questions about a truss rod design change

[Maple]

Hi,

I'd like to come up with a two-way truss rod retrofit to an existing neck.
The goal is to rescue one of my old and (sentimental to me) bass necks.
I built it originally with a fully serviceable truss rod that it is only capable of compression.

Over time, maybe from storage or possibly from fret choice (recent re-fret) the neck has developed a back bow that string tension hasn't been able to correct.

The original design had a removable anchor at the heel and a standard gibson style nut at the headstock.
What I'm thinking of doing is to replace the rod with a new one that has a square welded to the rod at it's headstock end and has a wheel nut at the heel. The wheel nut would be dropped into a new slot between frets 23/24. so that it can apply pressure in either direction and be clear of the mounting bolts.
The welded headstock end would get trapped into it's recess with some blocking so it can move or back out.
I'm thinking the wheel nut would be 3/16" thick and probably made of brass or steel.

1. Anyone see any issues result from this sort of design? I think it's similar to what Ken Bebensee does on his basses though it's been a long time since I've taken a close look at one.
2. Do you think 3/16" of thread on the wheel nut is enough to provide the needed tensions to adjust the neck without stripping?

 

[JustForSport]

Getting a dual-action truss rod to fit in an existing single-action compression truss rod slot may be asking for too much.

Removing the fretboard so a dual-action truss rod can be fit may not be what you want to do, tho.

 

[Maple]

Those were my conclusions too. I'm not taking the fingerboard off and there's no way that I see to ream out a slot for an aftermarket truss system.

So the new rod would be a single rod (also 3/16" dia) but be able to apply pressure at both ends.
My hope is that the rod when in "expansion" mode (along with string tension) would translate into forcing the neck into relief.

 

[Bruce Johnson]

I'm one of the few Luthiers around who works with double-acting single-rod truss rods, similar to what you are describing. Your basic idea is okay, but you'll need to beef it up somewhat. When you put a single-rod truss rod into reverse, to correct a back bow, the two ends push outwards with quite a bit of force. You'll need to make sure that both ends have enough surface area and shear area of wood to apply that force to.

At the headstock end, which will now become your anchor end, you'll need to attach a square end block, or something similar, to the rod to keep it from turning. Like you said. But that end block needs to be recessed into the neck far enough that you can back it up with a substantial glued-in plug, which can withstand that outward push load. That plug will need to be at least 3/4" long in order to have enough shear area so that the rod won't push it out of the hole. That anchor needs to be sunken way in there, partway to the first fret. But I think that end can be worked out.

At the heel end, you've got more of a problem. If I'm understanding your idea correctly, there probably isn't enough wood at the end of the heel to withstand the outward push of the spoke wheel. It would split out the wood. The point where the parts apply the load to the wood needs to be further inboard from the heel end, preferably about an inch. Also, making the wheel only 3/16" thick, with just 3/16" of thread engagement would be marginal. You very likely would strip that trying to straighten a neck. On 3/16" rods with 10-32 threads, I always use 3/8" of thread engagement.

The way to solve those problems is to make up a double-acting head assembly, which you could implant into the heel from the bottom.

Take a look at this thread, about Rick's Les Paul bass build. Starting on page 24, post #480, I show the whole process of how I make up and install one of these double-acting single-rod truss rods. On page 25, you can see how I make up the head assembly, and how it works.

My Les Paul Bass Build - I need a Gibby too...

That type of head assembly also works in a heel-access installation. The point of it is that the load, in either direction, is applied by the aluminum cross bar, and the cross bar is set inboard far enough to have enough wood surrounding it to take the load. The long neck brings the head out to where you can reach it to do the adjustment. And the long brass barrel has plenty of thread engagement length. The adjusting end can be socket head (Allen wrench), hex head, or spoke wheel. Whatever is more convenient for the arrangement of the neck.

That's the type of assembly you need to make up to fit into the heel, to make a single-rod truss rod into a double-acting single-rod truss rod.

I hope this gives you some ideas.

 

[Maple]

My Les Paul Bass Build - I need a Gibby too...

Thanks for the excellent feedback! The headstock end is going to be fine - I have room for about 1" of plug and the plug would have a positive interference - there will be a small step in the access cavity that it can drop into and push against.

But at the heel - If I have to provide more thread than 3/16" then I will need to re-think the design. The neck does have quite a bit more wood after the 24th fret. There would be over an inch of wood after the wheel. So I worry more about a failure of threads than of wood.

I plan to make the rod from titanium which can gall if you're not careful (I want to lighten the neck a little). If the wheel is steel and I use anti-seize compound on assembly, I might be in good shape but I don't know how much stress the threads will see.

 

[Maple]

I did a test that confirmed the truss rod will work as planned!
I replaced the brass nut with the wider fender hex truss nut at the head stock end,
I then clamped an aluminum backing block behind it.
the block was cut from scrap and had a hole drilled through to allow the allen wrench to pass through.
And sure enough, the neck went into a proper relief!

So next I'm working up a replacement truss rod to retrofit.
The fixed end of the rod will now be at the headstock.
and the nut will either be an allen head that is trapped at the neck joint or a wheel nut that fits between the last frets.

