How to Inspect a Guitar
Neck
...also available
in French at this location:
Manche
de la Guitare
This discussion applies to classical, electric and
acoustic steel string guitars. Although the classical nylon string guitar
usually does not have an adjustable truss rod this method of evaluation is
still applicable.
In
my terminology, "warp" always means a neck coming forward as a consequence
of the force exerted by string tension and usually produces high playing
action. "Tilt" means a neck which is set back against the pull of the strings
resulting in lower playing action. Throughout this article I shall
use the ideal neck set-up, which has a slight amount of warp (also called
"relief"), as a constant point of reference.
VISUAL INSPECTION
To examine a neck first perform a preliminary visual inspection:
Grasping the fully strung guitar with both hands, bring the neck up to your
line of sight (looking from the bridge toward the tuning keys) and aim it
at a diffused light source, such as a bright window, so the reflected glare
from this light illuminates the fingerboard. Concentrate only on the shadow
of the strings on top of the frets, not the wood (also, don't look at the
edge of the fretboard because playing wear can make it appear warped). With
the upper frets over the body reading straight as a rifle barrel, the shadow
will show a very slight dip around the 4th/5th/6th lower frets (on the ideal
neck).
Take note if a fret (or group of frets) is obviously higher than its neighbors.
This may indicate a fret rather than a neck problem. However, a group of
protruding frets resulting in a visual high spot in the lower frets could
be caused by a tilted or overadjusted neck.
Next, rotate the instrument end-for-end and repeat the procedure from the
tuning machines point of view. Do the string shadows indicate a neck which
is straight as an arrow; or, does the area of the fretboard over the body
seem to deflect slightly up, down, or remain in line with the rest of the
neck? Any abrupt or obvious deflections of the fretboard over the body indicate
a limiting flaw in the overall playability of the guitar. Make note of your
observations and recheck them.
STRAIGHTEDGE INSPECTION TEST
(actually a three-part test conducted using the 6th and 1st strings with
the goal of determining the extent of warp and the area of the neck where
the warp is concentrated)
Now try and confirm your visual inspection results with a straightedge placed
on the frets. The most convenient is the installed guitar string. Simultaneously
press the 6th string at the 1st and 12th (or 14th for electric and steel
string guitars) frets. Look carefully at the 6th fret...is there the tiniest
gap or is the string touching all the frets? A 1/64" gap (.015" or .4 mm)
indicates an acceptable warp for most guitars. If there is no gap the neck
may be too straight (or actually tilting backwards) and the truss rod in
need of loosening. But first you must check the rest of the fretboard.
Next, fret the string simultaneously at both the 8th and highest (19th, 20th,
21st, 22nd or 24th, depending on the type of guitar) fret. Is there a gap
at the midpoint (12th to 14th frets)? If so, then there is warp over the
body (the fretboard is rising) because this area should always read level
(no gap).
Finally, fret the string at the 1st and last frets. If the 6th fret gap
determined by the first straightedge test is unchanged then your warpage
is concentrated in the part of the neck away from the body and is controllable
by the truss rod adjustment. However, if the 6th fret gap increases then
you also have some warpage over the body, a situation not likely remedied
by truss rod adjustment alone. Keep the string down at the 1st fret (use
a capo if necessary) and confirm your results by sliding down the fretboard
starting from the highest fret, making contact with each fret as you slide.
If the 6th fret gap decreases as you move then there is definitely fretboard
deflection over the body.
VISUAL RECONFIRMATION
If the entire neck is showing warp the visual inspection will reveal a continuous
curve running the length of the fingerboard. The ideal neck will only
show warp concentrated in the neck area away from the body. Remember,
the upper area of the fingerboard should be dead straight and not show a
gap when examined.
Use the straightedge test to cross-check and educate your eyes. With persistence,
patience and practice you will be able to confirm the results. Check both
sides of the fingerboard by repeating all three straightedge tests on the
1st and 6th strings: lower area, upper area, and the entire fingerboard with
your finger pressing the string to the frets and sliding from the top down.
