Baritone 12-string neck bowing issues

[Professor Woozle]

Had precious little time recently to progress any of my projects but one thing I have been looking at is trying to fettle neck bow in my baritone 12-string acoustic (29" scale). When strung, it has a  pronounced bow (~2mm) in the neck, even though I'm using light gauge strings. I'd tried tightening the truss rod bit by bit but I've hit the point where there doesn't seem to be any more adjustment and the neck bow is still there.

With the tension off, the neck returns to straight so the bow isn't set in but I'm concerned that it will get that way over time and want to act before that happens. I'd prefer not to have to take the fretboard off and fit carbon fibre reinforcements unless there's no alternative so I'm wondering if there's a way to increase the tension available through the truss rod? The rod does run the full length of the neck, the adjusting nut is at the soundhole end. Would taking the nut off completely and putting some copper washers in front of it give that extra bit of tension needed to resist the pull?

[Bizman62]

9 hours ago, Professor Woozle said:

Would taking the nut off completely and putting some copper washers in front of it give that extra bit of tension needed to resist the pull?

That sounds like a viable solution. By your description it sounds like you've turned the nut down to the end of threading and taking the rod out for a few more rounds with the die equals to adding carbon rods. It's also possible that the shoulders for the nut have compressed a bit in which case I'd be tempted to glue in a hard wooden hollow plug as an extension. Fitting a ring of just a couple of mm can be tricky, though, but should be doable as the rod will act as a guide. A piece of straw would prevent the glue from sticking to the rod and a couple of washers along with the nut should take care of the clamping pressure.

That said, fibre washers glued in might work just as well as wood and they should be readily available. And of course a copper or brass washer as a bearing.

[Professor Woozle]

Took the nut off earlier this evening and discovered that it had a steel washer behind it already. The nut threads look fine and although I've not looked with an inspection mirror, the rod thread feels OK too so I'm more confident about tightening the nut further. So, I'll see what I've got already in the way of copper washers and if I've got some of a suitable size, failing that I can always make some up out of scrap copper sheet. 
I might take a bit off the bottom of the nut and bridge too while I'm about it, the action was a bit high even without the excess bowing!

[Bizman62]

9 hours ago, Professor Woozle said:

rod thread feels OK too so I'm more confident about tightening the nut further. So, I'll see what I've got already in the way of copper washers

Sounds promising. And starting from the easiest solutions until you reach your goal is always recommendable.

You said in your first post that you felt like hitting the end point of the adjusting range which made me think that the nut met the end of the threaded part of the rod. Further thinking the issue could potentially be in the rod itself. As I understand it's a single action one and if it's inside a bowed channel any corrosion might grab. If that's the case, banging the neck all the length after tightening the rod should release any snugs.

[Professor Woozle]

Found some fibre washers in the garage that were pretty close in size so I'll try one of those first, I think. Feeling round the truss rod end I think there's glue squeeze-out around it that may have been interfering with tightening so I'll try and give that a good clean before refitting.

Confession time... the bowing is in part due to me reprofiling the neck, I found the fretboard too wide so I took the binding off and spokeshaved/sanded the neck back, but in doing do I'll have reduced the cross-sectional resistance. Fingers crossed what I'm about to do works, as it's a great-sounding guitar!

Edited May 16 by Professor Woozle

[Bizman62]

9 hours ago, Professor Woozle said:

Confession time... the bowing is in part due to me reprofiling the neck, I found the fretboard too wide so I took the binding off and spokeshaved/sanded the neck back, but in doing do I'll have reduced the cross-sectional resistance. Fingers crossed what I'm about to do works, as it's a great-sounding guitar!

That may be part of the issue. Then again, given that 12 strings double the string tension but no 12 string neck is twice as thick as a 6 string one some bow is to be expected sooner or later. Slimming the neck most likely just sped up the process some.

Once I tried an Ibanez Talman acoustic-electric. I liked the profile but every time I strummed a full chord the neck flexed. And I didn't even hit the strings hard. If your guitar doesn't do that you're already on the winning side!

[Professor Woozle]

After two failed attempts to get the washers on, accompanied by swearing that made the start of Ian Drury's "Plaistow Patricia" sound like a sermon in comparison, I remembered the set of surgical clamps lurking at the bottom of my toolbox (bought  for a few quid from a Sheffield market stall many years ago, they were rejects from a specialist tool maker!). Washers refitted with the help of the clamps to hold them in the right place to push on with tweezers, nut coated with graphite and re-tightened and strings back on. So far, it's looking to have worked, the neck now has about the right amount of bow but I'll keep checking over the next few weeks.