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Just a tease...I will have more shots later, and more numbers. Note the second bronze thrust bearing was lost in grinding - the first is pictured, with the captive nut book-ended with these bronze bearings. The operating rod is highlighted in red, and the captive nut shown intalled in two of the shots and separated/ displayed below the appropriate rod in one shot. Note the fixed end shot shows the plastic filler strip above the actual StewMac Hotrod...this is inside the shrink-wrap, so is a part of the rod system.

Weights are 128g for the 12 fret Martin, 132g for the 14 fret Martin-style, and 86g for the Stewmac. By comparison, a Martin 1/2" depth T-bar is about 10g per running inch, or about 130g total for a 12 fret neck.







After this exercise, I can understand the preference for the Martin rod over other similar captive-nut designs, although the weight seems to me to be a by-product of that engineering effort, rather than a desired result.

Also worth noting that the operating rod on the Stewmac rod was installed off centerline and with a slight curve to one side, as well as having some weld spatter that would likely have sheared and allowed the full design length of the rod to be realized. We only had the one rod to tear down, so tough to say whether these issues are common in the design. At just 86g, this rod is about 2/3rds as heavy as the Martin, but it is easy to see where weight was pared.

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For the times they are a changin'

- Bob Dylan

Thanks for posting this. It clears up a mystery for me ever since a discussion in another thread about a year and a half ago comparing the merits/demerits of truss rods like the ones above versus the ones where the nut is welded to the end of the threaded rod. I asked about the workings inside the housing on the rods above because I couldn't see (literally) how they differed, but never got an answer. Now I get it.

Your photos remind of a monthly series of articles that Car and Driver magazine (I've been into cars since I was a kid) did for a while in which they literally sliced open various car components like shock absorbers, turbochargers, etc. to reveal their inner workings. Pretty cool.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I avoid the captive nut designs all together preferring a welded end nut like Allparts and other sell. An overwhelming majority of broken truss rods I have seen have been the captive nut type.

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Brian

You never know what you are capable of until you actually try.

https://www.howardguitarsdelaware.com/

One of the other students here - a retired process and reliability engineer for the pharmaceutical industry, - listening to the hour of engineer-speak from the boss (aerospace engineer) and Mr. Morelli (race car builder) on the nuisances of weld penetration and material choices, then suggested that for small shop builders, the cost of warranty repair and reputation risk should drive them to rods with the largest installed base and lowest percentage of failures. That was an interesting perspective, and one worth considering.

Part of my rationale for spending the time on this project was just why some manufacturers can see trouble-free use of a particular design (e.g., Martin's 1,000,000 plus 2-way rods in the field and zero reported failures), while others using similar trussrods are less successful (e.g., some Asian-made guitars using 2-way rods with 4mm adjuster nuts). Rather than lumping all similar-principal rods in one category, we decided to examine the differences in operating principal and execution, with the captive nut teardowns as a target of opportunity based on Stewmac's introduction of a Hot-Rod replacement and having both Martin and faux-Martin rods in the shop. Given that the overwhelming number of 2-way rods use this design approach, it also seemed to me to be of broader interest to the audience here.

Obviously, installation also plays a role in failures. Truss rods that are not fully wrapped can suffer from glue intrusion in the rod channel, so usually call for filler strip installation before fretboard installation. We've replaced several original Hot Rods (e.g., the version with the independent operating rod) used by other custom builders for just that reason, so the cause of the failure may be both related to the actual rod design or poor choices/poor technique on the part of the builder.

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For the times they are a changin'

- Bob Dylan

Excellent photos. Thanks Woodie!

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Steve Smith
"Music is what feelings sound like"

"The action is a little high. I'll have to give the truss rod a few turns". I heard someone say this the other day so I'm not surprised at the number of broken ones.

Realistically, in the life of a guitar, how many times does the truss rod need adjusting. Once? Twice? Maybe never.......... On a new build I more often than not never have the need to touch them once the guitar is strung up.

Does anyone need any Alan wrenches to fit Allied rods? I have a pile. I never put them in the customers case.

Thanks for the info pal!! Any thoughts on why they would build a 12 and 14 fret rod differently?

I think I was unclear. The Martin rod is unique in construction, and the same design is used for 14, 12, and for the out-of-production Size 5 truss rods. The use of the 'Martin Style' moniker refers to how StewMac has marketed their superficially similar but very different truss rod. From the StewMac site:



That copy not accurate and borders on deceptive, given that the rod was never used by Martin, and is significantly different in construction as can be seen in the original photographs. Perhaps I should have labeled it the 'Low Profile - Not a Martin' truss rod.

