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My first guitar has a three string 1 1/2 x 3/4" thick birch neck with an oak fretboard (simply what they had at home depot - oak was a bad
choice, lots of rough softer grain to mess up those string bends I don't
tend to do anyhow). I ended up leaving the neck in the original
profile, just light rounding of the corners. I actually like the
feeling, to my surprise, and I'm afraid to take any off because it bends
from string pressure just a tad already.

I'm building my second neck, this one a four stringer carved from one piece of 1 3/4" square hard maple. I'll add a decent rosewood or ebony fingerboard as well, just haven't decided.  With the tiny bodies of a cbg a truss rod would have to be longer than normal for a guitar but shorter than for a bass (unless I do some funky stuff inside the box). The cheapest rods stewmac carries are plenty long enough and are sold uncut and unthreaded, requiring a tap (extra $5 from them). That would work, but I'm unclear on a few things:

First, how may of you add truss rods?  Any advice, warnings, comments or preferences? What do you use?

How deep does the channel need to be routed for a traditional simple compression-only rod? If I make it deeper (farther from fretboard), should I glue in a strip of wood to fill in the void between fretboard and rod? Or should I just route enough so that it fits, no more? It seems it would work more efficiently if deep (matter of leverage) but that big channel seems to spell trouble for me.

How about using just a non-adjustable length of metal, like the square hollow stock stewmac and lmi sell?

Anything I forgot to ask?

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Stu mac sells carbon bar stock that you can embed under the finger board, usually a truss rod is not needed until you go to 6 strings. Usually a stick and finger board is enough strength for 4 strings, you may try a 3 or 4 degree neck angle built into the neck next time. An angle tipping the headstock downward just a little, to counter string pull. Or try Old Lowes idea of adding flat bar stock under the fret board. Another idea would be to laminate the neck with three strips of hard maple, with the center strip going a different grain direction the the outer strips.
One more idea, is to router out a half inch wide and and half inch deep slot under the fingerboard, and pack it with bondo, then embed a 1/4 threaded rod from the hardware store. That will stiffen it up. But normally, you would not need a truss rod for 4 strings.
Thank you both.
I had the impression that a neck needed just the right amount of curve - more than none but not much and very little room for error - to prevent buzzing up the neck. Hmmm, perhaps that is why I never liked how my guitars played. I kept having the friend who explained that set them up.
So, a truly flat neck, with correct angle (as in the 3 or 4 degrees back Mortimer Mudbutt mentioned, is actually desirable? (great name, not sure if I want to know, though!)

I wasn't clear: the first neck is birch from home depot, with an oak fingerboard glued on. I left it almost completely rectangular. I worried the slight bend would get out of hand if I carved it down.

THis one I'm carving by hand and want a proper profile. It will also have one more string. Two reasons it might bend more. Then again, perhaps it will benefit from better wood (hard maple this time (+ fretboard). I modified the circle guide/straight edge guide for my dremel so I can route out a channel and will see what size once I see what I can find at the hw store. You gave me a great list of things to try. Might as well add something as practice for when I do need one.

One question for MM: why the bondo? I've never actually handled the stuff. Is it just a filler in this case or structural - binding the bar to the wood?

In any case, I bought some hide glue in a bottle today so I can take it apart if it doesn't turn out right :)
Bondo is a filler to hold the rod solid, bondo hardens hard, it is an automotive repair product with many uses like duct tape. I assumed you knew what bondo was, don't bother with it then, you would have to buy a quart of the stuff.
I would focus on a hard wood neck and a thick finger board, and lighter strings. Heavy strings sound cool on a CBG, but to get them to tune, they pull harder then lighter strings.
We had a neck angle thread here, a couple of them actually, go snooping for them and you just might learn a tip or trick.
Sorry for the long response here...
I did know what bondo was but had no real idea of the physical properties. I have some silicone caulking which I've read about using in truss rod channels to avoid the rod vibrating at certain frequencies, but caulking would only deaden noise, not add any structure. I'm tempted to thread up this 1/8th inch rod stock I have for reasons unrecalled - if the tool library here has taps, otherwise I'll buy something more appropriate. Very thin, but as pointed out we're not talking all that much tension. Does seem too thin to add any stiffness other than via compression.

I tried trimming some angle stock I have with a dremel cutoff wheel, aiming to get a nice long piece maybe 3/8" wide by 1/8 thick" to embed. I even made a guide so I could get a really nice cut. Only I held the dremel vertically below the metal I was cutting and only after some 10" of cutting did I realize how hot it was getting and that the cutoff wheel was turning slowly to highly abrasive sand (as they do in use). Sand was sucked into dremel. Oops. Took it apart, seems fine after a thorough cleaning. Sealed bearings saved the day.
Lesson learned. Some shortcuts aren't.

