Dowel Stick Fail


One of the most common problems that I encounter with the antique banjos is that the dowel stick-to-heel joint glue had dried out and come loose.  The allows the neck to angle upwards too much, resulting in unplayably high action (1/2" or more at the 12th fret).  To fix this, you must remove the dowel stick (not always easy, see my article on it), and reglue it at an angle giving proper action.  I don't measure the angle, but lay out the banjo on a simple jig, blocking up the neck and head to give an action on the order of 3/16" at the 12th fret.

Now, there are various ways to get the correct fit of the dowel stick.  A common and fairly easy way to do this is to use Titebond glue mixed with sawdust to form a paste about the consistency of toothpaste.  The paste is then smeared liberally on the dowel stub and inside the receiving hole in the heel - some should ooze out when you assemble it.  This fills all the voids in the joint and dries very solidly in a couple of days.

Some say this is a half-assed approach to repair.  The best way is to plug the heel and redrill it.  I guess that's true if you like to spend your time building jigs and buying very large drill bits.  Another thing you can do it is glue thin shims on the dowel stick stub.  The idea is to have all solid wood-to-wood contact with no reliance on the adhesive as a filler.  I can see that, but as a pragmatic matter, the sawdust makes the Titebond into a good, reliable filler.

I wish to report that I did have one of these "glue & sawdust" joints fail fairly recently.  It was on an 1890-ish no name banjo that I sold to a guy in North Carolina.  Shortly after getting the banjo, he reported that the action sprung to about 5/8", which was much high than when I shipped it.  I got him to return it to make it right.  Upon brief inspection I easily saw what the problem was.

Here, it is easy to see that a chunk of wood had split right off of the end of the dowel stick stub.  The orientation of the split is determined by the grain direction.
In this end view, the problem is pretty clear.  There was not enough filler/glue in the hole.  One portion of the stick stub adhered to the heel, but the string tension eventually split the stick stub.
You can clearly see the split-off chip still stuck inside of the hole.
Here you can see that there is not much contact on this lower side of the hole.
I tried to work up some crude graphics to see if I could portray what I'm talking about a little better.  Here we see a stylized cross-section of the heel-dowel stick joint.  The black color indicates the location of the Titebond & sawdust paste (labeled"glue").  In this case, the fit is very nice with glue distributed evenly all over the stub and hole.  This is seldom realized in antique banjos.
In this second graphic, I attempt to portray a more common case, where the dowel stick is at a small angle (approx. 2-3° from horizontal) to the hole.  You can see the uneven but adequate distribution of the glue.  Here is where the sawdust is important - it converts the glue to a FILLER.
Shown here is the failure mode.  You can see how the glue is pushed up into the too-deep heel hole.  The film of glue holds the stub to the inside of the hole but allows the stress placed on the joint with string tension to concentrate on the upper surface of the stub, causing it to catastrophically fail by splitting along the grain.  Had the hole been shallower or had there been more glue, the failure would not have happened.  Now, I put a plug at the bottom at the bottom of the hole to insure a completely filled joint.

This is poor work.  What I discovered was that the heel hole was deeper than normal, so that there was a good bit of space between the end of the dowel stub and the bottom of the hole.  This is were most of the glue/filler went instead of all around the circumference of the stub.  The drawings hopefully illustrate this a bit better.

To fix this, I pried the chip out of the hole and glued back in place.  Then I cleaned the hole out and inserted a plug into the bottom so that the dowel stub was almost touching it. Then I glued thin wooden shims on the the upper and lower surfaces of the dowel stub, and then slowly filed the shims to get a good solid fit.  Finally, I mixed a new batch of sawdust and glue and assembled the joint as before.  Now it is solid and strong and will not be prone to failure.

So, the lesson here is simple.  Be sure you add enough filler.  Check the depth of the heel hole relative to the stub length.  If the hole it too deep, plug it.  Thanks for reading this far.