The Dovetail

[Paul Cooper]

Most of the neck dovetail joints I have seen have a gap, or "pocket" at the top of the joint between the end of the tenon and the neck block.  Certainly, this is handy for shooting steam into when removing the neck.  But it has always seemed to me that this gap constitutes a source of joint failure:  The string tension is pulling the top edge of the tenon towards the guitar body, and pulling apart the glue joint at the upper walls of the dovetail joint.  On the reset I'm currently doing with a cool old Harmony archtop (it's the birdseye maple one that they only made in 1940 and 1941), I'm going to try putting shims up at the top ti fill the gap (as well as in the other usual places) to fill the gap, so that the joint can't collapse in that direction.  I will cut a vertical channel in the middle of the shim, so that a steam needle could still reach through into the lower pocket.  Any comments on this idea?  Anything I haven't considered before I try it?

[Mark Pollock]

First, the obvious point is, if it works, why mess with it? Have you ever seen a dovetail joint that has failed because there was a gap between the dovetail end and the neckblock? We steam off necks because their alignment is bad, and the reason we have to steam them off is because they are holding so well! When a dovetail joint DOES fail, it fails by having the heel pull OUT from the body, tilting the neck further forward, which no shim will help with. (There is something keeping the neck from crashing in toward the joint, and that would be the entire tongue of the fretboard, glued to the guitar top.)

Second, shimming the whole thing is completely unnecessary. The only part that you can shim that would do any good would be the part right against the fretboard, since the fulcrum action of the strings pulling the neck will tend to pull the rest of the joint away from the neckblock. However, introducing a third part here may have unintended consequences, such as creating a hump in the fingerboard, or fracturing the neckblock.

The real strength of the dovetail is the pressure formed between the shoulders of the neck against the outside of the guitar, being drawn in by the dovetail on the inside of the guitar. Adding a shim at the end of the neck just creates a lot of complication in carving, and also a lot more pressure trying to destroy the neckblock. Broken neckblocks are a horrible mess to deal with. I've got one on a Favilla baritone uke that I'm dealing with, and everything just wants to pull everything else back out of alignment.

Love the old Harmony stuff. Just pulled the neck off of a small Regal from the late 60's, and will pull the neck off a gorgeous (but unplayable, thus its pristine condition) Sovereign tomorrow. Solid maple sounds like a killer guitar.

[Frank Ford]

The outside shoulders of the neck joint press up tight against the sides, and thus against the neck block as well, so I seriously doubt that there's any real movement possible that would tend to loosen the top of the dovetail, even in the event of glue failure there.  Even an archtop has the cantilever support glued down over the top in that area on the inboard side of the dovetail, so that would also add support.

In short, I'm with Mark on this - I see no need or advantage.