I'm using quite heavy strings - find it much easier to control the slide (not new to guitar, but new to slide). I guess if it gets bad enough I'll make an external truss cable with some steel cable and turnbuckles I have. ugly, but I built it fast and a bit rough anyhow. I'll just route a shallow groove down the back and cover it (or not). Plenty of room inside the box for the large turnbuckle (talking only of the already built neck here). I've seen photos of a cb mandolin with bracing done this way since the strings were distorting the box itself... just extend it up the neck.
Here's how my truss rods come out through the peg heads:

I had drilled a hole first so that the slot would be in the middle. The threaded rod has to sit deep enough so that a nut and washer won't stick out.

Side view. Looks like I'll have to file that truss rod back a touch... Not that I have left a bit of extra wood on the peg head, under the truss rod and nut. I will thin that back and it will look nicer than here.
For those of you making your own adjustable truss rods, what end hardware did you choose?

I bought a long fine threaded 1/4 " rod and some assorted nuts and washers to try. I got a long socket and a flex shaft socket driver (think flexible screwdriver w/ 1/4" square socket drive) to get in past the tuners and strings to adjust the rod.

Alan Roberts - thanks for the photos. I see how you embed it, the lines you have on the side give me an idea how deep. That looks like a steep angle on the headstock, but I;m not the one to ask. I ended up googling it, found the wikipedia article on headstocks lists the angle used by a few major manufacturers... but I ended up choosing what would fit in the blank I'm carving from, just 10 percent.
Fitzhugh said:

I have some silicone caulking which I've read about using in truss rod channels to avoid the rod vibrating at certain frequencies, but caulking would only deaden noise, not add any structure.

Truss rod vibrating???? There is no such thing. There is not a guitar company in the world using caulk on their truss rods. How would a truss rod vibrate when under the stress of the strings? I wouldn't believe everything you read in the archives, especially about truss rod vibration.
What you said seems logical but I knew I'd read it somewhere, and that they suggested using something to prevent rattle, but wasn't sure what. I think I read it here:

2. Set the truss rod into the slot. Although the rods are cushioned with PVC tubing, we recommend extra cushioning at the nuts to eliminate the possibility of rod rattle. Apply a small amount of silicone bathtub sealer in the slot at the double nuts, then press the rod as deeply into the slot as possible. Use only enough silicone for minimal squeeze-out. A spot or two along the double rods can also be cushioned by a little sealer.

Looking at the instructions for each of the rods they sell, they don't suggest this on the simple compression rod, only ones with a rod in a channel or two rods.

Seems to me a single rod would never vibrate if under compression: it would press too hard against the wood. On the other hand, the rod in channel might be able to. Reading the instructions above they focus on the ends. As I understand it, these rods work by bending themselves and pressing perpendicularly to the length of the neck, not by pulling on the neck in a direction parallel to the length of the neck. They still push against the neck, though.

I saw a number of references to doing this via. Perhaps it is a problem, perhaps one guy at Stewmac got it into his head that it was a great idea and added it to the instructions, and each person who buys one of their rods read that without questioning and it went from myth to "Accepted Truth"... no idea which. The rods would be in a near-perfect place to be vibrated by the strings, question is whether it is free to vibrate.

Re Naz's comment about locking up the rod: they're not suggesting using a hard glue, rather a soft rubbery compound, and not much of it. I'm still not arguing they are correct here, though!

I went with a homemade simple compression rod, and the fit is tight without any need to be forced in, so I can safely ignore the issue. I HATE using silicone caulk, anyhow. Stinks, very very hard to get off hands.
Oh, and I see that Alan Roberts' angle makes sense in more ways than just tension. I used the angle that would fit in my too-small piece of wood, only 10 degrees. It not only looks too shallow now that I've carved it, but the low angle means the hole where the rod can be adjusted is ridiculously long. A more obtuse angle would make it much smaller.

I haven't planed the neck to the final thickness so I'm going to shorten it and angle it the two more degrees I still can. Even that makes a large difference in that way. As it is it almost looks like a fender neck and like it would need string trees!

My neck looks much like yours did in the photo you posted, only with the poor angle choice. I've still got the large chunk under the transition - I left it so I could find a nice way to carve it.

Alan Roberts said:
Here's how my truss rods come out through the peg heads...
In the true spirit of CBG ... we build it ......

Home made Trussrod

I've gone with just a simple compression rod. Next time (and isn't there always a next time?) I'll try to make a double-action rod.

Matt Towe's link is great.

I've got to look over that site, apparently he goes into some detail on how he built a few different guitars using a minimal shop setup - which is what I'm facing.

LMI also says to use caulk. http://www.lmii.com/CartTwo/TrussRodInstall.asp - saw that just now when searching for a page I saw comparing types of truss rods, pros and cons of each. Can't find it.

Oh, wait - here it is. http://www.warmoth.com/Guitar/necks/trussrods.aspx
In case that helps anyone.

Frets.com has great info, including a bunch of pages on truss rods.
Why not just make a fat neck? and use good quarter sawn lumber. ?

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