I recently got my hands on a used schecter km6 guitar. It played great, except for that the action was higher than i like it. I checked the relief in the neck (fretted the 1st and 17th fret, measured the action around the 8th), and found the relief was around 1.5/64", which is high. Adjusting the truss rod I was able to get it to 1/64", but at that point the rod seemed to be almost as tight as it would go, and I wasnt comfortable leaving it in that state. Any idea what might be the problem? I read on other forums about guitars with similar issues, but most of these where very old. Mine is only a few years old, has a dual action truss rod, and also rienforcement rods apparently.
You should be very careful about pushing things with the truss rod. You can damage the neck and strip the truss rod if you tighten it too much.
You might want to loosen the truss rod a bit, and try lowering the action on the bridge and/or shimming the neck. Here's a good article about that: Guitar Shop 101: How to Shim a Bolt-On Neck.
Having said all that, unless you are familiar and comfortable with making such adjustments, you're far better off bringing it into a good guitar tech for an inspection and set-up. Particularly since it's used a guitar, that's a good idea regardless - then you'll know exactly what you have (or haven't...) got there.
You can do more harm than good trying to make such adjustments yourself unless you really do have the knowledge and experience. If you value that guitar, don't use it for a DIY experiment.
Agreed, be very careful when changing the tension of the truss rod as it can cause a lot of damage to the neck of the guitar. There is a good article about adjusting truss rods here: http://www.guitarlessonslondon.com/Adjusting-Your-Truss-Rod/ - however, if you are unsure I would recommend bringing it to a store to get a professional to look at it.
Another tip, although it might be not so relevant in this case is to make sure that the guitar is not being placed standing up against a hot radiator for hours at a time - this can also damage the neck and cause warping.
Adjusting string height by truss rod or bridge adjustments can add buzz to a guitar. The lower you go the more likely you will add fret buzz.
Remember guitar necks are not perfectly straight there should be a very slight bow. This is known as relief. You really need to use a feeler gauge to measure relief As the measurements are exceedingly small. So small that you cannot discern the difference with a ruler and your naked eye.
Adjusting the truss rod adds or removes relief (bow or warp) to a section of the neck. Truss rod adjustment does effect overall string height, but the goal of truss rod adjustment is to set the neck relief, not overall string height. The change in overall string height is merely a byproduct of adjusting the relief.
Adjusting the bridge height affects string height without affecting the neck relief. Bridge height adjustment is meant to set overall string height.
So the first question you should answer when your “action is too high” is: “Do I have an overall string height problem or is the issue with the relief on the neck?”. You can have both problems on a neck.
The check you did was for neck relief only. You would need to also take measurements to determine if the string height you are trying to adjust is due to warping/bowing (I.e. trussrod adjustment) or bridge height. Google “guitar adjusting bridge height” for details on how to measure and adjust bridge height. You should evaluate both relief and string height when setting up a guitar to ensure that you are making the correct adjustments.
If this is a graphite reinforced neck, it would be particularly surprising to have already used all the relief in the truss rod on a guitar that is only several years old.
After adjusting bridge height or the truss rod, intonate your guitar too.