Functional fix for a top plate crack

[supersoul]

I'm looking for advice or ideas about how to repair a crack in the top plate of an old archtop. I'm just interested in stabilizing it, it doesn't have to look pretty.
The guitar, when I got it, was in terrible shape, and I'm taking it apart to put back together into a playable form. It had been sitting in the front window of a record store for years. The poor thing was completely warped, unplayable, and it was covered with autographs done in black sharpie. I bought it for cheap when the store moved. The autographs mostly came off with lighter fluid, although some of the ink has gone through the finish checking cracks into the wood.
Which is why I don't care about how a repair will look!
From watching youtube videos and reading repair articles, I don't think humidifying the crack to close it would work in the long term. The split has been there for years; the wood likes it that way. So I'm thinking of a couple cleats to keep it stable.
My questions are: will cleats be enough, and I can just leave the crack open? Or if I have to fill the crack do I have to make a spline or can I fill it with some glue/sawdust combination?
As you can see from the pictures the back is off the guitar (as is the neck, for that matter). Which certainly makes any repair easier!

[Jimmy blue note]

Take it to a luthier. That crack has REALLY opened up. Lucky for you, the dry winter is the time to address that. A luthier (a good one) will open that crack into a V shaped channel and I would highly recommend a well matched splice be placed in there. That doesn't preclude the use of cleats, especially since the back is off. A splice will make the repair tight while the grain is dried out. Then, no matter what the humidity situation is or becomes, your chances of shrinkage (and further splitting) are greatly reduced.

That's what I'd do. It addresses all your issues and assures a sound repair from here on out. In my opinion.

[nevershouldhavesoldit]

My bet is that the crack is from severe dehydration. That guitar sat in a sun exposed window for years. I'm sure the record store didn't keep the HVAC running 24/7/365, so the temperature and humidity in the store probably varied widely - so the top shrunk. It looks wide enough that it couldn't even be clamped together & glued before cleating. The fact that the binding is still intact and the rest of the guitar is undamaged suggests to me that it's still solid overall and will be fine whether or not the crack is repaired. It's a shame you don't know how it sounds when it's together. If it plays well and sounds fine, there may not be any point to doing anything to it except humidification if the wood is really dry. This presumes tha the neck angle etc are fine and the top and back have not sunk from dryness.

You can get a decent digital humidity meter for about $20. I think it's well worth determining the moisture content of the wood before doing anything. If it's under 6%, it's seriously dry and needs to be rehumidified. Doing so will close the crack to some degree, possibly even making a traditional clamp & cleat repair posible. But even if it still needs a splice, it'll be more stable (assuming it's kept properly humidifed afterward). If it's really dry, rehumidifying will probably also help keep the binding in place and the rest of the instrument stable.

[RJVB]

German guitar?

Pity your pictures don't show if the split has been stopped by the brace or continues. If it has been stopped and the brace is still glued properly you could just add some cleats or, why not, a bit of carbon fibre mat to prevent the crack from growing wider/continuing. That might not be such a good idea if it has already continued past the brace.
If you decide to leave it open, do sand the 2 faces and put some finish on them before doing whatever else you do; untreated open cracks tend to make an instrument react more to humidity changes in ways that you'll probably notice when playing.

It looks wide enough that it couldn't even be clamped together & glued before cleating.

Plus it would have to be cleaned first to remove the oxydised wood so the glue would hold correctly.

[supersoul]

Thanks for everyones replies!

It's the repair of the guitar that is the project for me. I bought it two years ago and it's only been since this summer that I've had the time and a space to work on it. Everything about it was warped; the neck was bowed, the front and back plates are both warped away from eachother, part of the rim was cracked. I'll include a couple more pictures below.

I offered them a low amount and bought it, just as an experiment to see what I can do with it. A few years previous I had bought a similiar old East German hand carved archtop, in similiar condition, and after a years worth of thinking, researching, and actual work I was able to make it into what is now my favorite guitar.

This guitar has a Rellog pickup hidden under the fretboard. I've tested it and it works. Single coil, it looks like. When the guitar had strings on I could hear that the guitar has a nice acoustic tone.

The bowed neck: I removed the frets, sanded the fingerboard flat with sandpaper on a long straight level, re-radiused it to 10" with a radiused sanding block, put in new frets which I leveled, crowned, polished.

The top and back plates were warped, but I could push them back together and everything fit. I did some experimenting with applying boiling water with a brush to the warped areas, then clamping, which seemed to help. Then I just let them sit clamped in place for a few months, and now the warp is gone. Something about heat and moisture to reset the wood fibers. I dunno, I'm learning.

Weird hobby, I guess: buy an old busted up carved archtop for peanuts and set about making it playable. But the process is meditative for me. I don't do anything until I'm absolutely sure. Sometimes I just think about it for months before I can see a way forward.

Very few people seem to care about these old German archtops. If it was a Gibson, of course I would take it to a luthier. For me it's a way to learn about hand carved solid wood archtops without the expense and mystique of something collectible.

to RJVB, the split stops at the brace and does not continue beyond. It cannot be pressed together.

