How To Tell If You Have A Single Or Duel Action Truss Rod

Discussion in 'Restoration & Repair' started by rfxcasey, Jul 23, 2019.

  1. rfxcasey

    rfxcasey New Member

    Jul 20, 2018
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    Hi, I just acquired and old electric guitar apparently in decent shape. This guitar had apparently been sitting unstrung for several years. First thing I wanted to do was check to see how level the frets are. I checked the neck for straightness and found it had quite a bit of back bow. I loosened the truss rod, counter clock wise, a little at a time letting the neck sit for a few minutes between adjustments.

    Eventually I reached a point where the truss rod adjusting screw loosened up. The neck still had back bow so for the heck of it I gave the adjusting screw an addition 1/4 turn counter clockwise at which point I checked the back bow again and it appeared to be reduced even further. I stopped at this point because I didn't want to damage anything.

    I looked truss rods up on line was reminded there are both single and duel action truss rods. I had assumed this guitar would have a single action truss rod but now I'm not so sure. Are single action rods still in common use today and if so how can I be certain of which type, single or duel, is in this particular neck before proceeding further?
  2. bruce bennett

    bruce bennett Active Member

    Apr 14, 2013
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    Chattanooga TN
    First what make and model of guitar are we speaking about , that alone can answer your original question.

    I've been telling people about the 4 main types of truss rods for a very long time and still its not yet common knowledge.
    and well, if I'm gonna answer this question, then it needs to be a full answer so that folks can find it and not keep repeating the same question.

    1. the basic steel reinforcement bar.
    this is found on lots of guitars, starting from the early 1920s up.... and it just about any priceing range as well.
    until Martin finally made the switch to an adjustable truss rod system in 1974. that pretty much was the end of the non adjustable truss rod.

    they are what they sound like, a simple steel bar (or aluminum as found in most early Valco guitars)
    in many unique cross sectional shapes inlaid into the neck under the fretboard. some have filler stripes on top while other have nothing.
    they do a fair job of keeping the neck straight.
    but if there is too much space between the bar, the underside of the fretboard... and the instrument is subjected to some extended period of time to a higher than room normal tempratures. the glue joint between the fretboard and neck can slip under the string pressure, which some people will call "thermal creep" of the glue and once the temp comes back down to normal, the neck has taken a new set to it. and the only way to get it back is to reverse that procedure. which is tricky at best. but manageable.
    they are decidedly not the best solution and pretty much have been abandoned for that 50 years or so.

    2. the single action adjustable Truss rod.

    Terry C Macinturff is the authority on Gibson and their invention of the adjustable truss rod in 1922 as well as the varient on that same patent in 1924.
    so I won't go into all that history here as Terry has written a whole mountin of literture covering it and its Sonic properties, over at TGP. a good read but still controverisal IMHO.

    a single action adjustable truss rod is a single steel rod threaded at 1 end and anchored at the other in either a "deeper than center of the neck" straight bottomed slot
    with a filler strip of wood over it and a thrust washer used to impart the Compression force of the adjusting nut to the wood so as to compress the wood of the neck below the centerline and force the neck into a backward bow thus countering the forward pull of the strings. pretty simply really.
    the Improvement was to make the bottom of the slot have a curve that is toward the back of the neck and the filler strip forces the steel rod into that smae curve which when the nut is adjusted will make the steel rod try to "straighten out" under the pressure, and basically brings the neck with it.
    the advantage is sometimes wood will compress in a direction of the weakest grain which may not be the direction the luthier wants it to go. have a pre curved rod will overcome the woods natural procilvity and give more consistent results during adjustment. again this is a simple idea that just works.
    the drawback is this, if the wood at either of the two pressure points, were to give way at anytime then the effectivness of the truss rod could become moot. also if the neck wood has a natural tendency to bow backward due to it grain structure.. then you end up wth a neck that can't be made to work at all without resorting to super heavy strings to provide enough pressure to bow the neck forward enough to come into the truss rods range of motion.

    as a sidenote many Japanese and Korean guitars of the mid 70-80s had a single steel rod anchored inside a .500 X.500 aluminum channel that acted very much the same way. thought IMHO it was not as effective as the gibson or fender rod systems.

    which brings us to....

    3. the dual action truss rod system.

    this type of truss rod system uses 2 steel rods or 1 steel and 1 aluminum (rare) or 1 round rod and 1 flat bar or even 2 flat bars or even 1 folded flat bar (Rickenbacker used this method)
    to create an adjustable system where one pair of ends of the rods were welded together. and the other pair of ends had either a welded collar onto the top rod, that went around the bottom threaded rod (which is always on the bottom of this type system) or a through holed, thrust block (again Rickenbacker) that took up the pressure of the adjusting nut on the threaded rod. and with the other ends being welded once the adjusting nut was tightened, the top rod would bow upward while the bottom rod remained fairly straight.
    this system exerts more pressure than a single action rod and is not dependant on the strength of the wood at the anchor points.
    in fact I have personally seen that this sytem can exert enough pressure to pop the fret board or or crack it.
    you can also install it with, or without a filler strip, (I prefer to use the filler strip to keep down any extranious noises/vibrations)

    Charvel and Jackson are the most noted users of this system and might even be the patent holder (not sure on that) in the mid 80s Ibanez quickly followed suit with its JEM and RG series guitars as this system also has the advantage of being able to be set into the neck at much shallower depths than a single action system, which for shredder or "wirzard" type thinner necks, was a perfect match.

    the drawbacks are: its heavier. (roughly another quarter to half a pound)
    it does leave more "open space" inside the neck around the rod system, which many (namely Terry C Macinturff) will argue that this negatively alters the resonant frequency of the neck. some say they can produce dead spots in the tone, I can happen and I have heard it. but it isn't consistant or common.
    and lastly, of course its not a great idea for amatuers to adjust it without knowing what to watch for, as it does have the potenital to seriously damage the neck and fretboard if overtightened. and being as they are most often found in very thin "shredder" necks.. that probablilty increases.

