Dovetail-Installed Guitar Neck Reinforcements

1930s Kay with no steel bar and a new hole drilled just under the fingerboard

Why is this needed?

Old guitars were often built with a steel bar in the neck to help counteract the force of the string tension and prevent the neck from developing a bow. Acoustic strings can put up to 200lbs of force on a guitar and even maple can only resist that pull for so long. So the adjustable truss rod system was developed to reinforce necks and allow for an adjustable bow which has probably been one of the greatest advancements in guitar building. They were more expensive to install and make than a steel bar so often lower end guitars find themselves with steel or sometimes nothing (especially common in the 1940s because of the war).

Often times people will sand the fretboard flat again (removing more material from the ends than the center) and refret but this is only a band-aid. In some cases, sanding the fretboard removes patina, dyes, or physical wear that fit the instrument and make it unique. The best method for preserving the fretboard (besides a compression fret) is to remove the fretboard, install reinforcements, and reglue it. This is expensive as it takes time and care plus finish touchups afterwards. That often means that old inexpensive guitars are not worth sending through this process and end up unplayable. 

I seek to change that by creating a more economical method

What do you need?

• The guitar in question must not already have a steel rod or truss rod installed
• A 15" or longer spade drill bit
• A corded drill
• A jig to clamp the neck securely and also add back bow

How does it work?

First, I begin by removing the neck using the typical method of steam and clean it of excess glue

Second, I place the neck into my jig that holds it steady at the heel and at the peghead and begin turning my turnbuckle/neck-cradle which forces the middle of the neck upwards and into a backbow

Then, I put a 15" 3/8" drill bit into a corded (you'll want the constant power) drill, line it up, and begin drilling until I have reached just past the 1st fret

• I set magnets on top of the fretboard which glide towards the center of the drill bit and help let me know if I'm veering to the left or right
• I also use a flashlight and look into the hole periodically to make sure I'm going straight and not up into the board or down 

Finally, I get my carbon fiber inserts (recently been using a 3/8x3/8 bar), check the fit, pour glue down the channel, and gently nudge the carbon fiber in using a mallet. Saw the excess off so you're flush with the dovetail

This operation was done and the fret dots popped out
allowing me to see the channel I had cut.
Note the ebonized board which would not have taken
a planing and looked the same

Results & Improvements

It takes me about an hour to drill a hole through the neck, depending on the material, and much more care is needed for thinner neck profiles. The ideal neck would be a V or U shape (to prevent from accidentally drilling out through the neck) with a fretboard which is harder than the neck wood (to discourage the bit from wandering into the board)

I've had varied success depending on the neck/fretboard wood's rigidity and how much backbow I've put into the neck before drilling. I've also used this method in conjunction with a heat-press and had good results. 

I've dabbled in using this method to install truss rods and haven't quite gotten a successful result yet.

What I've learned

• If the neck already has a steel bar, they are not easy to remove through the heel
• Force the neck into a backbow (not straight!) before drilling. A larger bow is needed for softer woods
• Work slowly