Liutaio Mottola Stringed Instrument Design



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Bolt-on Neck Joint for Acoustic Instruments

One of the nicest neck joints ever devised is the one used in the previous generation of Taylor guitars. It is simple to implement, inexpensive, strong, and the neck can easily be removed for repair. Resetting of the neck is extremely simple with this joint. And years of use by many guitar makers small and large have proved its positive acoustic properties as well. Although used by many established guitar makers this joint is ideal for neophyte builders as it is probably the simplest neck joint to implement. The inside surface of the neck heel simply butts against the body. There are no tenons, mortises, splines or dovetails. This joint uses threaded inserts installed in the neck heel and cap screws with washers to attach the neck to the neck block.

Last updated: Sunday, June 01, 2014

Dimensional and Access Limitations

The size of the hardware components of this joint place a lower limit on the size of instrument for which it can be used. As a general rule the neck heel must be at least 7/8" deep, 2" high and 1 1/4" wide to use the hardware specified. Also please note that the instrument must provide access to the screws. This is generally guaranteed for guitars with round holes and typical rib depth, but access may be an issue for shallow instruments and/or f-hole instruments. Some folks that make f-hole instruments do use this joint, and they build themselves special long Allen keys that can be slipped through the end pin hole to access the bolts.

Parts Needed

Individual parts can be purchased from McMaster Carr and other hardware vendors. For each joint you'll need:

    (2) Self-tapping steel threaded inserts for metal, 1/4”-20 internal thread, 3/8” external thread, 31/64" length (part# 90240A001, package of 10). No substitutions - see note about the inserts below;

    (2) Stainless Belleville washers, 1/4" screw size (part# 91235A112, package of 10);

    (2) Alloy steel socket head cap screw, 1/4”-20, 1 1/2" long (part# 91251A546 for package of 50). See note about bolts below;

To install the inserts and the bolts, you'll also need:

    (1) 1/4”-20 hex nut;

    (1) 3/16” Allen hex key;

    (1) Socket wrench with long socket to fit the hex bolt;

    A drill press is absolutely needed to drill pilot holes for threaded inserts;

A note about the threaded inserts. There are many different types of threaded inserts. I have tested the strength of various threaded inserts, and the results of those tests can be found in my American Lutherie article entitled Testing Threaded Inserts1. The ones specified above are self tapping thread cutting inserts intended for use with metal. When driven into the end grain of the neck heel they will thread their way in, removing wood as they go, resulting in a strong and permanent insert installation. Other types of inserts will work fine in mahogany, and lots of fine luthiers use other types of inserts with excellent results. But the inserts specified here provide excellent holding power (the best in my tests), will work in wood harder than mahogany such as maple, will work in laminated necks with very hard center laminates, and are extremely easy to install. For these reasons I strongly recommend that you do not substitute any other inserts here. The instructions on this page are specifically for these inserts, and may not work right if you substitute some other type of insert.

That the inserts specified had the best holding power in my tests is a fact, but I can only speculate as to why these inserts came out on top. Inserts made for wood typically are designed for use with MDF or particleboard, or to be inserted perpendicular to the grain in soft or moderately hard wood. They are not self-tapping and they do not remove material as they are threaded in, but rather compress the wood around the threads. This compression probably tends to break away the wood around the threads when they are threaded into the end grain of hardwood. And when used in the end grain of very hard wood like maple these types of insert will simply strip out the pilot hole, resulting in a weakly installed insert that is almost guaranteed to pull out once the instrument is strung up.

The bolts listed above are the right length to use for a 1" deep neck block. Other lengths may be used for thicker blocks, or if the face of the block has a channel routed into it so that the heads of the bolts sit below the surface of the block. Also note that some builders use less expensive hex head bolts here, but doing so requires that you be able to get a socket wrench into the instrument to install and remove the bolts. This is a look at the neck block and neck attachment bolts of a great sounding Seagull guitar:

Hex head neck attachment bolts

You can see the hex head bolts and plain washers used to attach the neck. Note that the neck block has a wide slot to accommodate the heads of the bolts. When this guitar was new a label covered the block so you couldn't see the bolts.

