Truss rod advice

Please see the attached photo. On my 1915 Gibson A, I had previously remedied a forward neck bow by planing the underside of the finger board to match the neck bow. Now, wanting to refret and correct that thinned fingerboard, I bought a blackwood fingerboard and decided to add a truss rod, too. (My first rod job ever). The router chipped out a chunk of the head block, and I'm not sure I'm properly positioning the rod end dowel. I expect to have to hide glue a well-fitted replacement for the chunk, extending it to bear against the dowel. I guess an epoxy fill might tend to creep. Can anyone advise me on this, and the neck straightening? Thank you. (About .005 inch relief currently).

Normally the dowel nut would be in the neck itself rather than in the block. If you want to have the nut where you've drilled the hole, it looks like you would want a tight-fitting plug to fill that space between the nut and the end of the dovetail, as the rod will put that whole area in compression. It might be better to move the nut to the neck, though. Then the chip wouldn't matter too much because it's behind the dovetail where there's no force exerted by the neck. You could also use a two-way rod, which doesn't put the wood in compression so you could run it the full length of your slot. It adds more weight, though.

However, another option you could consider is using a carbon fiber bar instead of a trussrod. A 1/4" by 1/2" bar buried as deep as possible in the neck would prevent movement and would let you keep the headstock in its original condition.

I'm not liking the neck block routed out. I had a similar old Gibson in the shop a few years ago. The mandolin was in pristine condition except for a neck bow. The owner and I decided a carbon fiber bar would work well. We did not want it to be or look non original. I removed the fretboard, heated the neck to straighten it as much as possible and then, I routed the neck for a carbon fiber bar. The neck was straight as an arrow and you could not tell anything was ever done to it.
Good luck with your mandolin.

amowry and Skip Kelly: I'm very grateful to you for steering me in the right direction. Thank you!

Another vote for carbon fiber.

If you set the carbon fiber rod 1/16" or a little less below the surface of the neck, you can cover it with a thin hardwood strip. That will make the surface much easier to level after the rod is installed. That's because carbon fiber is really tough stuff and is not easy to sand, file, or scrape.

I would recommend grafting in new wood to any place you took it out of the neck block.

You are not the first person to do a little dance with a feisty router around here. Luckily it will all be hidden inside.

True African Blackwood, dalbergia melanoxylon, tends to chip out terribly if you ever have to pull the frets.

I'm of the school that if it was built with hot hide glue, it should always be worked on with the same. I don't ever see a reason to use epoxy in a 106 year old mandolin.

I had success with a badly warped neck on a old A by removing the board, heating the neck as much as I dared, clamping it immediately into a very slight back bow, and letting it sit for some days [I don't remember how long]. The old board was still full thickness, so I reglued it with hide glue [also in a very slight backbow], and finished it up with a good compression fret job. Let it sit for a few weeks under tension, then consigned it in Nashville. I checked back with the store some months after they sold it, and so far as I know, the neck is still straight. No routing, no added rod.

Quote:

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Originally Posted by rcc56

I had success with a badly warped neck on a old A by removing the board, heating the neck as much as I dared, clamping it immediately into a very slight back bow, and letting it sit for some days [I don't remember how long]. The old board was still full thickness, so I reglued it with hide glue [also in a very slight backbow], and finished it up with a good compression fret job. Let it sit for a few weeks under tension, then consigned it in Nashville. I checked back with the store some months after they sold it, and so far as I know, the neck is still straight. No routing, no added rod.

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I have done a very similar repair with guitars and mandolins. It has worked well for me too.

Thank you, everyone, for your input. Is there any reason why I shouldn't use two 1/8" x 1 /2" rods to make up a quarter by half rod? (I already have the thinner rod on hand. ). And, although I get the impression that it is customary to use epoxy with carbon rod, I understand the argument in favor of hide glue. I plan to replace all missing neck block wood, and extend the rod from the end of the dovetail to under the nut, and take my chances with the fragile, inauthentic blackwood.

I'm not sure about using two 1/8" bar-- in theory it seems like it would be okay, though it would depend on the strength of the bond between them. It might be worth spending a few $$ on a 1/4" bar just for peace of mind.

I wouldn't expect hide glue to stick to CF well, so I would use a good slow-cure epoxy for that. I always rough up the CF surface by sanding it first (the bars I get are usually slightly thick, so I just sand them to the final 1/4" thickness on my drum sander or belt sander, which also gives a good gluing surface). I would use HHG for the neck repairs and fretboard, though, as it will help prevent long-term creep.

Since the stress is only one direction, two bars side by side is mechanically the same as one…but if the glue embedment on the two outside faces is different, some strength might be lost, so glue the two together. Pretty sure epoxy will stick better to a less-porous surface.

Progress report & new questions:
Showing a photo of my chisel guide for fitting a wedge to replace wood that was chipped out of the neck block in the truss rod router episode.
Attachment 198286
Also, photos showing body part separations and a .012" sinking of the neck in relation to the top, as demonstrated by the carbon rod clamped to the neck.
Attachment 198277
It looks like the sinking neck would be exacerbated by gluing the separation between the neck block and the top edge of the instrument's side.
Attachment 198285
The heel cap doesn't seem to be movable even though it looks separated.
In the treble area near the tail, the side projects somewhat proud of the back edge. I'm guessing I could glue it as is.
Attachment 198287
Can someone please advise me regarding the body separations and sinking neck? And what is the translucent amber layer between neck block and top? Some of it is missing.
By them way, I had bored the headstock for a rod nut before I settled on carbon rod. Guess I'll have to cover that, and maybe fill with a dowel, too.
Thanks again, everybody!