THE VISUAL INSPECTION AND THE TILTED NECK
If we have the straightedge test then why bother with a detailed visual
inspection? If the neck is perfectly straight or tilted back the straight
string will give the same results because it will appear to touch all the
frets. The visual inspection therefore separates the perfectly straight situation
from the tilted (or over-adjusted) neck. The visual inspection is also
very important for detecting when the fingerboard drops over the body.
Rechecking with the straightedge test and also measuring action height changes
at the 12th fret with a fine ruler will track the actual effect caused by
small changes in the truss rod adjustment. Loosening (deadjusting) the rod
will raise the overall height of the playing action and necessitate lowering
both the string nut and bridge saddle height settings.
A six-inch metal ruler can find high frets when your visual inspection and
straightedge tests indicate a straight neck. Fret misalignment (a high spot)
of just a couple of thousandths will cause the ruler to "rock" on the protruding
fret. Notes played just below this area might "buzz" or be cut-off completely
by the high spot. If an individual fret is causing the problem it can be
reseated or levelled. Also, a truss rod readjustment might level a high spot
or, if the wood itself is distorted, the frets can be removed to allow planing
of the board. Flip the ruler to the other edge and get the same results to
confirm that the ruler itself is straight and true.
BOTTOM LINE
When you can reconcile and cross-confirm the visual observations with the
straightedge tests there is enough information to determine if the neck is
in need of adjustment or repair. Best of all, mastering this procedure can
empower you when shopping for a new or used guitar. New guitars can be seriously
flawed or just need a neck/fret adjustment.
Because the facets of a guitar's playing action are so highly interrelated,
simply adjusting the neck is a preliminary step. I always do this in front
of the client to show how much the guitar can be improved (and how well the
mechanism works) and why the other issues of fret wear, bad neck angle, neck
twist, popped fret(s), bridge height, etc., must also be addressed in a complete
setup.
Since most classical guitar necks are not adjustable the corrections must
be made in the wood by using heat and pressure or replaning and refretting
and/or a combination of both approaches.
If you can't make sense of this description then it is best to take
the instrument directly to a qualified guitar repairer. Unless there is a
structural problem (loose bridge, etc.) leave the guitar fully strung so
it can be playable when examined. Without the normal pressure exerted by
strings it is difficult for a repair technician to assess the guitar. Adjusting
necks is not a big deal and I usually don't charge for it although it is
polite for the customer to buy a set of strings for my
time.
For
any guitar, only an experienced repairer can methodically sort out the variables
and determine the most appropriate resolution. You should receive a
clear explanation of your guitar neck's condition, including it's good points
as well as the liabilities, followed by a brief step-by-step description
of the procedure leading to the exact results. If the repairer cannot
efficiently deliver this information the person lacks experience or really
doesn't know how the job will turn out.
Acquiring
the observational skills oulined in this article will help you understand
what must be done to restore your neck and fingerboard to optimal playing
condition. If you are in need of the services of a repairer such knowledge
can only serve to assist both parties toward the goal of achieving mutual
satisfaction.
Roger
Thurman
Nominal String Height - General low settings for
comfortable action
(At the 12th fret, measure with a fine ruler the gap
from the top of the fret to underside edge of the string.)
Classical Guitar
First String (highest string): 1/8" or 3.2 mm.
Sixth String (lowest pitch): 5/32" or 4.0 mm.
Acoustic Steel String Guitar(six or 12-string)
First String: 1/16" or 1.6 mm.
Sixth String: 3/32" or 2.4 mm.
Electric Guitar
First String: 3/64" or 1.2 mm.
Sixth String: 1/16" or 1.6 mm
Flamenco Guitar
First String: 5/64" or 2.0 mm.
Sixth String: 3/32" or 2.4
mm.
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Thurman Guitar & Violin Repair, Inc.
900 Franklin Av.
Kent OH 44240
Toll Free: 888-803-8693
330-673-4054
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© Roger G. Thurman 1998-2002
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Thurman Guitar & Violin Repair, Inc.
900 Franklin Av.
Kent OH 44240
Toll Free:
888-803-8693