Measuring for the Neck Reset on Regal Tenor Guitar

Oh wow.

I just realized that I've been working on a Princess banjo-mandolin, and now I have 2 Regal tenor guitars in process too!  Royalty is in the house. I suppose I should roll out some red carpet in The Dungeon! 

Monaco or San Marino, this ain't.

So I have some new tools from Stew-Mac to use on these neck resets.  One of the tools will be super useful I think, one will be medium-useful, and one may just be a bit overkill.We shall see.

So, when one does a neck reset, the first order of business is to measure the amount the neck is underset.  On the left I have an old picture I took when resetting an Ovation guitar.

The process is simple:  with the strings tuned to proper tension, you lay a long straightedge on the frets parallel to the strings and position the straightedge so that it contacts the bridge.

If the straightedge sits just on the top of the bridge, the set is correct.  If it contacts under the top of the bridge, as we see here, the neck is underset.  We then need to measure the vertical distance between the top plane of the bridge and the point where the straightedge contacts the bridge.  We'll then use that measurement in a formula to determine how much material should come off the bottom of the neck heel in order to correct the neck set.

In the past, I've put a pencil mark at that contact point, and then measured the distance as best as possible with a small ruler.  It's a small measurement, and it's not particularly accurate, when you consider there is the width of the pencil lead and the fact that there are tiny fractions of whatever measure you are using - maybe between 64ths of an inch or fractions of a millimeter.

The accuracy of this measurement will determine the accuracy of the amount calculated to remove from the heel.  If you have an accurate measurement, you'll spend less time taking off a small amount of material, testing the set, taking off some more, etc.

Enter the Stew-Mac neck set gauge.  This tool is essentially a dial caliper you bolt to the end of your straightedge and line up with the bridge.

The dial is calibrated in .001 inch divisions.  One thousandth of an inch.  That's .0254 millimeters.  We're talking a very small amount here.

It's very easy to take this measurement with the gauge.

Line the straightedge up as you would usually do.  The straightedge will naturally "point" to the place it would contact the bridge.

Then take a measurement from the top of the bridge with the gauge.  You can see this in the closeup picture.  I illustrated this on the image as well.

The gauge will record the difference between the underset point and the top of the bridge - accurate to a thousandth of an inch!

On the small Regal tenor, I measured an underset of .1026 of an inch.  No way I could get that accuracy with the pencil-ruler-and-eye method.  I think this is a super useful device.  Not too expensive either - it could pay for itself after a reset or two. 

I'll do the math for the actual wood removal after I get the neck off.  For now, I just need that 'golden' measurement.

Now for the tools that may be overkill.

I need to talk about a physical concept here.  When you release the string tension on an acoustic instrument, the top will tend to deflect downward, since it's not under tension.  The amount of the deflection depends on a lot of things - the string length, the number and gauge of the strings, and the size of the body. I would guess you'd see more deflection on an larger, longer scale instrument than on something like an ukulele, mandolin, or the tenor guitar we have here.

If you think about it, if the top of the instrument goes downward with no tension on it, wouldn't that affect the height of the bridge and also therefore, the measurement we just took for the neck set?

Yes, it would. If we just had some way to measure that deflection and then account for it when we do the reset.  Hmmm.

Which brings us to potential overkill tool number one.

A top deflection gauge.

Crazy.

What you do is put it on the top of the instrument at the bridge and zero the gauge - see the picture.  See how I used a square behind it to ensure my reading was accurate - and the location of the gauge was exactly repeatable.

Then take off the strings.

Take a second measurement with the strings removed, and you, my friend, have just measured the deflection of the top of the instrument!

In this case, it measured .019 inches - nineteen thousands of an inch.

So what now?

We remove the strings and employ the tool that will allow us to account for that deflection by raising the top of the guitar back up to where it would be if it were under string tension.  To do that, we place a scissors-type brace jack inside the guitar and crank it up .019 of an inch.

While I said the gauge and this tool may be overkill, this jack could be useful for things other than neck resets - for example, it would be good for regluing loose braces inside a guitar body.

I have a few of the inexpensive turnbuckle-style jacks, and I've had mixed results with them.  It's extremely hard to adjust them when they're deep inside a guitar.  Cheap, but you get what you pay for.

When I saw this brace jack in the Stew-Mac catalog, I pondered about getting one.  Then I decided to get one when I realized I had 5 (I think) neck resets waiting in the queue.  But the real impetus was after Toy Making Dad said to me, "That looks like a clever tool.  I can see where that would be useful."  Of course, he didn't offer to finance it.

It's a nicely made jack that adjusts with a 4mm machined square drive on the screw that raises it.  It comes with a long flex hose to attach to the fitting on the jack.  In theory, the long flex hose is a good idea, but in reality, it doesn't work well, mainly because it's hard to get a lot of torque on it when the jack is in position.  However, you also get a knob which you can attach to the jack instead - if you can reach the jack to turn the knob.  And, since we know the size of the adjustment screw...well, hold that thought for now.

The jack also comes with two neodymium boride magnets.  They're really strong.  I have some others around here, and boy, do then SNAP together.  Talk about opposites attacting - these things are the Tony and Maria of magnets.

They're so strong that there's a warning label in the box with the jack.  Not only will they demagnetize stuff, they could pinch your fingers if your fingers got between them.

Potentially dangerous, but very useful.  Sort of like a magnetic version of a table saw.

In this instance, you can screw one magnet to the jack, and use the second one to locate the jack within the guitar. 

It took me a while to figure out how to get the jack into the small tenor guitar body, and how to get something strong attached to the jack's adjustment rod in order to, well, jack the thing up.

Finally I came up with the idea of just using a 1/4 drive ratchet.  You can see I had to tape the socket to the jack - I say it's a 4mm drive, but not exactly.  Nor does it seem to be an exact Imperial measurement either.  All I can tell you is it is resistant to sockets or wrenches, which slide off easily.  Hence the tape.

I need to get the jack under the bridge of the guitar so I can raise the top up.

Took a couple tries, but I got the jack positioned under the bridge location.  In the picture on the left, I used a mirror to attempt to show the brace inside the guitar.

I did try jacking it up, but on this guitar, the bracing is so light that the jack actually pushed the top and the back outward.

What I'll do is take the neck off, and then clamp the body down so the back won't flex, then I should be able to jack the top that .019 I need (sounds a bit crazy to account for such a small amount of deflection, but there you have it) and go from there.

Stay tuned.  Will all these measurements may the reset easier and more accurate?  They better!