 

[Bruce Johnson]

If you are trying to save weight, think about using 7075 aluminum for the rod, rather than titanium. When you look at the numbers, their strength is about the same. The 7075 is lighter and easier to machine. I've made truss rods from both, and I don't see the advantage to titanium. Titanium is better for long fatigue life and withstanding high temperatures, but hopefully those aren't problems on your bass! In machinability and strength, 7075 is about like mild steel, at 1/3 the weight.

I recommend making the rod from 7075 aluminum, and the wheel nut from brass. The anchor can be aluminum, steel or brass. Join the anchor to the rod with threads and red Loctite. At both ends, make sure you have at least 3/8" of threads engaged.

 

[Maple]

If you are trying to save weight, think about using 7075 aluminum for the rod, rather than titanium. When look at the numbers, their strength is about the same. The 7075 is lighter and easier to machine. I've made truss rods from both, and I don't see the advantage to titanium. Titanium is better for long fatigue life and withstanding high temperatures, but hopefully those aren't problems on your bass! In machinability and strength, 7075 is about like mild steel, at 1/3 the weight.

I recommend making the rod from 7075 aluminum, and the wheel nut from brass. The anchor can be aluminum, steel or brass. Join the anchor to the rod with threads and red Loctite. At both ends, make sure you have at least 3/8" of threads engaged.

Hmm. I've used 7075 for many things but haven't considered it for a truss rod. Are you using 3/16" dia rod or something larger?

The titanium rod will cost me close to $150 in parts and machine time (threading is the bulk of the expense) but aluminum would certainly be cheaper overall and can be threaded easily in my workshop.
Thanks!

 

[Bruce Johnson]

Yes, I was using 3/16" 7075 rod with 10-32 threads. You can thread it yourself by hand; it's about like threading mild steel. Use a threading oil, like Tap-It.

Titanium can be a real bear to thread. The higher-strength T-5 work hardens real easily, and is real tough to hand thread. You usually have to thread it in a lathe to control the speed and heat. I've made titanium rods, but it isn't worth the cost or trouble.

These days I make almost all of my truss rods for my own instruments and for my luthier clients from 304 stainless. But I use 7075 aluminum for applications where the weight is really important. I always use brass for the nuts, because it's nearly impossible for it to seize up on either stainless or aluminum.

 

[Maple]

Materials are on order! Thanks

 

[Maple]

Ok, I'm several steps further.
I ordered aluminum, threaded it and then decided to take a second shot at threading the titanium. This time it worked out nicely so I figured I might as well stick with the Ti and save the aluminum for a later project.

I decided to forgo the wheel nut with this one - just seemed too problematic. So instead, I went with the Fender bi-flex nut

The rod has a small Ti plate welded at the headstock end and is now blocked with a wood plug so it doesn't budge. That's good.
the heal end has the 3/8" dia fender style hex nut used in their bi-flex truss rods. I made a walnut tubular plug to trap it in place and allow an allen wrench to pass through. It's glued in with Titebond III.

Now every time I try adjusting the rod in compression mode (screwed out), the plug breaks out. I must not have enough glue surface.
Should I use epoxy instead or should I cross pin the plug to make it extra strong?
(the plug is about .4" dia and is 3/8" long)

The obvious alternative would be to sink the nut deeper and put a longer plug in there but that would require a specialized allen wrench and would interfere with pickups. I'd have to remove the neck pickup to make adjustments. do-able but... yuck.

 

[Maple]

Alright, so with the advice from @Bruce Johnson, I have rescued this neck! I went with the titanium rod instead of aluminum. I bought both rod stocks but the Tap Magic and a good quality die, I was able to cut very nice threads on the titanium rod.

I made a long brass nut with a machinist grade shoulder bolt as the visible end you drive it with. The whole thing is threaded together and held by a roll pin.

1. Brass nut and truss went in first, with anti-seize compound on the threads
2. Then I slid the square steel backing plate into the slot at underside of heel.
3. Screwed the shoulder bolt through the backing plate into the brass nut
4. Secured it with the roll pin through the hole in heel.

I have more work to do on the bass - I'm re doing the electronics. But I strung it back up and did a quick neck tune - now it effortlessly went into the proper relief! Thanks!

 

[Bruce Johnson]

That looks great! I'm glad that it worked out for you on this neck.

Here's an example of a similar double-acting single-rod installation that I did this past week on a pair of guitar necks for one of my clients. This is the heel-actuated spoke wheel style head assembly, installed from the top before the fingerboard goes on. It works the same way as what you did; pushing or pulling on the cross bar.

The tricky part was sealing around the aluminum sleeve to keep the epoxy from leaking out around it during the casting process. I used a couple of little triangular scraps of foam rubber and some LMI yellow glue to make a temporary seal.

The bundle of carbon fiber strands go under the truss rod at the heel end to stiffen the back and prevent the dreaded 12th fret kink aka ski jump condition down the road.

 

[Maple]

Thanks! That looks well thought out