Acoustic Guitar Neck Reset – Strange Guitarworks

We tend to write blog posts about projects that we find interesting or unique in some way, but that’s not typically how our days go. Most of what we do is routine guitar maintenance. Instruments need to be serviced at regular intervals – much like oil changes, brake pads and tire rotations on your car. So, today I’m going to cover in detail one of my most common jobs at the shop: the acoustic guitar neck reset. Each instrument manufacturer has a slightly different take on acoustic guitar design, so there are some small variances in procedure when it comes to working on different guitars, but for the purposes of this post I’ll be focusing on a Martin HD-28 Dreadnaught, one of the most common and best selling acoustic guitars on the market.

Every acoustic guitar, regardless of brand or price point, will need a neck reset done at some point in its life. Going back to the vehicle maintenance analogy, this is kind of like the timing belt of acoustic guitar upkeep. It’s a lot of work, takes a lot of time, can be a real pain to get right, but once it’s done – and done correctly, it’s good to go for decades.

An acoustic guitar in standard tuning, using factory recommended string gauges, is subject to roughly 180 pounds of relentless string tension. These instruments are put together using very thin pieces of wood, precariously attached to one another with relatively little structural support. Trial and error by acoustic guitar manufacturers over the decades has proven that more structural support engineered into a guitar’s soundbox will choke the resonance out of the instrument, or one can build minimal structural support and it will chime like a bell. Basically, you can either build a guitar to last, or you can build a guitar to sound good – finding the balance is key.

Ever-present string tension is slowly but surely pulling the guitar neck into the soundbox, forcing the soft wood top (usually some type of spruce) to begin to collapse on itself, and causing the neck to list upward as it pivots at the body joint. This geometric shift ultimately results in the string action rising to the point where you can no longer cut the bridge saddle low enough to be able to comfortably play the guitar, or be able to make it play in tune with itself, as you’re now having to stretch (sharpen) the strings considerably just to fret the notes.

This is where the neck reset comes in. A neck reset is a procedure wherein the neck is removed by melting the wood glue with heat and/or steam, the angle of the neck heel is changed to compensate for the settling of the soundbox using exact calculations, and then it is put back together. Our shop does this differently than most: We use actual string tension on the guitar at various stages of the process, to account for compression and deflection of the neck and body of the guitar when it’s under the 180-pound-force of the strings. I developed this technique about fifteen years ago and I find it t be the best way to get the most accurate results.

There are a lot of steps that need to be expertly executed for this procedure to be a success. These are the steps I’m going to be covering here. So let’s get into it!

When this Martin came into the shop the customer was complaining that the action was really high and the guitar was difficult to play. The string action gauge shows the action to be over 4mm from the top of the 12th fret to the bottom of the low E string. This is what we consider be be very high string action – more than double our low action standard of 1.75mm-1.5mm (from bass side to treble side).

This guitar is due for a neck reset. The first move I make here is adjust the truss rod to get the neck into an optimal state of relief and allow for string oscillation, which is typically around 0.1mm distance between the top of the sixth fret (halfway point between the nut and 12th fret) and the bottom of the string, while being capo’d at the first fret and fretted at the 12th. I use a .102mm automotive feeler gauge to measure this gap.

Once I have the truss rod set where I want it I lay a straight edge across the entire surface plane of the fretboard all the way to the front edge of the bridge. If a guitar’s neck angle is correct the straight edge will lay just on top of the bridge and be able to slide right up to the saddle. In this case, the straight edge hits the front edge of the bridge. Way too low.

Using a long ruler as a secondary straight edge to rest on top of the bridge and another ruler perpendicular to the two straight edges I’m able to accurately measure the difference between the two. This neck is about 3.5mm underset.

This is the formula I use to calculate how much material will need to be removed from the neck heel to correct the guitar’s geometry.

(We use the metric system for almost everything we do in the shop. We don’t have to bother with fractions, which makes it a much easier system to use.)

Once I have all of the numbers I need, I can plug them into the formula to find X.

