Measuring the gap from the top of the 12th fret to the bottom of a straightedge to determine the need for a neck reset.

[Jeff Bolek]

Does anyone know the gap size measured from the top of the 12th fret to the bottom of a straightedge to determine the need for a neck reset?   One way of determining the need for a neck reset is to place a straightedge along the neck and see if it clears the top of the bridge.  If it does not clear the top of the bridge but goes just slightly over it there would be a gap on the 12th fret.  How big a gap can you have before you really need a neck reset?   I have a couple of guitars that do not clear the top of the bridge but are just slightly over it but not by much.  thanks  Jeff Bolek

[Len Biglin]

if the line of the frets is above the wood of the bridge then its overset , meaning you don't need a reset , but your saddle will need to be a bit higher than normal to get the strings up off the fretboard . I hope I understood you correctly , and assume the neck is straight when you checked this ? 

[Jeff Bolek]

I'm trying to determine how bad the neck is as far as needing a neck reset.  I am using the rule that when a straightedge is placed along the fretboard, it should just clear the top of the bridge.  On my guitar it does not clear the bridge but just barely hits it.  If I slide it over the top of the bridge there is a gap at the 12th fret.  I am trying to use this gap to determine if the guitar is bad enough to need to neck reset.  How big a gap is acceptable?  

[Mark McLean]

Jeff

Most discussions look at this measurement in a slightly different way, as you have probably seen.  The usual method is to place the straight edge along the plane of the frets and then see where this projects against the bridge.  Well illustrated in this tute from frets.com:

http://www.frets.com/FretsPages/Musician/Guitar/Setup/NeckAngle/nec...

If you did it by your proposed method you would get useful information, but different measurement.  If we look at one of Frank’s photos from that tutorial, of the bad neck angle.  It is bad because the ruler falls below the top of the bridge.  Lets say that the distance is 6mm below the top (that is 15/64” if you prefer to measure in archaic units)

By your method you would place the end of the ruler on top of the bridge and measure at the 12th fret how far it has been lifted off the fret.  That height will be exactly half of the distance measured at the bridge (3mm, and I don’t even know how you calculate that in imperial units, 15/128?).  Anyway, before I get distracted on a rant about the obvious superiority of metric measurements - my point is that seeing and measuring this phenomenon at the bridge will be easier because the magnitude of the effect is double.  But your method would work too.  

A few caveats - this methodology assumes a straight neck, and a standard bridge height.  Beware of the bridge that has been shaved thinner.  Good to also check the height of the strings over the soundboard (which should be something like 42/87ths of an inch- whoops, I think I am slipping into that rant again.......)

[Taffy Evans]

Hi Jeff, I do measurements with the strings at tension, and slack. This way I can see/record the amount of movement in the neck [bowing up] and in the top [bellying up] if any. I use dial indicators to measure the amount of deflection. This way I can better determine if it's the neck or top that needs the attention, or both.

Taff

[Jeff Bolek]

Sorry I'm so slow on this but....what is your best guess if the straightedge just barely hits the bridge.

[Taffy Evans]

Hi Jeff, for me, and it may not be correct, but what governs the need for a neck reset is the fact that the strings cannot be lowered enough to get the desired action height at the 12 fret when lowering the saddle, and whilst maintaining the correct, or suitable, back angle of the strings to the bridge pins. Assuming that the top is not out of shape, and at its correct radius.

If I aim for the straightedge to kiss the top of the bridge, then I find I can set up the guitar using the correct saddle hieght.

Taff

[Mark McLean]

If the straightedge lands just below the top of the bridge it is a fairly good sign that the neck angle is within acceptable limits.  That is as long as it is a normal height bridge, and not one that has been shaved down.  A well set up new guitar would have a bridge about 8mm high, and would have a saddle protruding about 5mm above that, which means that the strings are sitting about 13mm (half inch) above the soundboard just in front of the bridge.  If the neck angle is ideal and the straightedge is falling right at the top of the bridge, that 5mm saddle will be producing a string action at the 12th fret of 2.5mm which is about average “good action”.  As that guitar ages and the neck gradually pulls forward the action will rise, but can than be corrected by lowering the saddle height (up to a point).  It sounds like your guitar has progressed a little way down that path, but if there is still some decent saddle height left, and if the action is OK, it is probably not yet so far gone that it needs a reset.  Like Taffy and Len said, you know it is time for a reset when the saddle has been repeatedly lowered and has got so low that there isn’t much break angle left - but the action is still high.  At this time you will also notice that the strings are getting closer to the soundboard.  

[Jeff Bolek]

Thanks.  I have a couple of high end guitars and while I have performed work on some lesser guitars with no problem, I'm hesitant to do a neck reset on a really good one.  My rule is if I have not played the guitar in a while and I did work on it in the past, if I can't remember if I worked on it and can't tell by looking at it, that's the mark of a well done job.  So far that has been the case.  I would hate to screw up a Martin D41.