The Double Tenon Neck Joint

The purpose of this article is to explain how to attach the neck of the guitar to the body without any glue at all. I believe this neck joint was first conceived by Dana Bourgeois (hat's off to him!). However, as far as I know, no source gave the reader any specifics or measurements on the method. I though it would be a good idea to share this information with other Luthiers who would like to try the joint and, perhaps, incorporate it into their own instruments. Why would you want a neck joint that contained no glue? Simply put, it allows for extremely easy adjustment of the playing action at any age of the guitar. All instruments, at some time, require a neck reset. This joint allows that to be done in a matter of minutes rather than hours. The joint also eliminates the dreaded popsicle brace and replaces it with a piece of wood that is wider than the fretboard, thereby eliminating the possibility of cracks developing along the fingerboard line into the top (as seen on most instruments over twenty years old). How does it sound? I could detect no audible difference in the way my instruments sound with the double tenon joint as compared to a straight bolt on with a glued fretboard. I see no downside to using this joinery method. I have adopted it for all of my instruments. Enough of the history and reasons for the joint. Let's talk about how to do it.

This is a photo of the completed joint. Let's go through the specifics:

When building the top, glue a piece (I use spruce but mahogany would suffice) 2 ¾” wide and .425” deep (and with a truss rod slot routed in the piece if your rod is adjusted through the soundhole) from the horizontal brace to the neck block on the centerline of the top. Once glued to the top drill from the bracing side a locating hole to locate the position of the back (towards the neck) of the horizontal top brace from the top because when the top is glued to the body you can no longer see the horizontal brace's location. This is important because it defines the end of the rout for the neck.

After binding the body and a general cleanup, do a rough set of the neck. Insure the neck is centered on the top. Once the neck is right scribe a line around the outside of the fretboard. From your previous mark, locate the horizontal brace. This locating hole is the end of the rout into the top (soundhole side).

Now take the 1.85" template (shown below) and center it on the fretboard outline. The distance is to the back of the horizontal brace.

Trace its’ location on the top.  Line up the 1.85” jig (this piece is the exact size of the jig opening) on the lines and clamp in place.

Using a top bearing flush cutting router bit, rout the mortise in the body 1.85” wide. .290-.300 deep. Set the body aside.

Cut a piece of the tenon material (1/4” baltic birch plywood - 1.75" wide) so that one end buts up against the neck tenon. It should slide under the truss rod if your truss rod adjusts from within the soundhole as mine do. To determine the fit of the tenon, measure the length against the mortised body. Cut to length. Test fit to the body. Locate 2 places where the inserts will go as close to the soundhole end as possible. Make sure that you clear the mahogany neck block. Drill ¼” holes for the 8/32 inserts. Install the inserts. Sand the inserts flush with the 1/4" plywood.

Double stick tape the tenon to the fretboard. Check the fit in the body mortise. Make sure everything fits! If so, scribe a line around the tenon on the fretboard. Remove the tenon and double stick tape and glue the tenon on the fretboard using the lines as your pattern.

When you initially obtain the bolts that hold the tenon to the body, take two of the bolts, cut the heads off and grind a point on them. These become your locator pins to determine where to drill the holes in the body.

When the tenon is dry, take the pointed bolts and screw them point up into the inserts in the tenon. Fit the neck pressing hard on the area with the pointed bolts. This will give you the location of the holes in the body brace. Drill the holes in the body brace with a 3/16” bit (oversized). Although the photos show 4 bolts holding the fingerboard tenon I have now determined that only two are necessary - the two closest to the soundhole (01/07).

To install the neck, first attach using the two bolts that hold the neck to the body. Then, using 5/8” long 8/32 bolts and washers and lock washers, attach the fingerboard. Snug down tight but do not overtighten! If, for some reason, the fretboard is not flat to the top, undercut the bottom of the fretboard just like you would undercut the neck joint to get a good fit. Remember, only the outside of the fretboard will be actually touching the top and will be seen.

Lastly, here is a photo of a completed neck joint and all of the hardware that I use. The neck joint itself (which we have not talked about) uses 1/4 x 20 steel inserts and 1/4 x 20 bolts. I cannot tell you the length of the bolts for your application because it would vary depending on how you cut your neck tenon. All of this hardware for the double tenon neck joint is available either at your local Home Depot, Lowes or other supply store.