One last factor I need to consider is the small gap in the neck heel that I noticed when I was first assessing the instrument. I suspect someone has worked on this guitar before, and it looks like they improperly fit the dovetail joint when they were putting it back together. As the neck settled under string tension the dovetail wasn’t tight enough and the glue surfaces shifted, causing a .254mm gap to form. This seems like a small number, but it can make a big difference for the overall neck angle and it renders the stability of the job unreliable at best. This alone would be reason enough to take this guitar apart and fix this issue.

I will subtract .25 from X to get my new material removal number of .85mm.

Now that I have the material removal information I need I can start taking the guitar apart. The first step is to remove the strings and heat the glue under the fretboard extension so that it’s soft enough to separate the ebony from the soundboard. For this I use a purpose-shaped heating blanket controlled by a Variac (variable AC transformer) so that I can adjust the voltage going into the heating blanket. It’s very easy to burn wood and inlays with one of these if you’re not careful!

Once the glue has softened I work a pallet knife into the joint to release the extension, which usually takes 20 or 30 minutes, if responsible glue was used. This must be carefully done so as not to scratch or dent the guitar’s finish around the working area.

Once the fretboard extension is free I can begin the process of removing the neck. While the frets are still hot, I use a small pair of end-cutting nippers to remove the 15th fret and gain access to the dovetail joint beneath.

I very carefully work my way across the fretboard with the end-cutting nippers to lift the fret out of the slot without chipping out any of the ebony. This can be a very delicate process.

Just as I suspected, someone has been in here before me! The tell is the small hole they drilled for their steam nozzle. This would not have been done during production at the factory.

I will not be using this existing steam nozzle hole. Instead, I’ll use a drill bit specifically sized for the heating element I prefer for this step. We used to use a steamer to do this but we stopped doing it that way a number of years ago. We found that a steamer introduces way more moisture to the instrument than is necessary. These days we use a copper heating element attached to a soldering iron to apply dry heat the glue joint. That way we can control the amount of steam to release the glue joint without damaging other parts of the guitar or loosening unintended surrounding glue joints.

I drill these two holes in very specific places to target where the heat is going to go. The dovetail joint surfaces on this guitar are about 6mm from the edge of the fretboard, so I get those holes close to the glue surfaces without drilling into the neck block. The locations of these holes will vary depending on the make and model of the instrument, and the only real way to know where they are is with lots and lots of experience.

Before introducing heat, I drip a very small amount of water into each hole with a pipette to give me just enough steam to work the glue joints loose.

We’ve dedicated this soldering iron to be our neck removal iron. The copper heating element, which is long enough to reach all the way into the glue joint, stays in this iron at all times.

I periodically switch back and forth between the two holes so I can apply heat to each side equally until the glue starts to soften. This process can take anywhere from 20 minutes to several hours, depending on the type of instrument and the type of glue the factory (or last repair tech) used.

As the glue softens, I gently work the body of the guitar in a circular motion to loosen the glue joint. Forcing this stage can crack wood and cause damage to the instrument, so patience and care are paramount here.

Its always a satisfying moment when the guitar comes apart nice and clean!

Once the guitar is apart I let it sit overnight to cool off and dry out before proceeding. Then, using a purpose-shaped hardwood stick with 80-grit sandpaper on one side, I clean up all of the glue surfaces and prep them for the next steps.

This neck heel surface must be cleaned up and reshaped before I can set my new angle. The idea here is to create a very narrow perimeter surface to reduce the amount of wood I will have to remove by pull-sanding in the next step. I use a freshly sharpened chisel to cut into this mahogany end-grain.

Once my surface is prepped I use a digital caliper to scribe a reference line for material removal as I set my neck neck angle. Remember, my original calculation called for 1.1mm of material removal, but I have to compensate for the .25mm gap that was in the heel, so my new number is .85mm.

This next step is one of the most critical moments of the whole job. As you can see, only a very small amount of material needs to be removed to reset this neck angle. A little bit goes a long way and overshooting this cut is irreversible. Slow and steady pull-sanding is the key here.

Pull-sanding directs the wood removal to the back of the heel. No material can be removed from the front of the heel where it joins the fretboard. Doing so will make irreversible intonation changes to the guitar.

