After about an hour with a chisel, a flat file, a Dremel with a rotary cutting tool and a sanding block with 60-grit paper, the heel was about 1/8-inch away from being fully seated in the block. It was starting to look like a dovetail joint again.
Before continuing with the remaining work on the heel, my Dremel and I needed to be recharged. I put my tools away for the night, and got to thinking about the way the neck joint might have failed before I bought This Old Guitar. This was a mystery to the previous owner as much as it was to me. It was time to apply some forensic science to the situation.
The epoxy joint was clearly stronger than the wood. I thought a little bit about the physics of what could have caused the heel to split near the truss rod and through the heel without leaving any other obvious signs of damage to the neck or body. A force against the rear of the neck or excessive string tension would likely have caused some stress where the fingerboard meets the top plate, or some separation of the fingerboard from the neck near the body, or some damage to the bridge or top plate, so I decided those were most unlikely scenarios by the evidence.
A force against the front of the neck, midway or close to the headstock, could have produced enough bending in the neck to result in a crack through the flat part of the heel and traveled all the way through through the other end of the heel, splitting the heel wood through the dovetail joint. Or, there could have been a defect in the neck wood, caused or aggravated by excessive tightening of the truss rod.
Either way, I was determined to make the neck joint stronger than before, as well as to make it possible to make future adjustments to the neck angle. Thinking about how to take advantage of the geometry of the heel and neck block, I came up with three repair options. With a little more thought, I ruled out the first option, and a repair strategy appeared.
Count on the strength of hide glue in the dovetail joint and the surface area between the flat part of the heel and block to hold things together. Fill in the groove in the neck block and glue in shims to set the neck angle. Shim out the width of the neck block to close the gaps at the neck and block closest to the high and low E strings, and drill some small pilot holes beneath the 15th and 18th frets to provide a way to steam out the neck for future resets. The only problem was that there might not be enough surface area to develop enough strength in the joint, and there would be no way to test the joint and neck angle with actual string tension, as the majority of the strength and in the joint would be provided after the glue between the flat parts of the heel and the block was allowed to dry. Hmmm...
Better yet was Option Two:
Fill in the neck block groove and gaps, and use hide glue for the joint as in the first scenario. Install wood screws through the back of the bowl, through the neck block and into the flat part of the neck heel. These screws would also provide the clamping strength needed to hold the joint in place while allowing the glue to dry. Some ferrules would help distribute the load through the back of the bowl into the neck block, and the screws would be left in place after the repairs. Removal of the screws would provide ample access holes to steam out the neck for future resets, so I would not need to provide pilot holes beneath any of the frets.
Then there was Option Three:
If Option Two fails, install two pairs of black bolts, located within the 16th and 18th frets, through the back of the bowl, through the neck block, and into the flat part of the neck heel. Again, the four black ferrules would make this presentable, and I would install four metal inserts into the neck to receive the bolts. Use shims to make the angle adjustments, just like on the old-style Fender necks. I researched this repair method and found a bolt neck insert kit offered by Onyx Forge for this type of repair to bolt-on necks with worn wood screws.
Just like Page and Plant once wrote:
Ain't so hard to recognize - These things are clear to all from time to time.