All 2-way Martin rods use full-length fixed rod construction, multi-weld construction on the captive nut housing, beefier captured 5mm nut, and dual dissimilar metal thrust bearings, while the 'not Martin' rod has the partial length fixed rod, smaller captured nut, and uses the nut housing as bearing surfaces (steel-on-steel). The Hot Rod is as the 'Not Martin' rod, but does use a separate steel thrust bearing on one surface of the captured nut housing.

_________________
For the times they are a changin'

- Bob Dylan

How did the captive nut design rods that you've seen fail break? Was there a common type of failure? Were any of them true Martin captive nut truss rods?




What is the zero reported failures based on?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I'm not really sure what the terms y'all are using mean...

Is captive nut one of the terms?

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Indeed...

In the middle photo, the object below the end of each truss rod is a nut that fits inside the chamber in the truss rod end. The nut has a hex socket on the left end and threads inside it on the right end that engage the threads on the threaded rod. The nut is free to rotate around the end of the threaded rod. Since the threaded rod in this type of truss rod is fixed (can't rotate) and the nut can't move lengthwise because it is captured inside the truss rod end, turning the nut exerts lengthwise force on the rod that pushes or pulls on the rod and bends the truss rod overall.

The way the nut is captured differs between the Martin rod and the other two, but the principle is the same.

I hope this makes sense...

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Thanks...

been doing this for 20 years
Captive nuts are without a doubt the most reliable. I have seen to many welded nuts fail. When the nut is welded to the threaded rod , the threaded rod can get heated and this changes the temper of that rod causing them to fail.
I use the bitteroot rod as well as the Martin rod the only difference is the 5mm nut on the martin and 4mm on bitteroot. Being a martin repair shop and having made over 270 guitars I never saw a failure of the martin 2 way style .
I have seen too many of the welded nut design. When you study these pictures you can see the mechanical old of the Martin style there is no weld on the threaded rod that will apply a torque. It is a more forgiving mechanical design.

You pictorial is a good one to show the mechanical connections.
thanks for posting

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Obviously you are a builder, not a repair guy. Truss rods need adjusting at least twice a year in places where there are wild swings in humidity throughout the year. In Kansas, it is not unusual to do an adjustment (usually slight) in spring and again at the onset of winter. Rare indeed is the guitar that seldom needs a truss rod adjustment.

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"Act your age, not your shoe size" - Prince

I've used Gotoh (Martin stye 2 way) and Bitteroot for a while now. Both worked fine and no failures. I don't know if the Gotoh's are still available, i still have a couple in stock. Bitteroot have good prices and ship pretty fast. The Gotohs I was able to get locally. but if I can't get them, I'll get a bunch from Martin when they reopen. I almost always several of everything 1) in case something goes wrong 2) it saves money on shipping and 3) prices are always going up.

I haven't been able to find the Gotoh (Martin style) rods anywhere. The welded rods seem flimsy in comparison.

in 20 yrs I have seen few truss rod failures of the ones I did 2 were operator error The early stew mac 2 way snapped at the nut and others and 2 were gibsons 1 way the nut got rusted and someone twisted them off.
I now test truss rods before install

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Just checked and it looks like a captive nut welded into the channel. They're 2 way rods.

Thanks for the info on the different style truss rods. Great timing. I am ready to install one and am trying to decide which one. The one that is described as a Stewmac Hotrod is not like the ones that I have received from Stewmac. They list 3 or 4 styles, but none are like the ones that are pictured above. Where can I find the Martin truss rod you show in the first picture? I see them on the Martin 1833 site but I can't get signed in. UPDATE: OK, I got the site to work for the Martin truss rod. The Guitar Makers Connection is no longer coming up with the binding and other parts that they one listed. Anyone have any info on where to find the bindings, etc?

Here is the one that I have that is listed as Hotrod:
https://www.stewmac.com/luthier-tools-and-supplies/materials/truss-rods/adjustable-truss-rods/4mm-allen-nut-hot-rod-truss-rod.html

martin no longer uses that rod they switched a few years ago and GMC is now shut down so I don't think you will get them for a long time. I had gotted some a few months ago as a repair center the the rod they are using is like the ones at bitteroot.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Edit. Deleted. I got my info. Thanks