Restoration Stories - Necks
There are really three main issues here:
Let's deal with those one at a time:
Necks develop humps and bumps for all sorts of reasons, often these are so mild that the owner doesn't even notice – except perhaps for the occasional fret buzz in particular locations or an inability to dial in a decent action.
Mild cases can be treated with truss rod adjustment combined with some fret-levelling, if the bend/hump is too large to fix with simple measures then you're looking at plaining the fretboard flat and re-fretting. Unfortunately, while it is possible to “torture” a neck into a new shape with heat and clamps, this is a repair that has a reputation for not holding over the long run – once the wood has decided it wants to inhabit a particular shape it's quite hard to force it into another one with any certainty that it will stay there. It is a cheaper alternative though, so may be worth a try, especially on cheaper instruments.
Necks Pulled Forward
There's a lot of tension in a set of guitar strings, and all necks will pull forward somewhat over time. This can be due to the neck-body join failing, but more often the body will simply distort under the constant string pressure, resulting in an instrument with a sky-high action.
I'm going to skip over electrics with bolt-on necks – these are relative cheap and easy to fix by inserting a shim between neck and body. Please do note that this has to be done correctly – with a wedge shaped shim that supports the neck over it's whole length – simply placing a shim at one end to “prop the neck up” will work in the short term, but as the neck is now unsupported where the bolts are, in the long term the neck will get pulled out of shape making the issue much worse and harder to fix.
Let's move on to acoustics with set necks (dovetail joints). This nice old archtop had a neck that had pulled forward quite some distance:
Here's the reason why a neck reset is possibly the most traumatic thing you can do to a guitar:
Fortunately, those splinters will go right back where they came from, and afterwards we end up with a guitar that is now nicely playable:
This old Gibson mandolin had a similar issue, unfortunately someone had tried to repair this one before with epoxy:
Now, there are two things you should know about epoxy in a situation like this: nothing short of dynamite will get it out of there, and it absolutely will not hold a neck on: epoxy has many brilliant uses, but while it's very stable/resilient against knocks and bangs, it tends to creep/move under continuous pressure. After spending way too much time with dental picks trying to pick the stuff out, in the end I had to give up and saw the thing off, I'm glad I did too, the epoxy was all the way through the dovetail (the dark coloured sections shown here):
Since the neck already had some questionable repairs to the heal, I decided to take it off with a razor saw, and carve a new one that included a whole new dovetail:
With everything back together (along with a brand new fretboard to complete the repair) we now have a really nice old Gibson:
Probably the most fragile part of an instrument is the transition from neck to headstock, not only is the neck the thinnest here, but if there's a truss rod, then most of the core of the neck is effectively “missing” – in effect there are just two small “fingers” of wood either side of the truss rod joining neck to headstock. No wonder broken necks are so common!
Fortunately many breaks occur cleanly along the direction of the grain: these can be simple glued up, and should be just as strong as before. There's no need for pins or other inserts as these necessarily cut through wood fibres, and actually reduce the gluing area.
Things get more interesting if the break is either not clean, has splinters missing, or does not simply follow the wood grain (rule of thumb – gluing parallel to the wood grain is “stronger than the wood” as long as the join is really tight, where as gluing to end grain is unlikely to hold no matter what you do).
Let's start with an 1893 DeMeglio mandolin that had been badly repaired before, taking the joint apart there was a whole section splintered/missing:
The solution adopted was to graft a whole new section into the headstock, taking care to ensure there's a much side-grain gluing area as possible:
Now the whole neck can go back together:
Finished off, this became one of my favourite sounding DeMeglios, made in 1893 I suspect the neck was broken around 1894 (!), and this in effect resulted in an instrument in a time capsule. As close as we can get today to how a DeMeglio sounded “back in the day”:
You never know what lies in wait when you take a neck apart like this, this Leir mandolin had also had a previous repair that had failed:
Taken apart it's quite a mess:
No choice here but to take the two halves back to good wood, and then rejoin with a shim between the two parts. One of the nice things about hide glue is that it will pull the components together as it dries, so as long as the fit is good (I.e. “light tight”), a “rub joint” can be used. In a situation like this it avoids the need to try and clamp complexly shaped parts:
Note that while the shim is only 5mm thick, it adds a whole 13mm to the length of the neck, no way we can do without that! Here's the completed neck, with just the veneer and fretboard to go back:
Sometimes it's the headstock itself that breaks: most often this a result of the screws holding the machine heads in place being forced in too tight and splitting the wood.
In simple cases these can be glued up rather easily: more so the sooner the problem is addressed. More extreme cases look like this:
Clearly something more radical is required in this case! The solution in this case was to make a whole new fascia for the headstock, here it is testing for tuners fit:
The repair starts with some serious destruction, by removing the whole face of the headstock, with the net result that even more parts broke away. At least now we're down to sound wood:
Cleaned up ready for gluing - the blind holes in the new fascia provide clearance for the machine head screws:
With the fascia glued on and a bit of reconstruction round the back, things are starting to look much better now:
With a little bit of fretboard reconstruction, plus a full refret, the instrument can now be strung up: