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So... the distortion is below the UTB, around the soundhole? Did you take pictures of the bracing, or remember what the upper X legs and soundhole braces were like? Seems like a lot of people use puny flat grafts around the soundhole, which especially when combined with a thin top (something you won't see on those Gibsons) might allow that kind of distortion. Height and taper of X legs, distance from the edge where the scallops at the ends start, and final height tucked into the linings would be interesting to know. I'm not sure there's an easy way to judge what is "enough" in that area, without waiting to see if it sinks... I bet the builders of those guitars would appreciate to know what has become of them.

Another thing that I've wondered about is the width of the UTB in some bracing photos I've seen around here. Some of them have been pretty tall and thin. Seems like more width there would be a good thing, especially when combined with an L shaped headblock that is not only pushing down on the brace, but also pushing into it from the side, as it pulls toward the bridge... perhaps even "twisting" on the UTB and creating a depression below it, if the tensions work out that the UTB becomes a fulcrum for the neck/headblock unit, stretching some of the dome out of the back's upper bout to get the rotation around the UTB. C-shaped headblock and hefty back UTB should prevent that.

Do you know if any of those guitars have ever had neck angle adjustments before, or were in need of them? That might help in understanding exactly what kind of deformations have happened.

Also, do you know if some/all of the tops were domed or true flat at the time of construction?

Anyway, thanks for reporting. Good to know that L shaped headblocks do not equal instant stability.

I can't wait to see where this goes.
Filippo, what's with that trap door?

Alan,
When that little window opens, you`ll be able to watch Petticoat Junction!
Look it up, young `uns!
Jest don`t be pullin` up the ties fer tone-wood!
Coe Franklin

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Thanks for the pics. Wow, that is a deep dip.

Well, the UTB is certainly wide enough, so that possibility is out. The long scallops to very low height at the linings would be my first suspect. I agree, the headblock extension puts more strain on the UTB, sparing the rest of the upper bout from dealing with it in a more dispersed way. Good if the UTB can handle it, bad if not.

I do find it surprising that the dip is a smooth curve across the whole width, deepest in the center. The UTB looks tall enough (3/4"?), so I would expect it to dip sharply at the scalloped ends, and be flatter across the center. On the other hand, those soundhole reinforcements are definitely puny, so that's a rather large triangle between the X legs and UTB, with a big hole cut in it, and little to strengthen it back up. So the UTB is taking the full load of the neck pushing down on it, with almost no support from the plate. Maybe not tall enough after all.

X legs are difficult to judge... my instinct is "probably fine".

C shaped headblock would probably have been the best way to prevent that, transferring a lot of the strain that the UTB is dealing with over to the back's UTB. I would definitely have put taller soundhole braces as well.

Todd,

Interesting discussion. I'm sure you'll get a lot of varying opinions, so I'll give my $.02. Obviously the soundhole is retreating and there could be several causes. What I think is the fallacy of the L-shaped block is that many believe that this massive structural component (relative to the standard neck block) will prevent this sort of thing from happening. The head block is just one player in the structural integrity of the guitar and from my observation over a lot of years, it's not the determinate one. It's just a piece of the puzzle. In some cases IMO, it's the major contributer to the type of problem we're seeing here.

String tension wants to pull the top into the soundhole from both ends of the top and whenever you have a structural member tied basically to the front of the soundhole (not literally, but close), any rotation of the neck block is going to pull the top down with it. It doesn't take much movement at the front of the neck block (assuming the pivot point is the front of the neck block at the bottom) to cause a fairly significant movement at the end of the "L". In effect, this block design magnifies the problem once it starts. It's simple geometry.

I have a UTB prominently displayed in my shop from a dismantled early 30's Martin OM. It's .248 x .496 and it's straight as an arrow (as it was when it was taken out of the original guitar). I will say that guitars from this era had more massive neck blocks (thicker) than we're used to seeing which provided more glue area for the back. This does tend to at least slow down the block rotation. Obviously, it doesn't eliminate the need for resets, but I think it does contribute to the stability. It's hard to tell from the picture how thick the bottom portion of the block is , but it looks relatively thin.

