Spanish Heel and Truss rod

Construction and repair of Classical Guitar and related instruments
Alan Carruth
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Joined: Mon Dec 24, 2007 6:56 pm

Re: Spanish Heel and Truss rod

Post by Alan Carruth » Tue Sep 05, 2017 4:16 pm

I don't think you're likely to run into problems from wood expansion and contraction. The biggest issues I've seen have to do with the use of hardware, which can introduce stress risers. There are several different ways of doing this, some of which are less likely to be problematic. Ultimately, what counts is getting the details right. Try to work out a design that will 'fail well' if it does fail.

An example of what I mean by that is the use of what is sometimes called a 'barrel nut'. This is a section of steel rod with a threaded hole drilled through it sideways. This fits into a hole in the heel of the guitar, and bolts through the neck block pull it in. The hole for the nut is often drilled through the sides of a tenon on the neck, so that the nut is inserted crosswise, parallel to the plane of the fingerboard. One of my students found out the hard way why this is not the best setup.

The FAA has a rule that any hardware, such as one of those nuts or a bolt, that goes into wood has to be a tight fit in the hole, so that it is driven in. This insures that there is good contact between the wood and the metal all the way around, so that the stress on the fitting is taken as a shear load in the wood over a broad area. If the fitting is loose it puts all the stress on a line facing the bolt, which resolves itself into compression and a sideways splitting force along that line. Over tightening the bolts can actually split the wood along the axis of the nut.

We had done the nut on his guitar sideways in the tenon, as is not uncommon. When he installed the neck he over tightened the bolts. On lifting the guitar up he hit the headstock on a protruding beam in the ceiling. The heel split crosswise through the nut hole just below the level of the back surface of the neck, and the body dropped to the floor, causing some damage.

This is an example of something that failed badly. There were two main contributing factors. Aside from the 'wrong' oreintation of the nut, we had drilled the hole slightly over size. Here in the US we use Imperial threads most commonly, but these nuts are generally made of 10mm rod stock, rather than the 3/8" diameter that you'd expect. We did use the closest Imperial drill we could find, a letter 'X', but that is slightly over size.

Since then we have been drilling the holes for the nuts vertically in the heel, using a proper 10mm drill bit. This should insure the proper stress distribution. Also, the hole orientation means that a split in the heel will be vertical, and force the two halves out against the walls of the mortise. Since they will be captured the neck can't come off even if the part fails entirely. It's still possible to have acatastrophic failure if the bolts are tightened to the point where the nuts shear their way out of the tenon entiirely, but that's unlikely. The 'furniture bolts' we use have large round flat heads with a recess for a small Allen key, and I doubt you could put that much force on them.

I hope this is all clear. The point here was not to advocate for a particular system, or warn you off, but to show the need for proper though in design to minimize bad effects. I have seen successful examples of a number of such systems using different hardware, and other examples where things did not go well. The difference is all in the details, which have to be thought through carefully.

Amateur luthier
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Re: Spanish Heel and Truss rod

Post by MessyTendon » Thu Sep 07, 2017 4:28 pm

Here's another idea...I think it's pretty well, genius...Maybe it's not as precise or rock solid as using machined bolts and hardware..

But it doesn't get any simpler and cool than this...An ebony dowel, no glue. ... eset01.jpg

Alan Carruth
Posts: 2818
Joined: Mon Dec 24, 2007 6:56 pm

Re: Spanish Heel and Truss rod

Post by Alan Carruth » Thu Sep 07, 2017 5:22 pm

That's the system that Cumpiano and Natelson put in their book. They used two dowels, inserted from opposite sides. You need to make a special tool to get them out, which not every repairman will have. Also, if the pins are cut off short you might not see them looking in through the hole, leaving you wondering how the trick is accomplished. Finally, the force that pulls the neck mortise tightly together comes from the pin holes being slightly mis-aligned. If you re-set the neck by trimming the shoulder of the tenon you could lose that, and the neck will not be held in tightly. I don't know of anybody who uses that system any more.

BTW, I had to get a couple of those nuts at the hardware store yesterday; they're called 'cross dowel nuts', and are used in conjunction with 'furniture bolts'.

Rau Le Creuset
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Joined: Sat Jul 16, 2016 5:20 pm
Location: Toronto, Canada

Re: Spanish Heel and Truss rod

Post by Rau Le Creuset » Mon Sep 11, 2017 12:46 am

I wonder if laminate back and sides guitars distort as much as guitars with all solid wood.
La Patrie "Motif"
Martin 000-15m
Seagull Excursion Grand

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Location: St.Louis, MO- USA

Re: Spanish Heel and Truss rod

Post by radicicclassical » Wed Sep 13, 2017 3:21 am

Most of box distorting is caused because too thin sides are plugged into Spanish heel part of the neck. Even if the wedge is driven between sides and heel ("ala Romanillos"), the thin sides are still the thin sides! In long term, neck/body angle goes out of "wack" and level of the fret tops ends up projecting too low on bridge front. Making double sides in classical guitar with Spanish heel can make whole thing act differently, better all the way thru, much more rigid, more sustain and so on...Even though truss rod is not really needed in classical guitar necks, it may be applied and it lends a punch to a tone, all right, but also adds metallic zing to it. Question of taste. Yes, low profile truss rod will allow for a much thinner neck, some players like it and some don't.

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