that was question #0 Does Sigma use double-action truss rods.
Cannot find any info on web or in catalogs or online. Left a question in sigma FB group
AS to why? the truss rod is stuck and I fear it stuck fully clockwise. The Martin-style trussrod keys are expected to arrive Friday, promised. I want to loosen that rod. But if it is a double-action I won't know when I have reached the 'relaxed' point.
I would be extremely surprised if Siggys truss rod was not Double action.
If your truss rod is stuck fully clockwise, it may be maxed out now, do not turn it clock wise any more, the rule when adjusting truss rods is "DO NOT APPLY EXCESS FORCE" if you strip the thread it will cost you more to have repaired than the guitar is worth.
You still haven't answered the question "Why are you adjusting the truss rod".
Re DOuble-Action Ok i going to make some assumptions based on what I see on my Seagull. Because I could get a sideview via the Sound Port with the endoscope we observe a square metal thing below the the truss rod nut.
Like this image. SO i made a right-angle hammock for my endoscope camera and tried to get a view between the neck block and the top bracing. Very difficult to maneuver! I did see what looked like a slot or top of a square hole. So I am going to assume Sigma does have a double-action truss rod.
I think I need to re-measure it. Doubt it anything is the same it was in May before we dehumidified and oiled truss rod. Bu the laundry room is in use so my guitar work table (LG washer/Dryer) is in use. Measured while hanging (the guitar not me), not flat. Action: 3.25mm Relief: less than business card The clearance at 2ed fret with 1st capoed seems the same as 1st fret clearance from nut. But I would need to pull out thickness gauges to be sure. Don't ask me to check for belly below bridge, I nicked the top in May trying that. 6th string saddle height is ~5mm, so we got plenty if we sand. And i have a extra bone saddle if I screw up. Siggy has the tallest saddle of all the acoustic guitars. Is that normal? did the tech shim? So many unknowns.
From the information that you have given me this is what I would do. Best to keep old strings on for the time been.
Reduce the overall height of the saddle by 1mm. Then see if you are happy with the playability.
Tip. Put capo on first fret, slackened strings off pulling slack through the capo, untill you have enough slack that you can remove the saddle, first check if any shims are fitted, if so remove them, now remember you only need to remove a total of 1mm, so if shims are fitted take there thickness into account and take less off the bottom of the saddle. To make life easy, scribe a pencil line along the bottom edge of the saddle for reference. This is what I do place half a sheet of sand paper on to piece of glass (or other hard flat surface) then sand off the bottom of the saddle, now its important to keep the base of the saddle at 90°to the sides of the saddle, in other words when you stand the saddle on its bass it should be standing up dead straight,and not leaning. The base of the saddle must be dead flat, I check this by holding a metal rule along the edge of the saddle that you have just sanded, and holding it up to the light, there should not be any visible light between the saddle and metal rule. Now the golden rule is, you can always take more off, but you can't put it on. When you are happy, replace the saddle, put a bit of pressure on the strings, remove the capo, tune the guitar to what ever tuning you play the guitar in. It's possible that this is the only work you need to do, play the guitar for a couple of days, then see if you are happy to leave well alone. If you haven't sanded down a saddle before, take your time, it will take you longer than you might think, not to mention the sore fingers. Let me know how how you get on,and with a bit of luck you will be a happy chappie.