Once I have my new angle set, I go around with the chisel one more time in case more pull-sanding is necessary after my test fit.

Before I start putting this back together I clamp the neck heel in place and string the guitar up to full tension to check my angle and centerline.

The centerline of the neck is very easy to throw off during the pull-sanding process. Great care must be taken to keep the sanding symmetrical. If the centerline is thrown off, more material has to be removed from one side of the neck heel, which can affect the neck angle and possibly the scale length, which will throw off the intonation. This is a very critical part of setting a correct neck angle. Nailed it!

Once I’ve verified that my new neck angle is correct, I start making shims to take up the gaps I created in the reshaping process. While I’ve got the guitar strung up to pitch I measure the gap in the fretboard extension using an automotive feeler gauge and a straight edge.

To make the fretboard extension shim, I’ll use a piece of ebony to match the fretboard wood and sand it into a very thin wedge.

Using double-sided carpet tape, I attach both the ebony shim and the feeler gauge (as a stop) to a flat piece of wood and shape the wedge on a belt sander.

I use .5mm mahogany veneer stock to make the dovetail shims, then glue up all three shims one in one fell swoop and let it sit overnight.

Once the glue is dry I can pull all of the clamps off and trim everything down with a sharp chisel.

Fitting the dovetail is another one of the most crucial steps of the whole job. This joint must be tight enough to hold together under string tension without glue. The last tech who was in here over-sanded the dovetail surfaces causing the glue joint to shift and form a gap under the heel. Sanding these down for a tight fit can’t be rushed.

I start by test fitting the neck. Notice the gap under the fretboard extension. I carefully sand the dovetail shims little by little and keep test fitting until the gap disappears.

Each time I pull the neck back apart after test fitting I look for shiny spots on the dovetail shims to see where the mating surfaces are sliding against each other, and I sand those spots to create a full contact fit.

As I’m sanding I occasionally come in with a chisel to make sure my surface is totally flat from end to end.

I’ll fit the dovetail by hand until there’s about a .25mm gap, then I’ll seat it with a clamp for a tight fit.

Once the dovetail is fit I’ll tune the guitar up to full string tension without the heel clamp, double check my neck angle and centerline, and make sure the dovetail will hold tight without glue. I can’t stress enough how important it is for this to be a tight mechanical lock.

Looks good! Time to glue it back together. For this I use Titebond Original wood glue,

Once I clamp it up with glue I put the strings back on and tension them up to check the neck angle one last time to make sure nothing weird happened between the test fit and gluing.

Even after all the clamps are on I keep string tension on the guitar. I’ve found through experience that even this step can be affected by string tension, so I don’t take any chances.

After letting it sit overnight I remove all of the clamps and make two little ebony dowels to plug the holes in the fretboard that I drilled out during the neck removal process. I use epoxy mixed with black pigment to fill in any potential chips in the wood. I even made one for the hole the last tech didn’t bother to plug…

I let the epoxy fully cure, then I file and sand the area smooth again to make it match the rest of the fretboard.

Once the surface is smooth, I use a binding-friendly fret slot saw to recut the slot.

With the slot properly prepped, I use a brass fretting caul to press the 15th fret back in.

After the fret is fully seated I put a little dab of superglue on each side to help keep it from lifting when I level the frets.

Then I use a fresh razor blade to scrape off the excess glue and shine the board up with a Scotch-bright pad.

Since the geometry of the instrument has changed, the old saddle is going to be too low now. A new bone saddle is in order. We make every saddle from scratch to fit each individual instrument.

When the geometry of a dreadnaught acoustic guitar is correct, the saddle should rise between 2.5mm – 4mm above the surface of the bridge. If yours is lower than that, it may be time for a neck reset!

I compensate the saddle for string intonation with a single-cut file.

Then I polish it out to a bright, satisfying shine.

The final step to this saga is stringing it back up and running it through our Plek machine to level out any fret inconsistencies resulting from the surgery this guitar has been through.

After it comes out of the Plek machine with perfectly level frets, I polish the machining marks from the frets and complete the setup. This guitar is good to go for several more decades!

Thanks for reading!