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Mandolin nut string spacing

Q: I have recently acquired a "brand x" mandolin, no markings no serial #. I got it with only the bass half of the nut which had fallen off. Placing this piece where it should be, the nut seems too low as the slots are below the top of the fretboard. Obviously I have to make a new nut but I do not have a functional nut from which to copy the size and string spacing. It seems that the nut width will be 1 1/4 or 1 3/16. Are there general rules for the location of the slots and the spacing between the paired strings? I am a competent craftsman and would like to do this work myself but have been unable to determine the slot locations and string spacings. Any info is appreciated.
Tony B

A: On most instruments, you eyeball the spacing from the outside edge and then use a divider to make as many pencil marks as you have strings.  When you have more than one string per course, as on a mandolin, 12 string guitar, tipple, mandocello etc., it is more difficult.  You want the measurement between the strings to remain constant, which means that you must take into account the width of the individual strings.   On a mandolin, the outside strings should be 1/16" (.0625) from the edge.  There should be 1/16" between the strings in a pair and the spacing between the pairs should be equal.  The standard nut width on an F style mandolin is
1 1/8".  This puts the space between the pairs at 5/32" (.156).  Your spacing will be wider.  
Start by marking a light pencil line on the nut 5/64" (.068) from the treble edge of the nut.  This is .0625" from the edge plus 1/2 of .011 E string width.  Then mark another pencil line 5/64" (.068) in from the first line.  This gives you the position of the two E strings.  Next mark the position of the two G strings by making a line 3/32 (.0825) from the bass edge and then another line in 3/32 from there.  1/16" plus 1/2 of the .040 G string width etc.
Now enlarge your inside lines slightly to approximate the other half of the width of the inside strings.  Measure the distance between the two inside lines.  The A strings will take up 1/16 (.0625) plus 2 times their .015 diameter, or 3/32 (.0925).  The two D strings will take up 1/16 plus 2 times their .026 diameter, or 1/8 (.1145).  Subtract the A and D pairs from distance between your inside lines then divide it by three.  This will give you the measurement of the three spaces between the four courses.
Please note that the fractional measurements are very approximate.  The measurements in thousandths of an inch are much more accurate.  A dial caliper marked in thousandths is the tool you need.  But, a fractional ruler marked in 64ths is probably more accurate than the variables.   Mark your spacing on the nut with a sharp pencil.  Look at it.  Check for gross errors, like ones in my math or yours.  Remember that it should look a little funny with no strings on it because the wider strings will have wider spacing.
Using a razor saw, cut down into your pencil marks just deep enough to hold the strings.  Now string it up and look again.  Look hard.  The next step is to deepen the string slots with gauged nut files.  Any slight inaccuracies can be alleviated by cutting more on one side or the other of the slot.
In theory, the nut should be exactly the same height as the first fret.  With a capo on, the first fret makes a fine nut.  Some people with a rotten nut set use a capo all the time and are quite contented.  Forget everything you've ever heard about feeler gauges.  They are not helpful in setting a nut.  The easiest way is to belt sand a pencil in half lengthwise.  Then laying the flat sanded part on your frets you can mark the nut with the half lead point.  If you then cut down to the pencil line the nut should be set perfectly.  If the string buzzes against the first fret you cut too low.  Fill with bone dust and superglue and try again.  If the string doesn't buzz it's too high (just kidding).   Lots of cheap mandolins have a "zero fret".  If you have one of these just space it right and deepen the slots until the strings lay hard against the zero fret.  But, I've never seen a cheap mandolin with nut spacing as wide as yours.   Good luck with the project.

Steve Mason