Installation

The first step is installing the threaded inserts into the neck heel. Generally two bolts are sufficient for this joint, so two inserts will be used. Be sure to position the inserts so that there will be enough room to get at the bolt heads from inside the guitar. The inserts will be driven into drilled holes and the top surfaces of the inserts will end up about 1/8" below the surface of the heel that will mate with the body of the instrument. This is done to leave enough wood for final fitting and to be sure there is enough wood available for removal during a neck reset. There are basically three ways of making sure the tops of the inserts will end up below the mating surface of the heel when using the inserts specified above:

1. Drill the holes for the inserts 1/8" deeper than is needed for the inserts and drive the inserts so that their top surfaces are 1/8" below the mating surface of the heel. To do this, some sort of spacer will be needed when driving the inserts. I use another insert for this – its use will be explained in a bit. This option is the easiest to do;

Insert driven below surface of heel

2. Make a 3/4" x 1/8" deep counter bore for each insert, before drilling the holes for the inserts. The counter bores will provide clearance for the socket used to drive in the inserts, and this will allow them to be driven below the surface of the mating surface of the heel;

Counterbores

3. Route a 3/4" x 1/8" deep pocket in the center of the mating surface of the heel. This serves the same purpose as the counter bores described above. This is probably the hardest way to do this, but it has the advantage of thinning the mating surface, which makes it easier to fit the neck to the instrument body and to do a neck reset if it becomes necessary in the future. This is the way I generally do it;

Routed pocket

In all cases 11/32" pilot holes are drilled to accept the inserts. The holes should be an accurate 3/4" deep, measured from the mating surface of the heel to the tip of the drill bit. Note that if the counter bore or routed pocket method of sinking the inserts are used, these operations can be done either before or after the holes are drilled. If they are done before, then the depth of the holes from the bottoms of the counter bores or pocket will be 5/8". It is easiest to do the drilling while the neck heel is uncarved and still “square” and before the fingerboard is glued on. Drilling must be done using the drill press to be sure the holes are straight and clean and the depth is correct.

Drilling pilot holes

After the pilot holes are drilled the threaded inserts are installed.

Ready to install inserts

Thread the hex nut onto one of the cap screws so that about 7/16" of thread is showing below the nut. Thread the screw into the insert until the nut is touching the top surface of the insert. Then position the insert squarely over the pilot hole and drive it in using a socket wrench with a long socket.

Installing inserts

Be sure the insert goes in straight! Stop when the top surface of the insert is flush with the floor of the counter bore or routed pocket, if using one of those sinking methods. Do not drive the insert right down to the bottom of the pilot hole - doing so risks stripping the wood around the insert and weakening the joint!

If you are not using counter bores or a routed pocket the inserts will have to be driven below the surface of their pilot holes. To do this you'll have to thread two inserts onto the bolt for driving. The bottom insert will be threaded into the wood, and the upper one serves as a spacer that allows the lower one to be driven deeper than the surface of the heel. The setup looks like this:

Using a second insert as a spacer

After the insert is driven the nut and bolt can be backed out, leaving the insert behind.

Holes will also have to be drilled through the instrument body and the neck block for the attachment screws. In a non-production environment you can simply measure down from the fingerboard mating surface of the neck (or the underside of the fingerboard if it is now attached) to the center of the top insert, and then again down from that insert to the center of the lower one, and then transfer those measurements to the instrument body. The holes should be fairly large (I generally use 5/16" diameter holes) to allow for some positioning adjustment when attaching the neck. If the holes are drilled before the back is put on the instrument it is easier to back up the neck block with a piece of scrap to keep the exits through the front surface of the neck block clean.

