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I have never used a shim but I'm beginning to think it might be a good idea.
For those of you that do, can you share how big of a shim you use and if you glue it in?

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Joe Beaver
Maker of Sawdust

I use a wood cap over the truss rod and glue it in. It only ends up being about 1/16". I do this because I don't want to glue the truss rod to the underside of the fretboard.

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Nope, I don't. I want the rod right up against the FB...

I use a shim only to eliminate any free space between rod and FB -- rarely more than 1 mm thick. Glued in place with epoxy and clamped very lightly (particularly over mid section). Rough up truss rod with 80 grit to key in epoxy. Scrape down to neck surface when set.
If your slot is machined perfectly to exact height of rod, probably just fine without.


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If I left a gap I will use a shim, I usually route with the truss rod slightly proud and and use a chisel as a scraper to make the channel just deep enough. If I do use a shim I do the opposite I glue in the shim proud of the neck and use a block plane to take it level.

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Shim, as per Allieds directions.

I used a shim when I was using Allied rods. I have switched to Martin two-way and no shim is needed.

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Steve Smith
"Music is what feelings sound like"

I never have, preferring to have as much meat behind the truss rod as possible.

Mark Blanchard says to use one so I do. It winds up being very thin. I see it as mainly a way to prevent rattle. I had a couple of unshimmed LMI TRSDs rattle even with the recommended caulk in the canal. I had to inject rubber cement which fixed it.

Sometimes the Blanchard rod will be a little loose at each end as it is polished smooth without the blobs of weld the Allied and LMI rods have. I do shim the edges of the channel with wood shavings so the ends are very tight.

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I glue in a thin strip (maybe 1.5mm) over the truss rod. That way I don't have to worry about cutting the slot depth absolutely accurately. Also I feel it snugs in truss rod in. Lastly, if I have to true the neck before gluing on the fret board, the metal truss rod doesn't get in the way.
Mostly I do it because I don't need to be so accurate with the channel.

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+1

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Michael Lloyd

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Good information. I can see the concern some of you have over making a 'to deep' channel. My necks end up around 5/8" thick + fingerboard at the 1st fret. So with a 3/8" truss rod and an 1/16" shim that leaves me with 3/16 under the rod. Does that sound OK?

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Joe Beaver
Maker of Sawdust

i use da rods exclusively so for me i mill the slot in on a milling machine and so can set the depth very accurately. problem is, truss rods aren't as accurate in comparison. so i will cut the slot about .015 deeper then the rods actual height measured after removing the weld lumps and burrs.

i'll set the rod at it's neutral point and turn the backlash against string tension. next goes in a 1/4" wide strip of maple purfling stock that's been scraped clean on the rod side. in goes the rod with the maple on top.

then i'll measure the protrusion (just for entertainment) and scrape the strip to flush with the neck. next i'll use .500 x .001 kapton tape and cover the trench end to end to keep epoxy from getting on the rod.

with .50 wide tape and a .25 wide trench, that's approx .125 per side of protection. for me it's about maximum wood in the neck and i use the smallest da rods i can find. the kapton works great as well and compresses to nearly half it's thickness so there is no glue up trouble at all and keeps the trench totally clean.

I use a shim as per Allied instructions. Mainly to prevent rattle. I put the shims in with the rod at neutral and, after leveling to the top of the neck, they end up very thin or even non-existent at the ends but thicker in the middle. I think that's a point that hasn't been mentioned - the shims take up the slack in the middle of the rod that normally exists due to the gap between the flat bar and the threaded rod. I'm not saying to clamp hard on the shim during glue up thereby jamming the bar hard against the threaded rod. What I am saying is that you should start with a reasonably thin shim and clamp all along its length with spring clamps. If the rod is at neutral, the shim will go down a bit in the middle. No slack, no rattles.

Pat

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I've never put one in. LMI or Allied 2 way rods.

Pat, so when you use a shim thicker in the middle does that reduce to workings of the rod?

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Joe Beaver
Maker of Sawdust

I'm not sure I understand your question Joe, but I'll try to answer. If you just hold a Allied type truss rod set to neutral you can see that it's easy to squeeze it at its mid-point so that the flat bar will touch the threaded rod. That's the slop I am trying to eliminate with the shim. So when I make the slot in the neck, I make it as close as I can to the depth of the rod at the ends. This is another point that I don't think has been mentioned - the slot does not have to be deeper than the truss rod to use or gain the advantages of a shim. Then I make a what I'll call a shim blank that's only, say, 3 or 4 mm thick so it flexes easily. I put the rod in the slot and glue the shim blank on top with epoxy using spring clamps all along its length. Because the shim blank is flexible, the clamps push it against the truss rod closing up that gap between the flat bar and the threaded rod at the mid point. After the glue is dry and I plane and sand the shim blank back level with the neck, if I've done it perfectly, the truss rod is actually just showing again at the ends but there is a shim that thickens towards its mid point as it follows the curve of the flat bar that has been pushed down. So in the end there is a minimal of air space left in and around the truss rod.

I hope the above lets you answer your question for yourself. If I am understanding it correctly, I believe the answer is yes. Because you've eliminated the play in the rod associated with the gap between the flat bar and the threaded rod, that has reduced the amount of neutral zone that the rod goes through before making significant contact with the wood above it.

Pat

_________________
There are three kinds of people:

Those that make things happen,
those that watch things happen,
and those that wondered what happened.

I always use single-action compression rods and I always scrape out the bottom of the channel so the depth increases progressively toward the heel. I also scrape the channel so that it's not a straight ramp, but slightly curved. Then I wrap my rods in teflon plumber's tape to snug them against the sides and bottom of the channel and to keep glue off the rod. Then I glue in a spline, clamping the rod down snugly onto the bottom of the curved channel. I make the spline out of a scrap of neck material and plane it flush with the glueing surface where the fingerboard will go.

Pat,

Yes, that explains what you are going for with your installation technique. Sounds to me like a good method. I was thinking I needed a deeper channel but with your method I won't. Thanks for the info.

CP,
Also interesting. I've always used double action rods but I think your method would improve the working of a single action. Thanks for sharing.

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Joe Beaver
Maker of Sawdust