Repair of a 1931 Martin OM-45

[mirwa]

Originally Posted by Howard Klepper

Having the back off facilitates the repair, but it can be done through the soundhole. And the patch can be made trapezoidal so it fits within the braces if the bridge is further forward (or the braces R&R'd, which is a bunch more work).
When multiple methods can produce satisfactory results, which is often the case with guitar repair, people may differ in their judgment about whether it is worth doing extra work to create greater structural integrity.

Well said, I pull the back and scarf, I have learnt via this forum that we all repair differently.

Originally Posted by murrmac123

I would love to see, from the people blithely advocating a scarf joint on this repair, a step by step, blow by blow account of exactly how they would have gone about it. And a diagram as well.

One of your earlier statements to me.

Originally Posted by murrmac123

One of the criteria which I apply when deciding whether I can trust the advice of internet gurus is their ability to differentiate between "their" - "there" , and "your" - "you're".

Clearly, your question is not to me then, I believe others have more than adequately answered your question. One day, it would be nice to see some photos of your repair work.

Originally Posted by John Arnold

http://www.wikihow.com/Use-There,-Their-and-They're

John, I never responded to your little end of post snippet previously on the Taylor neck angle thread, but, as I have just addressed murmacs one and clearly some people yourself included have issues with my ability to spell and pronunciation, I thought I would use the opportunity to clarify.

https://en.wikipedia.org/wiki/Dyslexia

For note I am dyslexic, but that does not mean I am stupid or unskilled, it is a side effect from a brain injury (a car drove over the top of me, compleltey irrelevant to this conversation) . Auto correct on my computer fixes all my back the front letters but not "there" "their" or "they're", so it does not matter how many times I read it, or try to interpret it, it does not work that way for me

Steve

[D. Churchland]

I myself am slightly dyslexic so I understand where Steve is coming from. I've noticed a slight consistency in certain areas of work in which luthierie is different from other professions.

I also make tools (planes and tool handles) as well as furniture, and in those circles there are certain things that are not subjective and are truly objective no matter the argument or emotion involved.
One example is the scarf joint idea that was discussed earlier. If you talk to any furniture maker they will never advocate using an end grain to end grain joint unless adhesives like epoxy and polyurethane are used. On a guitar top that's not an option, we want to be able to work on the top in the future and it needs to flex and move. Not trying to just beat a dead horse just using it as an example.
Cut nails are another example. You simply cannot hammer a brad head 2" cut nail directly in the wood like a modern nail and not run a huge risk of it splitting. Instead the method you should use is to drill a small hole and then run your nail through it using the hole as a guide.

Things like this (and others) are commonly accepted in furniture building and tool making as objective, and thus to ignore them is slightly on the unwise side.

But it seems as if with luthiery there is much more emotion involved, and rightly so as music is purely emotional. I've told people that you cannot epoxy the neck back onto your Gibson Gospel that you're grandson dropped for some reason. But inevitably it shows up on my bench with a half inch of epoxy dried up all around the neck joint. Not saying anyone on here would do that it's just he didn't like the fact that a 26 year old was telling him a 68 year old that he couldn't do what he was proposing to do.

Like I said before, when I look at this repair I simply see a repeat of what was done before. Whether you agree or not, that's fine with me. Time will tell if it lasts and we probably won't see a picture of it 2-3 years from now. But again, that's fine with me.

[redir]

As my mom used to say, 'There is more than one way to skin a cat.'

[John Arnold]

Like I said before, when I look at this repair I simply see a repeat of what was done before.

I should give up, but I am going to try once more. A spruce patch (even one that is the exact same dimension as an oversize hardwood bridgeplate) is fundamentally different in 4 very important aspects:
1) Less weight. Spruce weighs about half what a typical hardwood used for bridgeplates weighs.
2) Stiffer in the direction of string pull. The spruce patch is grain aligned with the grain in the top. This makes it better able to resist bridge rotation than any hardwood of reasonable thickness installed cross-grain.
3) Less stiff across the top. This allows the top to vibrate better across the grain.
4) Tapering thickness of the spruce patch below the bridge further reduces weight, but full thickness above the bridge reinforces the top against sinking. Tapering is not advisable with a conventionally installed oversize bridgeplate because of #2 above. Spruce is stiffer along the grain than a hardwood is across the grain.

While a hardwood bridgeplate of the same dimensions could be installed such that the grain is parallel with the grain in the top, it would never sound as good as the spruce patch because of the increased weight and increased stiffness across the grain.

Time will tell if it lasts

You are welcome to reread post #19.
The OM in my previous post was repaired in 1994, and lived with medium gauge strings installed for around 10 years. I still own that guitar, and it is played regularly. The action has not changed, and the saddle height is unaltered.
To expound on what I said in post #19, I have repeated this repair dozens (if not 100+) times over the past 25 years, and I know of no unsatisfied customers or premature failures. Zero. That means none. Many of those guitars were considered unrepairable without a total replacement of the top.

[D. Churchland]

John, don't get offended if I don't agree, because I don't.

Your response to Steve has caused me to lose alot of respect I potentially had for you. As such, I have nothing more to say on the issue

[John Arnold]

I am not offended, just mystified. I am not sure where you disagree. If you are saying that the scarf repair is superior, then that is fine. OTOH, if you disagree with the following:

The guitars I have repaired by this method sound better and last longer than ones that are repaired conventionally using an oversize bridgeplate.

