Truss nut replacement for 80s Sigma DT-4 (Korean)

Hoping someone here might be able to help. Recently acquired a Made in Korea Sigma acoustic. 5mm truss nut is nearly rounded in the socket area so wanted to replace before using it further. The thread seems to be very close to 25 per inch (a M6x1.0 ‘seems’ to fit the nut for about seven full rotations then stops). M6 is pretty close to 1/4” but cannot seems to find 1/4 x 25 truss nuts, or M6x1.0 for that matter. Tried speaking with the usual vendors but no luck yet. Anyone here have a guess what size they were made with and where i could find a repalcement? Any advice gratefully appreciated. Thanks, Mark

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Hi Mark, my first thought was that if you can get 7 full turns of the nut then you may have the correct nut for the threaded rod, but something else ie stopping the nut from going further. Or, it could be meeting resistance from the neck. When I see a damaged rod nut, hex or Allen, I asked, why? 

Often I find that the nut has been forced "home' to try to straighten a bowed neck. This can result in the rod thread stripping, the rod breaking, or the "flats" on the nut getting rounded.

Does the neck respond to the tightened nut? 

I just read your account again and think I'm barking up the wrong tree. Are you testing the old nut on a new bolt/thread, or a new nut on the original rod thread?


Hi Taff

Thanks for your reply.

I should clarify some points:

- nut was on the guitar when I acquired it, went to pull it out to clean and lube, and noticed the hex socket was very rounded (see img 7549 below)

- figured that putting it back on in its current state wasn't a good idea, as applying any torque would likely finish it off and then it would be more difficult to remove or adjust

- trying to establish a thread/size as this is roughly 6mm or 1/4" in diameter and approximately 25 threads per inch

- when I mentioned 7 rotations, that was onto a M6X1.0 tap that I GENTLY tried threading into the nut, there was some resistance at that point, and I didn't want to apply any pressure at all for fear of 'destroying the evidence' by rethreading if that wasn't the correct size

Cannot find truss rod nuts either M6X1.0 or 1/4" 25, but a M6X1.0 bolt threads in pretty well and past 7 rotations to the full thread distance in the nut.

There is a bit more play than I would expect however when the truss rod nut is threaded onto the test bolt.

1/4" 25 bolts on the other hand, if they even exist, are not easily obtainable to try.

So, I THINK that I am looking for a M6X1.0 (6mm diameter, 1 thread per mm) sized Truss Rod Nut.

Philadelphia Luthier Supply, LMI, and some local dealers have been unable to help, and all point to Stew Mac.

I've sent SM two emails over the last couple of weeks and haven't received a response yet.

Any advise/suggestions greatly appreciated.

Could just cut a slot in the top of the existing nut and use a flat head screwdriver, but really want to get a proper fix if I can.

One of the attached photos shows the nut thread imprint on some silly putty I pressed into the side of the nut and then pulled out, I count around 12 1/2 threads in 16/32nds.




Just noticed that the attached images don’t appear in the mobile view, only the desktop view


Stewmac sells a u channel truss rod with this M6 nut, but does not sell the nut separately (part #1177). 

Nextgen guitar here in Canada might be in the same situation, and are currently checking.

One of the tech guys at a local dealer MIGHT have something in a drawer somewhere but I need to come by to have a look.

So, options at this point seem to be:

-buy a truss rod with a nut and just use that M6 nut for my needs (~ $18 cad either dealer)

- visit and root through parts with some local techs

The only consolation, seems to be that I cannot find a ‘close’ SAE thread size, so fairly certain that this is an M6.

Who knows though.

In any event more delays than I expected when I bought this DT-4, which may be headed for a reset once I source a steam needle.


Hi Mark, I just thought I would show a possible remedy if all else fails.

I used to do this 45 years ago when having to make my own truss rods. I used a brass threaded rod with the thread removed. I think the photos will tell the story. I did this recently when repairing a custom bass rod, only I had the new bolt tig welded.

Good luck Taff

Just to close this off, I visited a couple of friendly repair techs at local dealers and wasn't able to locate a M6X1.0 thread truss rod nut for a Korean DT-4 Sigma acoustic.

Tried many parts sources (NextGen and Solo Music here in Canada, Philadelphia Luthier, LMI, AllParts in the US), no luck.

Stew Mac sells a U-Channel truss rod with the needed thread size (part 1177) , but doesn't sell the nut individually.

Oh well, good thing I have that membership; with a November discount and free shipping it wasn't too much to get a suitable replacement.

The threaded section of the new nut is perhaps 2mm shallower than the original, but will add a washer if this causes a problem. 

Still hanging onto the old nut till I convince a friend to get a MIG setup, then can try putting a cap head socket onto the old rounded truss rod nut as Taff suggested.

Now just need to get some 1/4" fuel line and I can set up my low tech neck joint steamer (pressure cooker, water trap, and ball inflator needle)...

Hi, Glad you got it sorted.


I'm in very late on this but wasn't Sigma a CF Martin economy import line?  Maybe they can help with a direct replacement.


Just for those who might be facing the same search in the future, and to reply to Larry’s suggestion; I did reach out to both Martin USA and Sigma (now owned by a German company) and neither have any info on older Sigma (or any Sigma in Martin’s case). It seems to be solely dependent on communities like this one for info sharing. So similar to the Fender/Squire relationship brand-wise, but more detached wrt support.

Have most of the steamer parts now, but holidays are almost over and have been distracted with pillow bearings and cobbling together a pedestal buffer.... 

Im just wondering what a MIG setup is ? Maybe a PLEK ?

Metal Inert Gas welder.

Oh I took it to be a guitar setup ! Duh ...


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