So this split in the wood… it doesn't have to be a pretty repair, it just has to have structural integrity. I’m actually cool with leaving the split open, with cleats bridging between both sides. I'm thinking vibration transmission, basically.

[RJVB]

Very few people seem to care about these old German archtops

On here Hammertone does (if I'm not mixing up user names). And since you're in Germany: the people at german-vintage-guitar.com do too. They might even accept to give you occasional advice by email.

to RJVB, the split stops at the brace and does not continue beyond. It cannot be pressed together.

So this split in the wood… it doesn't have to be a pretty repair, it just has to have structural integrity. I’m actually cool with leaving the split open, with cleats bridging between both sides. I'm thinking vibration transmission, basically.

When you tap around the intersection of the crack and the brace, are there noticeable differences with the sound you get at the corresponding location on the other side, and that suggest the left brace is not properly glued anymore? That would be a source of rattle when playing (remember there will be downward pressure on the top that would make any gap between brace and top grow bigger), and it will also give the crack an opportunity to continue. I'm assuming the shop keepers were clever enough not to tune the strings up to normal tension while the guitar was sitting in the window and if so you don't really now how stable the crack really is.

The crack is a long way back from the saddle; I'm not convinced that there are many vibrations to transmit at that point. But instead of multiple cleats you could consider a single "longitudinal" one if you manage to make one with the proper curvature. If you really don't care about looks and "proper repair" you could then even run some wood glue into the crack that dies up transparently and can take finish.

[nevershouldhavesoldit]

remember there will be downward pressure on the top that would make any gap between brace and top grow bigger

Maybe I’m missing something here. But if the brace is directly under the top and positioned & shaped to abut it, why would downward pressure on the top increase any space between brace and top? It seems like any space would be closed by such pressure. If the brace is loose now (which is easy enough to determine with the back off), it would be reglued before reassembly.

Great pics of a really cool project, supersoul! But to be honest, I’d have left the graffiti alone. I don’t understand why anyone would mark up a decent guitar like that. But it was already done, and yours had a look that was both artsy and ironic. I’m anxious to see future pics of the project - it looks like great fun!

[supersoul]

the brace itself isn't loose at all.

nevershouldhavesoldit: I did think about keeping the graffitti, but theres no way I could play it in public with all that! Ugh, to me it was horrendous, and I was so happy when the lighter fluid cleaned it up.

[RJVB]

Maybe I’m missing something here. But if the brace is directly under the top and positioned & shaped to abut it, why would downward pressure on the top increase any space between brace and top? It seems like any space would be closed by such pressure.

Indeed, it would depend on where the gap is and what curvature there is at that point. Your reasoning is correct if there's a support under the brace (hah ) or enough properly glued brace length on both sides of the gap. In this case we're very close to the end of the brace. I've had an old German archtop where the end of the brace had come loose and you could clearly see the gap close when I removed string pressure. This was a tranverse brace so there was a bit more curvature (and the gap easy to see through the soundhole) but the principle should be the same in this archtop.

Another way to think of it is that pressure on the top will make it want to become flatter. The brace is shaped, so the deformation of the top can turn into some kind of lever. And an additional analogy (not sure if it's a good one but it may help you see what I'm seeing in my mind): workers once screwed a dark veneer of some kind of maybe 5mm thick plastic to the weathered wooden binding-like rim around the top of my parents' garage. They didn't use a lot of screws nor any glue. In summer that smooth plastic rim turned into a wave because the plastic expanded but couldn't go anywhere because of the screws.

[sgosnell]

I think a crack like that needs to be filled, and since you're doing this for fun and learning, it's a skill that you need to learn anyway.

[wintermoon]

If you can't get it closed don't fill it, splice it w a piece of spruce and clamp it.

[Hammertone]

A few East Germans built some fine fully-carved archtops in the 1950s, hidden among the garbage generally made the Worker's Paradise. These good ones are often stupidly cheap. This slightly inelegant Lang copy is definitely one of the good ones. My three cents:

1.
Use a splice to fill/repair the crack.

2.
While the back is off, install a cleat below the point of the bottom soundhole on each side. Preventative in nature.

3.
If there's not already a wide cleat installed below the two points of the upper sound hole on either side, install one on either side there as well. Preventative in nature.

[supersoul]

well, the overwhelming consensus seems to be to put a splice into the split, so that is what I'll do! And it's a good idea to do preventative cleats where the holes end in points.

Aside from not matching the existing wood, is there any harm in using wood that isn't spruce? I was thinking pine, as it's lightweight and easily available. I don't have any spruce lying around either.

Does it matter for the cleats as well? I think I understand about how the grain of a cleat should run perpendicular to the grain of the top plate.

I've been using hide glue for everything, so it'll be reversible in the future.

Edit: nevermind about the pine, i'm just being lazy. I'm sure I can find some small pieces of spruce at a lumber, hardware or art supply store.