Double Bass "Mortising" an old Kay dovetail joint

[KUNGfuSHERIFF]

Kay neck heels break for multiple reasons.

1. They’re flatsawn. I mean, come on, now.

2. The neck joint is sloppy, which happens when you build them fast at minimal cost a way you know is wrong and refuse to change. If there was less air space in the joint the tenon would be supported and stronger.

3. They made the blocks out of whatever was laying around and cheap. Good spruce is easy to work. Yellow poplar works like balsa and is about as strong.

Sure, string tension holds the tenon in the mortise. But that’s not the problem.

No mystery here.

 

[GretschWretch]

As I see it, when the joint fails it's usually not because the tenon sinks into that space, rather that the block gives way to the leveraged load at the rear of the mortise. Does that make sense?

Now this has got me wondering: if there is no space for the tenon to sink into, would the load leverage ever reach a level to make the block give way? I.e., filling that gap beneath the tenon eliminates the space it would sink into.

This would be similar to an archtop guitar with neck overstand and subject to the same forward-pulling tension as a DB. If a shim of some sort was placed beneath the end of the overstand, I would think that would prevent the neck from unseating due to string pull. The shim would prevent the neck from going in the only direction it could go for the joint to fail. Provided the shim filled completely the space beneath the overstand.

 

[Steven Ayres]

Now this has got me wondering: if there is no space for the tenon to sink into, would the load leverage ever reach a level to make the block give way?

I don't think it would change anything. With the dovetail the fulcrum for the leverage is the top front of the block. More wood under the tenon would look nice, but couldn't provide much engineering advantage there, if any.

I get that it looks like a good glue joint at the bottom of the mortise is important, but given the stress vectors involved it seems to me that the only important joints are on the sides, whatever the shape of the joint. I consistently find little ponds of hide glue doing nothing between the tenon end and mortise, but it's not deliberately put there, it's just seepage.

A block of more stable material than cheapo poplar would hold against that leverage much more effectively, and I'm wondering whether that might be the simplest solution as a dovetail restoration. I've seen one cello block (traditional straight mortise) with more or less the same failure condition as the dovetail here, and it was poplar too. In that case the block broke out the side, under the rib, so there was no option other than a new block. This tells me the problem is at least as much the material as the design, maybe more so.

 

[james condino]

The main reason for the gap in the front of the dovetail is simple: above everything else, Kay was a guitar company. (I've built about 200 guitars over the years, so I have a little bit of understanding into the guitar world mindset...)

In the guitar building world, it is almost universal that the dovetail ends under the 14th fret and you leave a small gap there- some to allow glue squeeze out and also so that you can steam the neck off when needed later. The corporate guitar nerds just used the tools and knowledge they had on hand. If you've ever removed or done a neck reset on an old Kay guitar, you'll see that the dovetail is almost identical to the one on the basses, just a couple of inches longer and almost identical in mass and shape.

I daily enjoy the "Kay theater" here and elsewhere...we all agree that they were not the best ride out there, but they made thousands of them, so as a repair shop, you get a whole lot more business befriending the regional Kay community that trying to survive on the four people in the state that own old Prescotts!