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Truss rod wrenches

Q: Hello Steve.

Could you please tell me what size allen wrench it takes to adjust the nut on a Collings guitar.
Thanks, Josh 

A: The wrench you seek is a 5/32 Hex.  Larrivee includes, with their guitars, a 5/32 wrench with a fancy bend that allows you to adjust the guitar with the string tension still on.  If you can find one of those, great.  Otherwise you are going to need to loosen the strings enough to get your hand in there, adjust, retune, repeat until perfect.

There are two basic styles of truss rods.  The first truss rods used, invented by Gibson, were a single rod of cold rolled steel pushed into a curved slot by a curved maple or mahogany filler.  The rod curved so that it was closest to the back of the neck at around the 7th fret.  The shortest distance between two points is a straight line.  As you would tighten the rod it would straighten out, pulling the bow out of the neck.  Your Collings is of this type.  As are: Mossman, Gibson, Larrivee, Thompson, Taylor, Santa Cruz, Fender (electrics), and Guild.

The second type is the "over/under rod."  As you tighten the bottom rod the top rod bows up, bending the neck back.  An early user of the over/under rod was Harmony.  The adjustment was at the peghead.  The other end was held together by a spot weld.  The spot welds were notoriously weak and once they broke you had a cheap guitar needing an expensive new truss rod.  A tip for collectors: most Harmony truss rods are already broken or are about to break.  Always remove tension, and always sacrifice to your favorite god before adjusting.

A subset of the over/under rod is the 2 way rod.  The bottom rod can be shortened or lengthened.  Guitars have a lot of string tension on them (180lbs with medium strings).  The necks bow forward and the truss rod pulls them back.  Banjos have much less tension on them and, once in a while, they will warp backwards.  A double action rod can pull out a back bow as well as a bow.

The most common truss rod is another modification of the over/under rod.  The top rod is a U shaped piece of channel iron (or aluminum or sometimes carbon fiber).  The bottom rod is inside the channel.  The channel iron stiffens the neck and discourages twisting.  Martin, and all of the import guitars use this rod.  The single rod requires some fancy router work under the fingerboard.  The curve must be optimized for your neck shape and the filler must be machined to fit the slot perfectly.  With the over/under rod you just cut a channel and drop it in.

Truss rods are generally cold rolled steel.  The truss rod nuts are generally brass.  The idea is that if something is going to strip it should be the nut.  You can just back off and replace a stripped nut.  A stripped truss rod would require major surgery.  Modern truss rods all seem to work.  Broken rods are no longer common.  Especially on major brands, you can tighten and loosen your rod with impunity.  But, feel for coming to the end of the threads.  Sometimes a rod will not be tapped far enough to do the required pulling.  I f you feel the nut stop turning, don't force it and strip the nut.  Back the nut off and stack on a few washers (available at your local hardware store).

Sight down the edge of the fingerboard.  Use the straightness of the strings as a gauge.  The neck should be perfectly straight from the nut to the body.  If the fingerboard tongue (the part on the body) doesn't line up, there is nothing you can do about it with a truss rod.  That is the province of a neck set.  A truss rod is anchored under the 13th fret on a steel string guitar.  It can only effect a neck between the nut and 13th fret.

When you are dealing with very light guitar strings, you might need some relief in the finger board.  If you pluck the string 5" from the bridge there is a reflection 5" from the nut.  When you fret the third fret there is a reflection 5" from the third fret.  If the fingerboard is concave there is always a little extra clearance 5" ahead of where you are.  With medium strings you don't need relief.  If you have some high frets and your saddle is cut too low you can effectively raise the action by loosening the truss rod.  The better solution is to get your frets filed into line and your saddle set properly.

If your saddle is too high, and/or your neck set is too high you can "S curve" your neck.  By overtightening the truss rod you can set the neck so that it plays fine in the first three of four frets and plays terrible from the 7th fret on down.   People who never play above the 4th fret can sometimes save themselves the cost of a neck reset with an S curve. 

While we're at it: The Pre-fire Mossmans and the Santa Cruz adjust with a 3/8" nut driver. The Post-fire Mossmans use a #10 box end wrench cut short enough to turn in the space allowed.  The Taylors and Guilds and new Gibsons use a 1/4" nut driver.  The Old Gibsons use a 5/16" nut driver.  Huss and Dalton use a 1/8" allen wrench.  Martin and all the imports use a 5mm allen wrench.  Good luck.

Steve Mason

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