Presented for your consideration...........
I'm getting around to resetting the neck and cleaning up an old repair on a Peavey SD-9 acoustic I purchased. The guitar suffered a top crack in the cedar along the edge of the fingerboard, down the grain of the top to the sound hole. I'm guessing string tension, heat or a fall compromised the area allowing the compression to let the neck slightly tilt back into the binding and neck block. The truss rod adjusts through the sound hole. The mahogany neck block remains solidly glued in place. The wood, binding and cedar top compressed.
The crack was previously repaired and a large spruce cleat was glued inside to reinforce it. See the image with the paper shape to see the size.
After removing the neck, I found that what I thought was the extended truss rod block under the fingerboard, was actually an extension of the neck. Not a separate block glued to the top and routed to receive the rod.
So I have is a neck with a dovetail at the heel and a rectangular extension of the neck back to the top brace along the sound hole.
But no additional top brace or popsicle brace between the neck block and the sound hole. The neck was glued in at the dovetail wings, Aliphatic resin, and under the fingerboard to the top only. No glue on the extension along the sides or bottom where it passed through the neck block.
I think this design was structurally weak. So now, I'm looking for suggestions on how to modify and stabilize this area to prevent any additional failures.
The extension is 5/8 tall, so a single brace with a channel for the extension across the width of the top may get tall.
Would a traditional 3/8 wide brace butted up against extension the on each side of that slot be sufficient? Maybe a wider 1/2 wide brace.
Or maybe a flat cleat like on the previously repaired side be sufficient?
Any real-time experiences anyone could share would be helpful.
I have reset a Landola neck like this a long time ago. I think I put a wood screw through the top part of the neck block into the neck and replaced the brace with the hole with a thicker brace with smaller hole. Can't remember exactly what I did actually. Adding patches of wood with the the grain cross the crack and the same thing on the other side is a good idea.
I did one similar to this not long ago. It was a Furch, built in the Czech Republic.There were some differences in the construction. The extension had a horizontal slot in the end which interfaced with a brace under the top as I recall. The fit was clever but left no room for neck angle adjustment, as the end of the neck was held firmly in place once the neck was bolted in. The guitar neck was bolted through the block à la Collings or old Taylor. But, unlike Collings or old Taylor, it had that neck extension under the fingerboard like yours. The neck angle was quite bad on my patient and my main concern was that, after resetting the neck to the correct angle, the end of the fingerboard was going to protrude excessively away the top (at the soundhole end) being held up firmly by that neck extension to which the fingerboard was glued solidly. There was no possibility of the fingerboard bending down to meet the top after the neck was reattached. The slot and the brace described earlier were easily an eighth of an inch apart.
So, I simply cut away the neck extension under the fingerboard entirely, freeing-up the fingerboard, then glued the bare fingerboard down to the top. Violà! A bold move, I grant you. It could not have been done had the truss rod extended out to the end of the fingerboard. But in my case the truss rod extended about an in beyond the dovetail, and a ball-end hex wrench was all that was needed, as before, to make the adjustment. Also, the neck extension wasn't glued to anything except the fingerboard. There was no involvement with the body at all except for the unglued interface involving the slot. It simply hung in the space created for it.
There was plenty of top surface for the newly-freed fingerboard extension to adhere to so it was an easy thing to score the top around the extension, remove the finish inside the lines and glue the extension to the top (with hot hide glue, of course. Man, I have come to LOVE hot hide glue. Thank you, Frank, for the insight regarding this miracle adhesive). The repair was a success, the customer was very happy, and I got to learn about a "new-to-me" neck joint.
I believe this approach would solve your problem. I'd also consider converting this to a bolt-on for ultimate flexibility in adjustment.
And, bonus points for the challenge! It's what keeps me in this business.
Thank you Roger and Mark for your insights.
I've added an extra brace on each side of the slot to reinforce the top from future stress cracks along the fingerboard.
I'm going to modify the neck joint to a bolt on system. I'm going to use 2 bolts, 8/32 or 10/32 hex cap heads and anchors. I'll dial in the reset, and see how much the fingerboard may rise on that neck extension. The truss rod is single action in a aluminum U channel that adjusts at the sound hole. I measured and the wood neck extension has about .8 of an inch beyond the end of the channel/washer/adjuster nut that sit in. If it would help bring down the fingerboard extension back down to the top I'll cut back that last .8 inch of the neck extension. If it doesn't allow the fingerboard to come down to the top, I'll make a shim to fill in the gap.
I'll use hide glue in the dovetail and under the fingerboard extension in the final assembly.
The guitar has a solid cedar top and I'm interested to hear what potential to sound maybe.