    4. The Dual action 2 way adjustable truss rod system. (please note that there is a HUGE difference in the terminology saying this wrong will cause a lot of confusion.)

    presently, this is the system of choice for most modern builders. and even has found its way into many new Chinese and Korean guitars.
    IMHO. I LIKE IT! & I USE IT! but to those that are pure traditonalists... they resist it mostly for the reasons listed in the dual action section.

    but seriously, this system has the best of both worlds and makes neck building very easy as it also can be installed with ot without a filler strip.
    it often comes with a truss rod "Condom" or plastic shrink tubing to keep down any noises or vibrations.
    it has the power required to adjust even the most stubbon neck woods and can overcome even the densest exotic woods like purpleheart or wenge.
    it s main claim to fame is the fact that it can adjust a neck that has bowed backward on its own due to grain structure by virtue of its using a "null point" style of adjustment. where you have a null point where the rod does nothing, then if you turn clockwise it will bow the neck backward as normal or if you turn it counterclockwise it will bow the neck forward with equal ease. this gives the luthier the control he has been longing for for decades.

    the system is not too complicated. but does differ from the dual action rod system

    you still have a pair of steels.. 1 round rod, usually 3/16" dia. ,
    the other is a steel flat bar that is 1/4" wide by 3/16 thick.
    the round rod will be threaded at BOTH ENDS. one end in standard right hand thread, the other end a left handed thread.
    the flat bar wil have two threaded collars welded onto each end with the right and left hand thread corisponding to the round bar.
    the adjusting nut or allen is welded to the round rod so that when you adjust the nut the entire rod turns and because the threadeing is opposite oriented.
    clockwise will give you an upward bow and counterclockwise will give a downward bow.

    the advantages of this system are obvious.
    the disavantages are negatable IMHO. but you will always have your pure tradtionalists that prefer the older more romanticized methods.

    Hope that helps
    John Nicholas likes this.
  3. larryguitar

    larryguitar Member

    Jul 16, 2013
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    To check if you have a 2-way truss rod, adjust the nut 'out' (what would be loosening on most rods) a half turn, and check the neck with a straight edge. If it shows decreased backbow, try another half turn and check again. If it continues to straighten (and you'll likely feel an increase in turning resistance), you've got a double-action truss rod.

  4. bruce bennett

    bruce bennett Active Member

    Apr 14, 2013
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    Chattanooga TN
    larryguitars answer is exactly why I posted that half Novel above him..
    His description is fine, but terminology is incorrect. and it leads to confusion.

    he's describing a D.A. 2 way, and calling it simply a "2 way" but at the end of his description, he calls it a "Dual Action"..
    they are NOT the same thing at all.
    and considering the power of both of those rod systems to seriously bow a neck, its rather important not to get those 2 confused.

    a Dual Action, CAN NOT correct back bow.. its also not quite as powerful as a D.A. 2 way.
    but a D.A 2 Way CAN correct back bow.
    BOTH systems use 2 rods in a stacked, or over/under configuration.
    it might seem trivial to many, but its IS an important distinction.

    here are some photos of the difference.

    this is a Rickenbacker Dual Action folded Truss rod system, note how the aluminum spacer block has no trheads in it. and it acts a a thrust block for the top section of the rod. while the bottom section, which does have threads.. simply passes through the thrust block and the adjusting nut pulls the rod toward the nut, and the thrust block pushes backward on the upper section of rod which bows the upper rod section upward. the bottom rod will usually stays fairly straight while the top rod bows upward.
    This system has no way to bow downward at all. it was popular in the late 70s,80s and early 90s but began to fall out of favor in the late 90s. when the D.A. 2 Ways began to come onto the scene.


    this is the D.A.2 Way and you can see the difference really quickly.
    this happens to be upside down in this photo.
    the adjustment nut should be on the bottom just like the D.A. rod above.
    but you can see that both ends of the rod are threaded.


    Here are the 3 basic adjustments that a D.A. 2 way can make. from top to bottom is Neutral, Clockwise, and Counterclockwise.
    its not wise to turn these type of rods more than 1 full turn in either direction, and that is a Maximum.
    you will likely hear some cracking sounds before you reach 3/4" of a turn. these dual rod systems were made to help overcome the exotic woods that Custom luthiers were playing around with in the 90s. I know this because I was one of those many various luthiers, and actually helped get these rod systems into they're final forms by requesting all manner of design changes to the early prototypes.. now the world has a vastly improved Truss rod system that is cheap, extremely powerful, and darn reliable.


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