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I may have filed into the end of the fingerboard slightly while preparing my Yamaha acoustic guitar's nut channel for a new nut.  It's not really visible (to my eye), but with a caliper I can see that the distance from the first fret to the new nut (which I have already completed) is very slightly greater on the treble side than the bass side.  With the new nut and new strings on the guitar, there seem to be some intonation problems beyond the inherent imperfections of a guitar.

I noted a slight wobble (end of fingerboard wasn't flat) while fitting the nut which foreshadowed this, but I couldn't tell where the glue ended and the wood began, so I wasn't sure exactly what to do.  In an effort to get a flat surface on the end of the fingerboard for the nut to sit tightly against, I filed it just a little, but now the fingerboard is less than square.  In my defense, the nut channel was already messed up!

If I want to shim the channel in order to move the nut back toward the peghead and situated at the proper angle, is there a way to figure out exactly how far it should be from the first (or another) fret?  Is there a material and adhesive that would be good for this?  Or is this a stupid idea?  Hoping I haven't ruined my guitar.

The scale length (nut to center of 12th fret multiplied by 2) seems to be very close to 24 15/16" (which I have not heard of as a standard scale length).  I took this measurement on another Yamaha guitar with similar fret spacing which (I am assuming) had not had its nut channel botched in this manner.

Thanks in advance for any help!  Also if this has been discussed, I apologize.  Searching for answers to specific questions can be difficult when the keywords always seem to be the same few dozen guitar related terms--fingerboard, nut, file, etc.!

Tags: channel, fingerboard, nut, shim

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We have no way to tell how much you shortened the 1st fret space. You can determine if that is the cause of the intonation error by slapping a capo at the 1st fret, tuning the strings exactly, and checking the octave note at the 13th fret (as if the 1st fret were the nut). If the intonation is still off, your nut is not the problem (which I suspect is the case, unless you really went crazy with the file).

Russell, 

measure the distance between the first and second fret and then put a couple of "test" scale lengths into the fret scale calculators  available everywhere on the net.  When you get a match for the "known good" reading of the 1st to 2nd fret distance read off the 0 to 1st  fret distance and you have a measurement to use for reconstruction.  There's probably a lot easier ways of doing this, but its early here.   Jig up the nut and glue it in place and then fill the taper area with rosewood dust and superglue (the repaired area will look like a shadow to the casual observer).  

Once again there are a hundred ways to fix this, all better than these mumbles I'm sure,   maybe make a tapered nut etc.   Its no big thing - you haven't ruined your instrument.  

Rusty. 

Rusty nailed it. Make a nut that fits, and you can always file the slot openings back until the intonation is dialed in. Or, you could add a small shim and file it to the correct dimension.

Get an accurate measurement of the distance from the fifth to the twelfth fret.   That should be identical to the distance from the nut to the center of the fifth fret, regardless of scale length.

It's a trick I've used for nearly five decades now. . .

Im gonna have to write that one down somewheres.

Thanks a lot for your replies.  Sorry for the extremely long post to follow.  I don't presume any of you guys should take the time to wade through all this, and if you don't, I certainly won't be bitter because I know you have work to do.  I appreciate all the information I've already been able to get online from Frank, Dan Erlewine, and all you guys on the forums as it has helped me to do so much on my own.  Anyway, here goes:

I've taken some measurements as carefully as I could using my vernier calipers, sometimes in combination with a 6-inch rule for longer measurements.  I am very careful to measure as closely as I can from the center of the frets, and the plane of the front edge of the nut, but there appears to be a small margin of error (maybe around .005" to .010"?).

I measured 3 guitars for comparison, which all seem to have the same scale length.  A Takamine G330, an old Yamaha FG-280, and the guitar in question, a somewhat newer Yamaha FG-340.

For the 'control group,' the 'Nut to 12th' measurements were:

Takamine:  12.475"

Yamaha FG-280:  12.477"

Doubled, these are around 24.95", which I originally was calling 24 15/16" before I pulled out the calipers.  It is definitely not 25".  Why have I never heard of this scale length, yet it's common to all 3 of my acoustics?  Or have I measured wrong?  

My measurements for 'Nut to 5th' and '5th to 12th,' to apply Frank's test, were:

Takamine:

Nut to 5th:  6.239"

5th to 12th:  6.228"

(Total:  12.467")

Yamaha FG-280:

Nut to 5th:  6.233"

5th to 12th:  6.236"

(Total:  12.469")

Looking at these pairs of numbers, which are within .011" and .003" of each other, respectively, and comparing them with the 'Nut to 12th' measurements, which are (for both guitars) .008" off from the sums, I get some idea of how much margin of error I may be looking at with my measurements.

When I plug 24.95" into a fret calculator (http://www.harpkit.com/category/fretcalc.html), though, it tells me that the 5th fret should be 6.259" from the nut, which doesn't make sense with Frank's rule, and it doesn't match up with either guitar's measurement.  Other frets don't match the fret calculator distances either (with the exception of the 12th).  Am I missing something?

For the guitar with the issue, the '5th to 12th' measurement is 6.232", which looks good (assuming it should be 1/4 of a 24.95" scale).  For 'Nut to 5th,' it is 6.235" under the high E string, and 6.214" under the low E.  The nut is clearly a little slanted.

And in general, should I be trying to get the guitar to the correct measurements, and then address remaining intonation issues at the saddle afterwards?

Thanks again, guys.  Let me know if I left anything out.  This is much more confusing to me than it probably is to someone with more experience.

Thanks for the heads-up.  I'd been using that little trick to estimate the fret position when I was adding a fret to a dulcimer, one of the only jobs where I have to add a fret in to a fingerboard.

As I compare fret charts, I see that my measurement trick results in a nut that is a bit closer to the first fret than Id expected.  Just about the right amount to cut off for nut compensation, I suppose, so it wouldn't be too much of a mess, but it isn't the level of accuracy I'd thought.

So, back to the ol' drawing board, I guess. . .

But the measurements on my guitars from nut to 5th, and 5th to 12th do seem to be close to equal, being just .006"-.008" off which could be my error.  The calculator is telling me that for guitars like my Takamine and Yamaha FG-280, with 12th frets at 12.475-12.477", I should have a 5th fret at 6.259", but in reality (well, as measured by me) they are at 6.239" and 6.233".  Closer to halfway (6.238") than to the calculator's 6.259".

My first frets are also closer to the nut than the calculator's 1.4", given for a 24.95" scale.  I'm wondering if I have the wrong scale or what.

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