Attaching the neck is a simple matter. The neck is fitted to the instrument body after sanding out and before finishing (it is also possible to finish body and neck separately and attach the neck after both are buffed out). If the mating surface of the heel was flattened before the inserts were installed and it is still flat, it is generally easiest to flatten the mating surface of the body for fitting. But if there is too much wood to take away that way then the heel will have to be worked instead. It is easy to do this by the sandpaper pull method – putting a piece of 60 grit sandpaper between the heel and the instrument body, holding them together with some pressure, and then pulling the sandpaper out. A few passes will mate the surfaces. You can also slightly undercut the mating surface of the heel (except at the edges) so that when the bolts are tightened those edges will “knife” into the body.

Although I don't want to go into the details on this page (because it varies a bit for each type of instrument) fitting the neck to the body also involves centering the fingerboard centerline with that of the instrument top, and making adjustments to the neck angle so string action is in the ball park.

After fitting, the neck is bolted on using the cap bolts and Belleville washers. Snug up the bolts just enough so that you can still make final alignment adjustments. When you are confident everything fits OK the neck is unbolted and a dab of glue is put on the underside of the fingerboard. The neck is then re-bolted, again just snug, and then re-positioned. The fingerboard is clamped for gluing to the top in the usual way. The neck bolts can be tightened after the fingerboard clamps have been tightened. Do not over tighten the bolts, or you risk stripping the inserts out. Remember, they are threaded into end grain. One reason I like the Allen head cap screws here is that, if you insert the long end of an Allen key into the screw it is difficult to get enough leverage on the short end of the key to over tighten the screw. Do not attempt to make up any gaps between body and heel by cranking down on the screws. If the heel and body don't fit right, fix it.

If you just don't know your own strength when tightening the screws, you may consider a modification to the technique described above. Before drilling the holes for the inserts, drill a long hole down through the heel from the top (fingerboard) surface, near the surface that will contact the instrument body, and glue a hardwood dowel into it. The dowel should be of a diameter and so positioned so that the holes for the inserts go through the dowel. Now, the inserts will thread not just into the end grain of the neck, but also into the long grain of the dowel.

If you do manage to yank out an insert, the repair is easy. Remove the insert, drill a larger diameter hole, and glue in a short length of dowel or, better still, a wood plug. Then re-drill and re-insert the threaded insert.

Attaching the Fingerboard Extension

The instructions above have the fingerboard glued down to the top in the traditional manner. But there are a couple of options for dealing with the extension other than gluing it down. These make for a completely removable neck, which has a number of advantages. One possible construction is to allow the fingerboard extension to “float” above the top. This option works very well in cases where the extension is very short. If the extension is long it may look a bit strange floating way above the top at the end. This is of course a matter of aesthetics only. Mike Doolin uses this approach for his double cutaway guitars. You'll probably need to stiffen the fingerboard extension if you use this method, either by continuing reinforcement rods under it as Mike does, or by extending the neck shaft (or some part of it) under the extension.

Another approach is to bolt the fingerboard extension down. This is the approach Taylor uses in their new neck joint. This is also the approach I use in my latest instruments. On my instruments a drilled and threaded boss is glued to the underside of the neck extension.

The extension is screwed down to a tab on the first transverse brace using a short screw.

Neck Reseting

There is no simpler neck reset than when this style of bolt-on neck is used. If the instrument is lacquered, a knife is used to cut through the finish at the neck/body joint. The neck bolts are removed but the fingerboard is left glued (or otherwise attached) to the top. The neck is bent forward enough to allow a sheet of 60 grit sandpaper to be slipped between the heel and the body, sandy side toward the heel. The neck is levered back while the sandpaper is pulled out of the joint.

Neck reseting using the sandpaper pull method

This is repeated until enough material is taken off the heel to effect the new neck angle. For information on just how much material needs to be taken off here when doing this style of neck reset see the page on calculating heel displacement for a neck reset on this site. After the heel is "trimmed" the neck bolts are then re-installed and tightened.




1. Mottola, R.M. “Testing Threaded Inserts”
American Lutherie #101, 2010, p. 54.