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I tried a double acting rod, and was troubled by how much force it took to get the neck to move. I think that in principle, it's a great idea, but I see two problems with it.
First, you have fine threads on each end, but when you have to move both threads to make an adjustment, then the thread pitch is effectively doubled, and the force necessary to push up or down in the rod's center is also doubled, plus twice the loss due to friction in the threads. This is in comparison with the force necessary to move a conventional compression rod up in the center. I suspect this may be a major factor in breaking them, welds aside.
Second, once you have torqued the adjustment nut to gain the proper relief, then one end of the rod wants to rotate clockwise, and the other, counter-clockwise. This leaves a permanent twisting force on the neck, which seems wrong to me from an aesthetic point, if nothing else. I have no idea if it is enough torque to ever twist the neck in response.
What is unique and desireable about this rod, of course, is that it fits in the slot that Hesh describes, and I use a 1/4" round bottom bit. The main advantage of this rod over the single acting rod is that it doesn't need a larger radius hole at the headstock end to accomodate the thrust washer the nut pushes against, weakening the neck/headstock joint. And I am unaware of whether anyone uses the single action rod adjustable at the soundhole.
When I was at the Taylor factory in 2000, I saw that they use the standard compression rod, but when they glue up the fretboard to the neck, they clamp it to a caul that has a .050" rise in the middle of a long curve, which gives the glued neck much more relief than needed. Then, the nut easily moves their neck to the desired shape, without the need to ever bend it the other way.
Looking to have the best of both worlds, I made a curved caul that only rose .020" in the middle, as it seemed enough for me.
I bought the LMI two way rod and the nut they sell separately, with right hand threads. I then proceed to butcher the rod by sawing off the nut (which is on the left hand threaded end) and sawing the main bar just above the threaded nut/end block. I then unscrew the right handed end and drill out the threads in the nut. The threads on the rod are enough to allow the end to stick out about .350" past the nut end, and I then screw the extra nut down to what has become a thrust washer, basically. But one that is contained within the size of the rod, and needs no wider hole in the headstock for a washer. I then have a local guy weld the other end solid, and sometimes shorten the rod, since I prefer the 24 5/16" scale neck. And since neither end block has any torque on them when adjusting or resting, I can grind the bottom end round to fit the channel the router bit leaves.
When I rout the slot, I use a fence and router bit from underneath the table when the neck block is still even width. I just continue the slot up into the headstock while leaving the fretboard surface flat on the table. I then glue on the head plate, and saw and file it to match the slot underneath. This gives enough room for the new nut end to lie below the surface.
I end up with a single acting compression rod that causes minimal loss of strength to the headstock joint, and requires much less torque to raise the middle of the neck due to only turning one set of threads. The .020" relief I glue into the neck seems about right for the necks I've made this way.
And when you've adjusted the relief, there is no unresolved torque in the rod.
I hope this doesn't offend anyone who believes in the double acting rod. It's only my intention to suggest an alternative that may not really be necessary, but satisfies my requirements.
Cheers,
Scott
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