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After completing this step on my current build I got to thinking this is a bad idea. Seems like this would be the last place you would want to weaken with a hole. I'm considering headstock adjustment on my next build because of this.

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Hutch

Get the heck off the couch and go build a guitar!!!!

Both ways work, but I'll share why I put the adjustment through the soundhole (where you have yours now).

I like having the headstock free from a hole I have to cover up. Plus, I like to use slotted headstocks, which are not really compatible with a headstock truss rod adjustment. Finally, some headstock designs are already prone to some weakness at the nut, and placing a hole up there just adds to that weakness. So, I like sticking with adjustment through the soundhole.

The hole in the upper transverse brace is not a huge weakness, if you handle it right. Once the upper transverse brace is glued to the top, you are adding strength right there, even if the hole were to be right on the edge that is glued to the top. If you have the hole slightly away from the edge that is glued to the top, and leave a little meat between the hole and that edge, then I think the hole is inconsequential to the overall strength of the brace.

I would avoid having the hole close to the exposed edge of the UTB. For most truss rods, you don't want it there, anyway. You want it close to the top.

If I'm wrong about that, then a lot of builders are wrong about it. But I don't think we are.

I think the engineering is pretty clear (not that I could prove it) that a hole near the centre doesn't weaken the structure much, hence why you see so many holes in bridges.

I leave 1/8" between the edge of the hole and the top.

I think if it were a problem, the millions of guitars built like this would have shown it...

Engineering wise the hole is fine. The same engineering allows us to drill holes in floor joists for plumbing when building a home. A brace underload has one side expanding and one side contracting, so it stands to reason that near the center not much at all is happening. All of the stress is toward the edges.

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http://www.Harvestmoonguitars.com

I'm with Don on this. Very little effect on the brace strength and I do not want to carve a great big hole in the headstock which will weaken it.

Dave

Don already said all the things I was about to type.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Actually putting a hole in the center of the brace or joist is about the worst place you can put it, which is why most building codes don't allow it in the center third of a joist (unless it is very small or the joist "overbuilt"). However hollowing out half the wood in the neck/ peghead transition area is even worse (as many an LP will attest).
The UTB is in a relatively dead area of the top so you can beef it up to where drilling a hole through the center will still leave enough wood to support the loads it has to carry. You could probably run a block from top to back in that area without effecting the sound of the guitar too much. It's not how massive you make the braces - it's where they are located and how they are attached to the top.

Just make the hole real small, just big enough to get a little hex wrench in there. That helps align it too.

You could get those truss wrenches that Stew Mac sells that are angled to get in behind the UTB too. I used to do that but now I just make a small hole.

We need to define center, The center of the span is the worst place to put it. In the center top to bottom of the brace is the best place to put it. Building codes allow holes but not notches in the center of the joists.

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http://www.Harvestmoonguitars.com

Yes John, you are right.
When I wrote center I should have clarified I meant center of the span where we drill the truss rod access hole. And again you are right that they do allow some holes in the center of the span (the smaller the better)
As Don mentioned truss rod access holes are often drilled near the top of the brace. Were it not for the fact that the brace is glued to the top (which is usually glued to the FB ext.) this would almost be considered a "notch". Luckily for us it is only a few dozen luthiers and clients with mirrors who inspect our work and not building code enforcers.

I don't drill a hole in the brace. The truss rod nut is flush with the neck block, and can be accessed with a standard Allen wrench. While the strings must be loosened to adjust the rod, I dont consider it much of an inconvenience. Truss rod adjustments are infrequent, at least in my shop.

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John

Search on "Castellated Beam". Pretty standard engineering since the 1950's.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

If this works, the hole in the UTB works

My substantially less experienced opinion validated by someone who actually knows! Thanks for your contribution John.

I can't claim expertise, but on my parlor guitars with slotted headstocks I had to access the truss rod through the UTB. Since the UTB is typically to be the largest brace in the entire guitar, I don't skimp on it's size. However, every one of my guitars has the upper portion of the head block extending to, and glued to the UTB. So, when I route a little channel in the block and drill a small hole in the UTB for a hex wrench to fit through, I am confident it makes no appreciable difference in the integrity of the upper bought or neck joint.

Here is the main argument against the Gibson arrangement of the truss rod adjustable at the headstock. It weakens it rather alarmingly - the reason why Gibsons seem susceptible to broken headstocks.

Another big factor in the Gibson problem is their use of one piece necks. It creates a short grain issue on the headstock, as the above photo shows. A scarf joint would help. But yeah, I disfavor putting a hole in the headstock at the nut, for this reason, as well as the others I mentioned above.

Since the UTB acts pretty much as a cut off brace for the main part of the soundboard there really is little penalty for making it over size. The primary load on it is a down load from the neck and the bridge, so if the truss rod hole is toward the upper surface of the brace there can be plenty of wood to take it. I can't say I've ever seen a UTB that broke at the truss rod adjustment hole, but I've fixed my share of broken Gibson heads.

That photo is brilliant.


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The other factor with Gibson is the use of a hex nut, rather than an Allen head. That means the cutout must be larger to create clearance for a socket. Gibson also used a 17 degree peghead angle on the older models, and has gone back to it. That combined with a one-piece neck is weaker than the 15 degree (Martin angle) or 14 degree that Gibson used from the late-1960's into the 1980's.

If the truss rod nut is recessed close to the neck block and accessible above the UTB, the hole in the UTB can be made smaller, since it only needs to accept the Allen wrench.

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John

I’ve been following the comments here with interest. I use a soundhole adjustment because I don’t care for the look of a panel on the peghead, or the weakness it creates in the peghead transition. I use an Allen head adjusting rod and put a small hole in the UTB close to the top side. I honestly never gave it a second thought figuring that the UTB is large (I tend to go fairly beefy), the hole is small and it is towards the center of the beam (if you consider the top as part of that structure. I was starting to over think this after this thread. Then Alan mentioned that he has never seen a failed UTB but had fixed lots of problem pegheads. That put me squarely back to where I was before I started reading this.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I didn't have any of Bryan's concerns (maybe I'm not open minded enough ) , but I do think that Alan's comparison of the two potential repair issues is the best version of common sense. We don't see broken UTBs, but we see plenty of broken headstocks. So, we should design future guitars to prevent the actual problem that arises in the field, not a theoretical problem that never arises in the field.

I do the same as Terry Mitchell - my UTB's are very beefy - 18mm x 18mm, and I build my head block with a "C" profile so the top of it extends to, and is glued to the UTB. (The bottom extends to and is glued the first back brace.)