I'm always looking for better ways to do things, but I'm constantly pulled back to the old ways of doing things. I've tried almost everything including the L-Shaped block arrangement and while it worked fine for me, I don't think it contributed anything structurally to my guitars, and for me, I can't justify the weight of the arrangement.

I'll get off my soapbox now. I know this works for others who I have a lot of respect for.

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Jimmy Caldwell
http://www.caldwellguitars.com

Looking at the UTB and X brace thickness where it inlets into the linings seems thin to me (maybe 1/16"), assuming the back brace inlets are 1/8". To me, this seems to be the_weak point_. There are a ton of Martin guitars out there with the L shape headblock, and I believe they are still using them? (John Hall will hopefully let us know if that is correct). I would be curious if Martin is having any trouble with the L shape block?

Also, as Jimmy Caldwell had noted about the smaller size of the 1930's OM UTB, it must be noted that there isn't a hole in it for truss rod adjustment....

Chuck

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As Jimmy said. That big extension of the block under the top is transferring the downward force of any block rotation to the area of the soundhole where the top is weaker, and magnifying the strain it by putting it out at the end of the long block. In the traditional design, the rotation can be resisted by the UTB and absorbed higher up in the bout.

And the UTB and X arms are tapered down too low before inletting to the liners.

Are those wood screws holding the neck in?

Is the builder's name a secret?

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Those are scary pics, Todd!
I use an "L" block, and have been concerned with the flat portion being forced away (glue joint failure) from the main head block. Not only do we glue that thing on, but we screw in on as well--a mechanical as well as chemical bond. It wouldn't take much tipping (or breaking of the glue joint) to move that flat piece downward. Any way you could check that scenario out on your patient?

Steve

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Looks like you took the picture with the guitar totally de-tensioned. How much worse does it get under string tension and how far down does the straight edge hit the bridge? If you reset the neck angle to make the geometry proper will you have to shim the fingerboard? Keep us informed on this toughie, I'm interested to see what you wind up with.

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Interesting thread
it looks to me like the neck block just wanted to warp on its own regardless of the string tension

Does any of you guys split your neck block stock ?

Also looks like the UTB was weakened by the off center trussrod adjustment hole. Does the L block extension touch the UTB? And if it does, does it appear to be glued to the UTB?

Chuck

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Todd: When I think of neck block rotation I think of the side /top juncture as the fulcrum.As Howard has said the neck block extension is a cantalever tending to pull the top down. In my mind the rotation happens because of how the back is arched.I have had the problem of some my guitars ending up needing a neck reset with in a relatively short time. Others are as solid as a rock. Can't figure it out,but quite sure it's back related. I wonder if Howard's point along with something with back arching is causing the problem.
Tom

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I didn't and wouldn't call the neck block extension a cantilever.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Jimmy Caldwell said


I bet that's at least part of it. I (think I've) seen some newer factory guitars where the back of the neck block was cut at an angle so that the glue line is the width of the kerfing. I don't like this. I believe it's to prevent telegraphing the outline of the neck block in the back. I normally have my truss rod adjustment inside the body, but I drill the hole in the center of the UTB. I also use a laminated UTB with .030" carbon fiber. I think I see a popsicle brace in one of the pictures. I've always thought they were worthless. If that's what I'm seeing I guess I was right. The ends of the UTB look really thin to me also. The side braces, if they're made the way they appear to me are also scary. Are they just butted against the linings?

Disclaimer: I intend no ill will toward the builder, or builders of any of these guitars. I'm sure these builders along with pretty much anyone on the OLF could find things they don't like when looking at guitars I've built. I like looking at these pictures and trying to decide what went wrong. If I built a guitar with similar (or any kind of) problems I'd like to have Todd Stock, as well as the entire OLF membership looking at it to decide what happened.

