Yet another soundhole crack - cause and repair

[eliya]

This crack is on the same D-28 I mentioned in the other thread about the lifting bridge, and for what it's worth, there's a very good chance I won't attempt to fix this soundhole crack myself, but I do want to understand its cause and what would be a proper fix.

As you can see from the pictures. The crack is on the bass side of the soundhole, but it's not to the side of the fretboard. The crack goes under the board.

I know some of these cracks are inflicted by the neck block shifting inward, but I think this one was caused by dryness, for the following reasons: 1) the fret ends over in this area (fretboard extension) are sticking out, so that suggests the fretboard had shrunk there. 2) the crack doesn't continue all the way to the end block. 3) the treble side isn't cracked. I assume if the neck block was shifting, it would cause a crack on both sides of the fretboard. At the very least, it would happen on the treble side first as the higher strings have a greater pull. 4) Slackening the strings completely doesn't seem to cause the crack to align itself, nor does a gentle push on the headstock.

I couldn't find any loose braces in that area, and I wonder if it's possible for this crack to develop without any brace becoming loose. Any thoughts if the cause is indeed dryness or if it culprit is the neck block? EDIT: I forgot to mention I believe the neck is over set. I don't know if that can contribute to these cracks or not, though.

Now for the appropriate repair. If it's the neck block, then I know Frank has a page that shows the right technique. However, how can (and should) the crack be aligned if it's not a result of loose braces/shifting neck block? It seems to me that without relieving the fretboard the crack won't align. Perhaps only the tall soundhole brace and the popsicle brace should be removed to align and glue the crack?

Thank you!

[Howard Klepper]

This is not the fretboard crack that requires realignment by pulling the neck back.  You don't have loose braces or loose neck block.  Just low humidity causing the end grain at the soundhole to lose moisture and crack. 

Don't remove or separate anything.  Humidify the guitar in an environment of about 75% RH for a few days.  Then glue and cleat it.  The wood will probably close up or nearly so.  The plastic probably won't.

[eliya]

Thank you! I will crank up the humidity in the case!

[eliya]

The guitar's been humidifying for a week, and in the last couple of days I finally managed to get the RH to 75% and it did wonders to the crack.

Do you guys usually glue it up when it's in 75% RH environment or do you bring it back down to 45-50%? My thinking is that if it's glued at 75%, then the wood will shrink considerably when brought down the humidity the guitar is stored at. Is it correct to think that? Would the crack open again when I bring down the humidity? I guess I could try doing it and see for myself, but I figured I'd ask here first.

[eliya]

Bump. I can't find information on whether other repair persons glue the crack at 75% RH or bring it down to 50%.

[Greg Mirken]

You're right, you should not glue it at 75%RH; that's inviting a crack opening at more typical humidities. Let it acclimatize to 50% or so for several days and keep an eye on the crack. It should be fine.

[eliya]

Thank you!

[eliya]

I actually started bringing down RH in the case a day or two before your reply, Greg. I figured I'll give it a go, and can always bring it up again if it should be glued at 75%. RH is at 55% now, and the crack had opened a little bit (but not as bad as before). When the guitar was at 75% you couldn't even see the crack, now it's visible, and that's what I'm aiming for. Any idea how to keep the crack closed while bringing down the RH? Should I bring it down slower?

[Ned Knepp]

Think of it this way, Eliya; If you had repaired the crack earlier, the gap you see now would represent tension in the fibers of the sound board. If you can't close the gap,you may need to make a spline to fill it. 

[eliya]

The problem with a spline, is that the crack goes through the abalone and is therefore jagged, so it'll be tough to get the piece of spruce in. I'll have to think more about how to do it properly.

[eliya]

So the guitar is humidifying again, and hopefully it'll take this time. However, I found this article about violin crack repair. The technique is quite intriguing. Two studs are glued on either side of the crack, and then a small clamp pushes them together to help the crack close. It seems like this could be applied to crack repair with guitars. The thing I'm worried about, though, is removing the studs. It'll be hard to remove the studs with steam without also perhaps heating the glue in the crack (which will then cause it to open). Perhaps prying the studs off, but I'm worried I'll take parts of the top with it.

Anyone tried this before?

[Ned Knepp]

I've looked at that technique but I don't think I would want to use it in a case like this. I've experienced too many cracks that I though were closed up very well when I glued and clamped the thing up only to have the open up again later. If it's open even a little when I want to glue it up, I spline it now the first time around. That goes double if it starts/ends on a sound hole. I think they are more prone to breaking again because they are not so well supported on one end and I think a properly made spline fills the crack without inducing stress. 

Just my opinion.

[Mark Pollock]

I was about to concur with Howard, but I can't tell if there is a shift happening from the pictures. I would want to be very sure that nothing is moving before saying just humidify and cleat. I might look very carefully at the sting alignment on the neck to see if anything has changed, and I might also grab the body and neck and give a good wiggle to make sure that nothing is moving. Then, since likely nothing is moving, I'd go ahead with Howard's advice.
Edit: I really think Howard is right, I'd still check. Trust, but verify.