fiber rod was easy to do, and it just happened to be in the StewMac newsletter when I was at that stage of the repair so I decided to try it.

Here's the link to the UMGF with all the comments. As it turned out, the guitar sounds great and the relief came out to be about .010".
I just installed a neck today on my 1978 D-28, and I used Stew Mac's info on the carbon fiber reinforcement, on this neck.
I did it a bit different. I glued a strip of hard maple, with hot hide glue, onto the carbon fiber rod, let that dry, then sanded it down until I had a snug fit. The Stew Mac tip shows them using plastic binding as a filler.
Next I warmed the neck good, poured some hot hide glue in the tube, swabbed glue on the reinforement, and slid that into the tube.
The neck was pretty much dead straight when I installed the carbon fiber rod. I let that dry for a couple of days. It also need a refret, so I did that too when the neck was off the guitar.
Once carbon fiber rod was installed, refretted, and the neck is back on the guitar, the neck is totally straight with no relief.

Being I just reinstalled the neck this AM, I won't string it up until tomorrow sometime. I can then give you more info. on relief. I'm sure there should be some. How much, I don't know? Before I did the work the relief was about .018".

If you plan to do the same thing, I'd make the neck as straight as possible, when you glue in the reinforcement. I clamped a 2 foot level to the fretboard, on this particular neck.
I'm sure when Martin originally builds a neck they make them as dead straight as possible. Once you install strings, that will normally add some relief. Personally I like really straight necks. I just raise the saddle, if need be, to eliminate rattles or buzzing. Using medium strings, seems to always add some bow to a neck.

I totally agree with a straight neck..Strings usually pull just enuff relief in a neck from my experience..I also question the relief theory..It is supposed to give strings wobble room in the middle of the neck, but if you are fretting in the middle of the neck, would it not be uphill from there to the end of the fingerboard?..I've heard that Dan E. favors a staight neck as well..But then again, what the heck does he know about guitars? I say perfect fingerboard prep and great fret work cures most ills..
I already did the repair back in August. Ron Frazer just resurrected the thread.
I'd be real interested in hearing how much relief you get after the neck stabilizes. Like I said, the neck was flat unstrung and .016" under string tension before I did the neck job. I set .004-.006" relief when I installed the carbon rod and it increased under tension to .010".
I am surprised to learn you poured hot hide glue down the truss rod. I thought the idea was to bind the carbon rod to the steel tube, which is why the use of epoxy? Wouldnt the hide glue bond to the steel tube break under stress? Hope it doenst cause a rattle.

Hi Mike,
I just strung the guitar up this morning. The relief is .007" to .008". I hope it stays there, I'll monitor it over time and see what happens. I'm happy, cause I had .018" relief before.

Hot hide glue should bond the carbon rod to the tube as well as epoxy. I'm confident it will work fine.

One more thought about adjustable truss rods. I can see how a neck with an adjustable truss rod in tension would have to vibrate differently than one with a non-adjustable rod. The non-adj truss rod neck would seem to be able to vibrate more "naturally". But the question that probably will be debated long after we are gone is whether that different vibration characteristic affects the sound of the guitar. Perhaps one way to test it back-to-back (I'm a former test engineer) is on a guitar with a bolted-on neck...install a neck with the adj truss rod and then swap it with a non-adj truss rod neck. Not a perfect test because you'd be using 2 necks and it wouldnt be a dovetail joint but maybe some sensitive instrumentation could measure the result.



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