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I've got a Harmony arch top (H62) in front of me, and along the way someone stole the truss rod nut. Any idea from the machinist crowd out there about how to figure out what this thread size/rod diameter is? I can't seem to get a replacement Gibson brass acorn nut to fit, and likewise with a fender heel replacement nut. (Which is what I have on hand). The Gibson nut may actually fit, but I'm having a heck of a time getting it all lined up square to thread it back on the rod.

StewMac sells three different thread chasers as part of their "truss rod rescue kit"

#6191 8-32 thread for vintage Fender (approximately '61-mid '64)
#6192 10-32 thread for Gibson (all years) and vintage Fender (approximately mid '64 through the 70s, and US-made vintage reissues)
#6193 10-24 thread cutting die for vintage Fender (approximately '54-'59

The difference in size between the number 8 (0.164 diameter) and 10 (0.190) should be fairly apparent, the threading is not

Conor, I don't think Gibson nuts will fit that Harmony. As I remember they are smaller. Sorry I don't have a part or answer for you.

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

Yikes! Pricey.

I've got a set of thread detectives (https://www.stewmac.com/luthier-tools-a ... itars.html). Simplifies figuring out thread pitches. I use them for guitars and toy trains.

I have one of these types of things. Pretty handy and cheap. You’d need clear access to at least a few threads to use it effectively.

https://smile.amazon.com/OSOF-Multi-Pur ... NrPXRydWU=

I realize the diameter of those things will be different depending on which piece you're using, but can you tell me what the diameter of the 10-32, 10-24, 8-32 ones are? Those are the most likely culprits, and for them to be useful I'll have to be able to fit them into the round opening of the truss rod channel, something I haven't been able to do with any of the sockets I have. Thanks.

Just search for a thread size chart online.

I *think* he was referring to the OD of these thread detectives that would need to fit through the truss rod access hole. At least, that’s how I read it.


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You could make some quick-and-dirty thread detectives using a length of mild steel rod small enough to fit your truss rod access, then drill and tap the end of a piece of rod. You’ll have to buy some taps, but these can be bought cheaply at any hardware/automotive store.


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How about using various diameter tubing or 1/4 " ID pipe for that?

Lets make a couple of assumptions. Since Harmony is a US brand from the 50's lets assume it is NOT a metric thread. I've given Conor three possible Imperial threadings - 8-32 which is 0.164 OD, and 10-24 and 32 (coarse and fine thread) on 0.190 rod. He should be able to tell the diameter pretty close even if he can't get calipers in. The most common truss rod for single acting rods is 3/16 which threads to either National Coarse or Fine. NF is used by Gibson, it is the recommended size for threading rod in Cumpiano and from an engineering standpoint, makes the most sense. My guess would be 10-32.

The problem comes if the threads are boogered up and have to be chased. Normal dies don't fit, you need the special one that SM sells.

A thread gauge will solve the threads per inch problem. The diameter you can look up on a chart . One of these is what I'm talking about.

One of the fun things about this particular job is that the access channel to the truss rod angles down to the rod and so I can't even get a wrench I would normally use on a Gibson to engage the rod squarely. I did manage to get a socket in though it was extremely tight.

Here's a picture...


So, now I'm trying to imagine what kind of nut was there originally. I found this picture online, (I have no idea what the guitar in the picture is, but this is the only other style of nut I can think of that might have been on the guitar I'm working on, I doubt either an allen key, or phillips/fender style nut was used.)



This guitar is from the 50's, and the rod appears to be 3/16" So as Freeman says the likely bet is it's either 10-24, or 10-32. Gibson used 10-32, and Fender during the 50's seems to use 10-24, so who was Harmony taking their cues from? (Frank Ford on his website references an 8-32 rod on a Harmony he was working on, but my rod is clearly thicker than 5/32".)

I have some 10-32 brass Gibson nuts, but they're not fitting, (but I also wouldn't dismiss the idea the threads are mangled.) Finding a 10-24 nut that is shaped appropriately for head stock truss rod adjustment is proving very challenging, (out of curiosity, what is the name for a nut that is thicker than a typical machine nut? When I search acorn nuts, only cap nuts come up on all the fastener websites I've looked at. Coupling nuts look like a better bet, but they are 3/4" long).

I'd prefer if the rod were 10-32 with mangled threads. I guess next step is buy a thread gauge, and hope it's 10-32 threading, then I can buy the appropriate stewmac truss rod rescue kit, and replace the nut with a readily available Gibson style truss rod nut.

Conor, there are long nuts called spacer or standoff nuts that are sometimes used for truss rods, but if a Gibson acorn nut won't screw on either its the wrong thread or they are screwed up and need to be chased. I use a thin walled nut driver to turn these - it barely fits the space around the nut.

Exactly the kind of short answer my long winded post was looking for! Thanks Freeman.

A SWAG: Find a couple of standoff nuts, a couple of regular nuts, and some bolts threaded for the nuts (10/32, 10-24). Use a standoff nut/jam nut to lock the nuts in place leaving enough exposed thread on the standoff nut to chase the truss rod threads with this composite tool. A wrench can be used to turn the tool on the truss rod. A shortened standoff nut can be installed as the truss rod nut. A bolt can be used as a gauge to determine thread pitch by laying it on the truss rod and seeing the threads engage.

Maybe?

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Peter Havriluk

I recently worked on a no-name Japanese guitar from the 70s which had a truss rod nut with holes drilled through it - insert a metal rod and lever it round. So if you can get a longish nut to fit, that might be a possibility if the channel is still too tight to get a wrench on the nut. A brass nut might be too soft to hold up to the levering I guess - this one was steel.

Let the record show it is a 10-32 rod. And apparently the nut is the same as what one would find in a similar vintage Guild guitar.

Good to know, Conor.

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