If that is the case, I have to say that you don't understand the repair that is being discussed here.

[David Farmer]

I don't understand what is confusing about the advantage of the John Arnold repair. If you agree with it's appropriateness or not for any given situation, he has articulated what he sees as it's advantages clearly. Probably many times.

A scarf joint long enough to give significant strength may not fit within the footprint of the bridge. You can put the long points of the scarf on the inside of the instrument, but in addition to the difficulty of making a good joint especially through the sound hole, the scarf must be covered by a larger bridge plate. In many circumstances much larger.

The goal is not just to fix the guitar structurally, (that's easy), but to do it while minimizing any alteration to it's voice.

To that end, a butt joint, spanned by spruce, with a smaller bridge plate, is more effective. A long scarf, covered by a long piece of hardwood, regardless of grain orientation, may be stronger but so what? A butt joint is strong enough, facilitates a smaller bridge plate, and will most likely sound better. The essence of instrument building, and repair, is to make things as strong as they need to be. Not as strong as possible.

I don't know John, or if he is a nice guy, (I suspect he is), but if you spend any energy analyzing where your technical information comes from, I think his wisdom should be given considerable weight. There is a very small group of people who have been doing repairs on fine guitars and paying as close attention to the outcomes as long as him.

[Howard Klepper]

Originally Posted by David Farmer

the scarf must be covered by a larger bridge plate. In many circumstances much larger.

The scarfed patch has no need to be covered on the inside by an oversized bridge plate. And it will not show on the outside.

[Bruce Sexauer]

In boat building, where lives depend on the successful outcome of the joinery, a scarf is generally thought strong enough if it has a 5 to 1 aspect ratio. On a top 1/8" thick then, the scarf need to be 5/8" long. This is not all that hard to achieve if one has skill with sharp tools. Because the "hole" is entirely under the bridge, a scarf cut on the inside would be entirely invisible, at the risk of repeating unnecessarily what Howard already said.

I rarely find anything about John Arnold or his work that I disagree with, and I do think his repair concept should work well, but it is a band aid, and the scarf that Howard and I have described is closer to true restoration, and has the added advantage that it would absolutely perform more like the original guitar. For me, performance is the main thing. Nothing wrong with John's/Paul's method, it is just not ideal, especially on a rare and stunningly expensive guitar such as this one.

Kind of like putting Bondo on an irreplaceable vintage Bugatti, it looks okay, but it isn't going to cut it at the Concours d'Elegance.

[murrmac123]

Originally Posted by Bruce Sexauer

I On a top 1/8" thick then, the scarf need to be 5/8" long. This is not all that hard to achieve if one has skill with sharp tools. Because the "hole" is entirely under the bridge, a scarf cut on the inside would be entirely invisible, at the risk of repeating unnecessarily what Howard already said.

Just to clarify, Bruce, am I to understand that you would be quite confident of getting a perfectly matching scarf joint working from the top ? ie without removing the back ?

You would be cutting an angle of under 12° , working against the grain on thin spruce, with little visibility.

"Intrepid" doesn't even begin to describe it ...

Originally Posted by Bruce Sexauer

For me, performance is the main thing. Nothing wrong with John's/Paul's method, it is just not ideal, especially on a rare and stunningly expensive guitar such as this one.

If, as John Arnold said, Tony Rice played another vintage Martin with a similar repair, and declared that he could detect no diminution in tone, I would be inclined to give some credence to his opinion.

[David Farmer]

I have equal respect for Howard and Bruce as I do for John Arnold and nowhere near the experience of any of them so my arguments are necessarily more theoretical than experiential.

I have used Johns patch method but never had the courage to attempt Frank Fords scarfed version through the sound hole. Franks version used a slightly larger plate but didn’t cover the entire scarf.
If the plate is left as small as an original and does not span the scarf joint, it would obviously be near perfect in terms of light weight but I would be concerned with long term health.

Bruce, mentioned boatbuilding and the safety required. It’s that very safety margin that is the difference between boats and guitars. I am pretty sure high quality guitars, especially the area just in front and behind the bridge, are much closer to the edge of collapse than even the most exotic racing shell.

An inside scarf behind the bridge is pulled together buy string tension but the one in front experiences repeated flexing and peeling strain. I would say this is one of the few strains that hide glue is not outstanding at resisting. Braces are tucked under linings to resist the tendency of glue joints that flex to eventually break free.

The scarfed piece has two long grain joints that are not supported by the bridge. If these glue joints were the result of cracks in this highly strained area, would you not cleat them? In very expensive violin family instruments where a sound post patch is employed, a cleat is frequently applied across the leading edges of the scarf. I think a soundpost patch is under much less strain than the area in front of the bridge on a guitar.

The very high value of the instrument in the original post was mentioned as a reason to take the time to do the most appropriate repair. Even if you feel the un-reinforced scarf is superior structurally and sonically, isn’t it much more invasive? Even if it is done through the sound hole? The un-scarfed Spruce patch can be removed and even replaced without the loss of any top material.

I’m not being gratuitously argumentative. I have a gut feeling that an unsupported scarf joint, directly in front of the bridge may not be durable enough over the long haul. Cutting one into the top is invasive. Has anyone done a large number of these and seen them last for decades?