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Truss Rod question
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Author:  Dharper [ Sun May 21, 2017 9:07 pm ]
Post subject:  Truss Rod question

Hello all, I am new to the forums, I am also new to guitar building! I recently bought a Martin Mahogany Dreadnought kit. So far everything is going well and I am about to put the back on and close up the body. Before I do that I have been looking ahead in the process. This is where my question comes in.

I am using both the instructions that came with the kit and I also bought an online manual from When I compare the truss rod in both of these instruction manuals and the one that came with the kit they are not the same. So, before I close her up I would like to get some opinions from some veteran guitar makers. Here's a picture:
truss rod.JPG

The picture in the upper left is the rod that came with the kit. The other two pictures are pictures from the manuals. The manuals show a rod that extends well beyond the dovetail of the neck. The one that came with the kit is flush with the dovetail. Now, I will admit, I am building a kit guitar and I know NOTHING about truss rods. I don't know what they do or how they work. I kind of pictured them sucking the neck into the body by tightening the alen key. But now that I see mine I am at a total loss on its purpose. So I guess my question is is the rod I have the correct setup? Am I missing any parts that extend it beyond the dovetail? Or should it be flush like it is? Also, if anyone would like to give a brief truss rod 101 I would greatly appreciate it. Thank you so much!

Author:  Hesh [ Mon May 22, 2017 5:05 am ]
Post subject:  Re: Truss Rod question

Flush is fine. The purpose of a truss rod is to induce back bow (more on this in a minute...) to counter string tension inducing forward bow so that the instrument can be set-up with a tad of relief (forward bow) on the treble side and hopefully.... a tad more relief on the bass side.

Some modern truss rods are often "two way" rods that can also induce forward bow if desired. When an acoustic (or electric for that matter) guitar is exposed to very low humidity for a period of time the dissimilar woods of the fret board and neck combined with string tension and maybe even compression from fret tangs may.... create a perfect storm and may cause permanent back bow. This can render an instrument unplayable and impossible to set-up without refretting and manually leveling the fret board.

Modern truss rods that are double action can counter this with a twist or two and can save the day, literally..... by inducing forward bow. I know you didn't ask about double action rods but I get carried away...;)

Anyway flush is fine just be sure that you have a hole in your upper transverse brace that fits the truss rod wrench that will be necessary to reach the adjuster. It's not uncommon for some rods to protrude beyond the neck block and it's also not uncommon for the adjuster to be flush with the neck block.

A good idea too is to clamp the truss rod in a vice and work it a bit with the wrench off the guitar being sure that the adjuster is properly fixed to the rod. Truss rods do break with the adjuster welds failing and this can be a nightmare if it happens after the guitar is built.

Welcome to the forum and building, sounds like you are doing a great job!

Author:  Dharper [ Mon May 22, 2017 11:03 am ]
Post subject:  Re: Truss Rod question

Very nice response hesh, I appreciate you taking the time to explain things a little better. I read about the "two way rods" that you mentioned. I wonder if that's what mine is? When you turn the alen bolt there's this kind of "neutral part" where the nut is loose and somewhat free. From that point you can tighten it either clockwise or counterclockwise. Two way right? [:Y:]

I just got confused when the pictures in the manual were different from the kit. I mean, it's obvious as heck that the truss rod that i got is correct. It fits like a charm inside the precut groove on the neck. I'll probably have to do more research once I get to that point in the build.
Btw the alen wrench can reach the bolt once the neck is attached. I already tried that out. I guess that means I can close her up soon!

Author:  Freeman [ Mon May 22, 2017 11:51 am ]
Post subject:  Re: Truss Rod question

Devery, first welcome to OLF and guitar building. Second, there are a number of different truss rod configuration and Martin has used at least two over the years (actually more if you go back in time). The truss rod is there to strengthen the neck and give you some control over its shape as the strings try to pull it into an bowed arch.

Martin used single acting rods for quite a few years - as you tighten the nut they bend in one direction which is intended to straighten the concave shape caused by the string tension. Most single acting rods end up with their adjuster very close to the edge of the soundhole. Here is a Martin single acting rod (there are other permutations) ... s_Rod.html

The plans on the right side of your post are definitely a single acting rod

Double acting rods bend one way when you tighten them (CW) and the other way when you turn the nut CCW. They go thru a neutral position where they are dead straight. Usually they consist of two rods or a rod and a metal bar with little threaded pieces welded to the ends. One end is thread normally, the other backwards (like a turn buckle) so as you turn the adjuster the ends move towards or away from each other. They are almost always installed with the threaded rod DOWN into the neck - again the idea is that tightening the rod tries to remove the concave shape from the string tension. Most double acting rods end up with the adjuster way up inside the guitar body, often flush or just slightly sticking out of the neck block. Here is one type of double acting rod - notice the picture of where the end is relative to the rest of the guitar ... s_Rod.html

Depending on the rod there may or may not be some sort of filler between it and the fretboard. Unfortunately Martin kit instructions are not noted for being the best and you are wise to use others, however remember they may not be exactly the same as your parts. You might want to download StewMac's (free) instructions for their kit - again, the parts might be slightly different but it will give you another idea of how to do it ... tions.html

Author:  Hesh [ Mon May 22, 2017 1:06 pm ]
Post subject:  Re: Truss Rod question

Hi Devery. We are happy to help.

Your truss rod is two way if when you turn it counter clockwise and observe the rod it bends slightly upwards at the ends with the low part being the middle when in the position that it would be in a guitar on it's back. Conversely when turned clockwise in the same orientation if the middle of the rod is higher than the two ends it's double action.

Or, in other words clockwise bows the neck back and counter clockwise bows the neck forward. You should be able to see this happening in real time.

Many truss rods will turn counter clockwise but the adjustor is backing out of the rod and no change in the rod is observed. This is one example of a one way rod.

That loose area in the middle that you rightly observed can be a source of rattles in a completed instrument and it's a good practice if the rod ends up not needing any adjustment at all once the strings are on and tuned to pitch to just barely snug the rod in either direction to prevent rattles.

If your truss rod is a Martin two way rod that's one of the best truss rods IMO available.

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