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Guitar Anatomy Basics: Scarfed Joints

Fundamentally, two types of guitar neck construction exist; single and two-piece. In a single-piece neck the headstock is cut into the same piece of timber as that of the rest of the neck. In a two-piece neck, a separate headstock part is joined onto the longer part comprising the greater length of the neck using a scarfed joint.


Origins Of The Term

The term "scarfed joint" reaches back to traditional timber building and ship construction to denote a type of joint used to produce a long piece of timber where one single piece would not otherwise be possible. The joints themselves were often complex and varied, not glued and instead used a combination of friction, mechanical locking using wedges/pins, fasteners, gravity, etc.

The development of strong adhesives beyond simple protein glues changed joinery and produced a whole new book to describe them. The term "scarfed joint" was appropriated and used to denote both the a lengthening strategy for joints glued at an acute angle for increasing mating surface area for glueing and as a method for improving the cosmetics of joints that would otherwise require weak and aesthetically poor butt joints, such as veneer wrapped around a cylinder.

The term has again evolved in usage to describe how a two-piece neck is constructed for a guitar, derived solely from the increased mating surface areas they produce for glueing. In some ways it has become simplified over its original base meaning. So much so, the more descriptive and correct term "scarfed joint" has devolved into "scarf joint". Both are correct in the context of guitar neck construction through adoption (language and meaning evolves) however we'll stick to the more specific term "scarfed" for this article....we'll see why this is meaningful later....


Headstock Angles

Most people will be familiar with the difference in headstock angle of a Gibson Les Paul versus that of a Fender Stratocaster. Whilst Fender-style designs have a headstock that lays in the same flat plane as the rest of the neck (zero headstock angle), headstocks of Les Paul-style headstocks typically fall away at an angle anything from a few degrees up to 20°. A few terms exist such as "angled headstock", "tiltback headstock" however they all represent the exact same thing; that the plane of the headstock is at an angle with respect to the rest of the neck.

Angled headstocks offer benefits over flat headstocks, such as greater string pressure over the nut and the elimination of string trees. A side benefit is that they can also allow for easier access to the truss rod for adjustment.


Why Use A Scarfed Joint?

The main two reasons that scarfed joints are used is for the strength that they add to the finished workpiece and also one of economy. Many builders also incorporate aesthetic values to their scarfed joints as a secondary aspect, which in many ways also distracts from the true purpose of the joint being there in the first place.

Wood Strengths/Weaknesses

Wood is not an isotropic material; it is weaker parallel to the grain direction and splits along this easily. For necks whose length is more or less in line with the grain, (flat zero angle necks or very shallow angles) this is rarely a problem. The grain travels uninterrupted along the entire length from one end to the other. Headstocks angled against the direction of the grain build in an inherent weakness called short grain. This is simply the path of least resistance through the grain into and out of the wood, in this instance from the rear of the headstock a short distance through to the front. The greater the angle of the headstock, the shorter the grain distance and the weaker the headstock.

short grain.jpg
Areas of short grain (red) in a single-piece angled headstock against normal grain direction (green)


Short grain is one of the main reasons that Gibson guitars and basses are so prone to headstock breakage. Decades of adherence to their traditional building style maintains this weakness in spite of a simple solution having been around for far longer. The scarfed headstock joint.

By glueing a second angled piece of wood to the first to produce a scarfed joint, short grain is virtually eliminated



It should be fairly apparent that a headstock angled against the plane of the timber increases the minimum dimensions that the timber is required to have for a single-piece neck, increasing the waste factor. Several strategies are possible for turning this into a more economical venture by employing a simple scarfed joint. The following diagram shows how shorter material can be used economically by recovering material wastage underneath the neck and using it as a scarfed headstock:




In guitar terminology, a volute often refers to a strengthened area behind the union of the neck shaft and the headstock. These are useful for a number of reasons, not all of them related to scarfed joints. In this context however, they can be useful to lengthen grain when scarfing on a thicker headstock component that would otherwise invite short grain under the first two frets. Volutes also offer cosmetic options for hiding the join line; same as how a headplate can be added to the top of the headstock, a bent "backstrap" can be added to the rear face.

A thicker scarfed headstock can add short grain from under the headstock

The addition of a volute lengthens the area of short grain, adding vital strength


Types Of Scarfed Joint

As touched on earlier, the term "scarf joint" is less meaningful than "scarfed" since we're really discussing the idea of two-piece construction over single piece rather than any specific type of joint. There's no language police here, and both terms are just as correct through widespread usage and basic adoption. It is however useful to consider that there's lots of ways to achieve this end. The most common type of scarfed joint is that shown above; an acute face milled into the end of the neck, and a second flat piece of wood glued on.

Several common methods exist to create a scarfed joint, however they all have two common aspects; they increase the mating surface areas of the two parts to ensure a strong glue bond whilst eliminating as much short grain as possible.

The two most common types of scarfed neck joint; over and under the neck


Notable exceptions to the common approaches on scarfing a headstock joint are the V-joint employed by Martin and many classical guitar makers, however this is seldom found on electric guitars or steel-strung acoustics. Cosmetically, this style of joint can be accentuated into a "dart" or even a diamond-shaped volute.

v joint.jpg
Making a V-joint - https://www.youtube.com/watch?v=KocJHchKVZQ


....or the far more weird finger joint as employed by industrious inventor Bob Taylor:

finger joint.jpg
It might look strange, however as well-engineered as joints go this is near perfect in all but cosmetics


In Closing

Scarfed joints are an important building technique to increase the reliability and durability of guitar necks with angled headstocks, plus afford us opportunity to use raw materials more responsibly. However the joinery is approached, a well-planned and executed scarfed headstock produce an end product that can be both beautiful and superior to those made from a single piece of timber. They simply need to be applied to the problems that they are intended to be solving. Over in the tutorials section, we'll look at the different techniques used to produce basic scarfed joints plus jigs to simplify the various processes....


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User Feedback

It seems our beloved Aria Pro II basses are not made with scarfed joints. Do you think their lack of breakage, a la Gibson, is due to the thickness of their necks? Does the added mass in bass guitar necks help in preventing breakages? Then again, I assume there is more tension exerted on bass guitar necks.

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It's a combination of two things, perhaps three. Gibson headstocks with the traditional compression rods remove a huge chunk of wood for the rod adjustment nut, plus use a higher headstock angle of ~17deg which shortens the length of grain from headstock face to behind the nut. That makes them exceptionally weak compared to lower angles of 11-13deg (I forget what angle the SB-1000, etc. had). The best Aria Pro II necks came laminated, which means that each pieces that forms the neck has a slightly different growth ring alignment with respect to its neighbouring pieces, providing a less clean path through any short grain. They'll break, but the resistance to it is far higher than a single-piece neck.

Without exact numbers, I recall guitars as being something like 120lbs of tension and basses 160lbs? It's a far amount of difference, but I don't think there's a lot of correlation between basses being more prone than guitars to headstock breakages. At least, not from string tension as a factor. It tends to be more about shock from being dropped.

I would hazard that it's more about headstock angle. Basses don't seem to use anything nearly as high as Gibsons, and I would think that is where most of the difference lays rather than simple cross-section.

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