Question about truss rod on Yamaha acoustic guitar.
 

Hello,

I have a Yamaha acoustic that I'm working on and I've run into something I have a question about. I set up my own instruments but haven't had much experience with acoustic guitars. I am currently trying to figure out this guitars truss rod and am 75% sure I know what's happening but would like to be 100% before proceeding. With the strings off of the guitar, I turned the truss counter-clockwise in order to totally loosen it. I felt it get loose but then noticed resistance as I turned it further to the left. There seems to be a point in the middle of this where it's loose and then turning in either direction has resistance. I am 100% sure that turning it clockwise makes the neck more convex so as to remove relief from the neck when strung up. I am not so sure about turning it counter-clockwise. Is this counter-clockwise resistance the truss rod "maxing out" after totally loose or is this actually making the neck concave in adjustment? In other words, how do I know for sure this is a single action truss rod? Once I know this, I can confidently work to get the neck to flat before starting a fret dress.

It sounds like you have a double-action truss rod, which allows for tension in two directions - convex and concave realtive to the fingerboard. Counter-clockwise puts more relief into the neck beyond what string tension provides. You shouldn't be able to turn a single-action the truss rod past its "resting" untensioned point.

I think fjeff has it

I bought a FX800 for my grandson for Christmas.

When I went to set it up, I had to remove some bow in the neck

So naturally you'll want to tighten the TR a bit, which only made it worse

That's when the light went on that it was probably NOT a standard truss rod

I set it in the mid-point, and the neck went straight as a rail and has not moved since.

Figured it out.
 

I took a fret slotted straight edge and checked the neck for straightness. There was a back bow in it so I turned the truss rod counterclockwise and it took out the back bow until it was straight.

It usually should not be straight but should have a slight dip in the middle of a few thou, measured when tuned to pitch. If it's not buzzing anywhere though, you should be good. Look up neck relief.

It’s for a fret dress not playing action.
 

I was setting it straight in order to do a fret dress which worked out great.

a thought
 

Double-acting truss rods are a current 'standard' configuration, not unusual at all and not an outlier. Tens of thousands of guitars are made with these truss rods every year.