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Filippo's picture distinctly shows the grain orientation of his neck block....which is the way I think it should be for structural reasons when being used in the manner it is...

I can't really tell on the guitar this thread is about, but is appears to be showing side grain on the end, which to me means that it is quite possible the extension did the warping and caused the problem..

I thought I'd post a quick pic of one of my top & block setup just to provide a visual context of a more traditional way of neck attachment. Nothing revolutionary here, I only post this to show the relative size of the neck block and the trapezoidal spruce plate that goes between the UTB and the neck block. This plate reinforces the area under the FBE and also helps prevent the block from rotating towards the soundhole. The plate is an idea I borrowed from John Arnold (if you don't know about John, check the UMGF) and he's about as traditional a guy as you'll find (remember my rant about the old ways being the best). This is an example of learning from past issues (many old Martins had cracks in the FBE area) and improving on them.

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Jimmy Caldwell
http://www.caldwellguitars.com

Howard: Sorry to misquote you...!! But I will call it a cantelever.
Tom

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Notable builder? The top looks like birch, the finish in the last pic looks like it was put on with a corn broom, is this a joke?

Birch? It looks like the spruce and tattered finish one would expect to see on an old, well-used guitar.

Looks to me like a guitar with a spruce top that has been played.

I take back asking the builder's name. Todd was right to omit it.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Oh, I figured it out,
the trap door is a sound port shut off gate.
Cool!
Do you think the rh had anything to do with the "cave in"?
How does the lower bout look?
Does it have a radius to the top.
I knew this would be a good thread.

I wondered if the wood under the heads of the bolts is compressed, there's not much surface area under there without washers. Is there any evidence of the neck heel trying to pull away from the body? It seems like if there's not a solid connection at the neck heel that it would distribute the load from the neck a lot differently, making the upper bout carry a lot more of the load. Also, how tight are you really going to get those bolts initially with a phillips head through the soundhole?

I know Somogyi uses a massive L-shape block, and seems like he would have a long enough history with his guitars to know if that by itself is likely to cause problems. Likewise, I think if Martin would have suspected that the L shape caused problems they would have changed it by now to reduce warranty issues.

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Jonathan Kendall, Siloam Springs AR

I don't like the look of the UTB hole. It looks too big compared to the brace. Is the hole larger than the usual 1/4" . If so that could be a big part of the problem. I also don't like the thin soundhole braces. And lastly, the upper bout x brace arms could be not stiff enough. They must also be deflecting too much.

I believe the main contributing factor is the lack of built-in arching in the upper bout. That is what distinguishes the traditional Martin design from Gibson and Larson. If the top is totally flat, it takes very little force to cause the sinking, especially considering the compressive loading parallel with the strings. On the other hand, a little bit of arching is all it takes to balance the compressive load and the downward pressure due to neck block rotation. I use a very slight arch on the UTB, probably around a 60 foot radius.
This balance of loading is why violins can survive very high string tension in relation to their overall weight. The downforce at the bridge is counteracted by the compressive loading, which tends to force an arched top upwards.

As others have pointed out, the upper bout bracing in that guitar is rather weak at the ends. I prefer a rounded convex taper (parabolic?), rather than the scoops in that area of the top. And I leave the bracing about 3/16" tall where it enters the kerfing. OTOH, I thin the lower ends of the X-braces to about 0.09" where they enter the kerfing, because the loading on the south end of the bridge is in the opposite direction.
This comes from repairing so many vintage guitars (particularly Gibsons) where the sunken upper bout is due to the failure of thin bracing where it enters the kerfing.

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John

Just a question.... If the idea of the L shaped head block is to help alleviate top distortion and help stave off a neck reset, would it not make more sense to have the tongue against the back rather than the top? Wouldn't that actually help brace the string tension against the more rigid and non-stressed structure of the guitar?
Brian

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Brian

You never know what you are capable of until you actually try.

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