This applies to Yamaha FG’s (made 1966-1981) with the adjustment in the headstock, although you can apply the same technique if the truss rod nut is inside the body. The necks on vintage Yamaha’s are old and hard, many times you can’t just crank on the truss rod nut to adjust the neck relief, sometimes the wood won’t bend without a fight, and you will bury the nut into the neck (and run out of adjustment) or possibly strip or break the truss rod.
First I try to adjust the neck relief with the truss rod nut. I recommend removing it first and lubricating the threads with a light grease or oil. After reinstalling the nut and snugging it up, make 1/8” turn adjustments and check the neck relief. If it improves consider yourself lucky. If it gets tight with no movement in the neck, STOP before burying the nut into the neck or breaking the truss rod.
If the nut doesn’t seem to adjust the neck relief, I loosen the truss rod nut, and use a custom back bowing block (made from a 2×4), and a screw clamp (with a cork pad to protect the back of the neck) to back bow the neck 3/16”, then tighten the truss rod nut, and remove the clamp & block. The guitar must be tuned to pitch before starting. The block has slots in the face to allow it to be used without removing the strings. When the clamp is released the neck should stay just slightly back bowed and will require a slight release of the truss rod nut to get a little neck relief.
Tools required for truss rod adjustment:
1. A Yamaha truss rod wrench. I’ve made a drawing of the wrench and did a search but wasn’t able to find an exact match. I will look some more. The length is critical to be able to fit it in the pocket in the headstock. YAMAHA TRUSS ROD WRENCH PDF
2. A back bowing block. NECK BACK BOWING BLOCK PDF
3. A screw type bar clamp. (A squeeze clamp won’t have enough pressure.)
4. A small cork pad. (To protect the back of the neck.)
5. A steel 6” scale graduated in 32nds to check the back bow.
6. A 18″ or 24” straight edge to check the neck projection to the nut.
Here’s a picture of the block being used on my FG-300.
A few pictures of the back bowing block.
THE PROCEDURE:
If you haven’t already, remove the screws holding the truss rod nut cover to the headstock. This can sometimes be difficult because the 2 front screw can be directly under a string. You may have to loosen the string. If the screw is stripped in the wood, get something under the head of the screw to provide upward pressure while unscrewing.
Positioning the block is critical. Normally the block is placed at the first fret. Sometimes the bow in the neck is farther up the neck and requires positioning the block farther up the neck.
With the block in position (be sure none of the strings are pinched under the block), place a piece of cork on the fixed lower pad of the clamp and position the clamp halfway between the gap in the block (about the 4th fret). Be sure the block and clamp are central to the neck, otherwise the clamp can slip off the back of the neck.
LOOSEN the truss rod nut. If the neck is bowed forward, back bowing it will make the neck very slightly longer as it straightens. That will put pressure on the truss rod nut. Failure to loosen the nut could bury it into the neck, or worse, break the truss rod.
Tighten the clamp until there is a 3/16” back bow, measured at the front of the nut with the 6” scale and straight edge. There must be enough back bow, otherwise when you remove the clamp the neck will spring back to a forward bow. NOTE – A 12 string neck (such as a FG-230) is thicker and stiffer than a 6 string neck. I use 2 clamps (one on each side) on them and it’s still extremely difficult to back bow the neck. NOTE – If the guitar requires a neck reset it’s possible (before back bowing) the straight edge may only contact at the first and last frets. You may need to use a 12” straight edge to check the back bow.
Tighten the truss rod nut (just beyond snug) with the truss rod wrench.
Remove the clamp and block. Retune the guitar and verify the neck relief. If done properly, there should be a slight back bow, requiring a slight release of the truss rod nut to set the proper neck relief.