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Is there a rule of thumb for deciding whether the guitar needs a neck reset or just a truss rod adjustment? many thanks

A neck reset is totally different than a truss rod adjustment.

The truss rod only controls the bend in a neck. It is adjustable because atmospheric changes and string tension will act on the neck, the truss rod will let you adjust the bow or lack of it. It only acts on the neck between the 2nd fret and the 12-14th frets.

A neck reset is needed because the actual joint has moved or the soundboard has sunk down. The tension on the neck from the strings wants to pull the neck up and in turn it will suck the soundboard down. Think of the tension on a guitar from the strings as wanting to pull everything into the sound hole.

You can tell if a neck needs to be reset by placing a straight edge on the fretboard (on edge parallel to the strings), slide it down toward the bridge and the edge of the straight edge should just clear the bridge. If it hits the bridge there is a very good chance you need a neck reset.

However... Humidity can effect a guitar and make it appear to need a neck reset.

Bob

It's not that the soundboard has sunk down it has bellied out much more then normal. This makes the action extremely high and unplayable. A neck reset removes just enough wood from the necks dovetail or tenon area to angle the neck back enough to compensate for the bellied top. The first thing that many do is remove some material from the bridge saddle lowering the action. this may work for a while but eventually a reset is necessary since their is only so much saddle that you can remove before you start running into other issue like low break angle over the bridge. To low a break angle effects the guitars sound overall....Mike

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@Shaw,

I should have explained my statement better than I did.

Yes you are correct about the belly of the soundboard. However what I was actually referring to was in front of the bridge. That area will be sunk in and the belly will be larger. Most guitars have a belly that is introduced by string tension and they can occur early in a guitars life, that is a normal thing and is sometimes difficult to tell what is "normal" for that specific guitar. But a concave area around the sound hole and in front of the bridge is not normal and a sign that something could be wrong.

But the best way to tell is to use a straight edge on the fretboard to see where the bridge is.

@enricopg, Sorry for the confusing answer, I hope that clarified it.

Thanks Russel, yes I think I understand. Anyway, the first thing to do is make sure the fretoboard is completely straight by adjusting the truss rod, right? Then you can try to evaluate if a guitar needs neck reset. Is that so? Otherwise, how could you please a straightedge on top of the fingerboard if it had an upbow because of the truss rod not being optimally adjusted?

It isn't really necessary to adjust the truss rod before but it can't hurt. A neck bow is measured in the 1/100ths of and inch. The measurement at the bridge would be measured in 1/16ths of an inch.

A note about a sunken soundboard and a belly. I have had to do neck resets on guitars that do not exhibit those symptoms so that is not a true test if you need a reset. The straight edge is the best way to measure it. Also make sure you have your guitar strung up and in tune before checking either the neck or the bridge.

Hope this helps.

Bob