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I'm sure this is in a thread somewhere but I couldn't find it. Using the LMI 2 way truss rod and mahogany neck - what is the minimum "safe" distance between the bottom of the slot and the back of the neck? Does anyone think it matters whether the slot bottom is round or square? Thanks.

Hi Richard,

I believe that rod is meant to be set flush with the neck blank or slightly bellow if you are using a filler strip on top. Provided you are carving your necks at a standard thickness then you will likely have around .2 - .25" of material at the shallowest point, if I remember the dimensions of that rod correctly. There is no point in setting the truss rod any deeper than necessary. Yes it does matter if the slot is round or square. Some rods are meant for square slots (like the one you are talking abouthttp://www.lmii.com/products/mostly-not-wood/truss-rods/neck-parts/truss-rod-double-action-welded-nut ) and some for round slots. If you try and put this rod in a round bottom slot then it will not work properly.

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Josh House

Canadian Luthier Supply
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Could you explain why?

I've used it in round bottom slots and it appears to work fine. I did square up the bottom where the flat ends are.

Kevin Looker

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" I did square up the bottom where the flat ends are."
Then you didn't have a round bottom slot, did you..;D
Ken

I believe the question is the minimum thickness of the wood between back of neck and the bottom of the truss rod channel. We use .180" as the minimum, which works well with the .360"-.370" depth of the Martin double action rods and a nominal .250" fretboard.

We mill the slot on a route table, using a 3/16" carbide bit with a round end called a core box or round nose bit. The cut ends up to be 'U' shaped, versus semicircular, which eliminates the stress risers but provides enough room to get a good fit around the end welds and heat shrink cover should the bottom rod be slightly off center versus the top (rectangular) rod. As the amount of wood left under the rod decreases, it would seem to me that minimizing the material removed for the channel and eliminating stress risers probably becomes of greater concern.

The only blow-outs I have seen were with very deep rods in shallow necks (a Hot Rod in a very shallow electric neck of a former student here...).

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Hey Kevin,

Squaring up the ends is the key! If you were to use a round slot for the square rod without doing that then you would have a lot of empty space under the rod. That space would serve no good purpose. Your method of using a round slot and squaring the ends should work fine. I'm not sure that there is a big advantage to your method over just using a square bottom slot to begin with, and it requires an extra step. I've used the square 2-way style rod from Allied or LMI on over 80 builds with a square channel and never had an issue.

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Josh House

Canadian Luthier Supply
http://www.canadianluthiersupply.com
https://www.facebook.com/canadianluthiersupply?ref=hl
House Guitars - Custom Built Acoustic Instruments.
http://www.houseguitars.com

I've gone as low as 1/8in but that's it.

I use the Matin style aluminum channel rod and epoxy the rod into the slot. no worry about break through.
Tom

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