First of all, allow me to explain what a shim is. A shim is a piece of something (usually cardboard) that is used to elevate the angle of your guitar neck, so that the upper frets are angled slightly higher. This allows you to adjust your bridge's height higher while keeping the action on the fretboard low.
After straightening the neck and balancing the spring of the floating bridge of my friend's Ibanez, I realized that the action of the guitar is still ridiculously high. Somewhere around 2mm on the 12th fret. The worst part is, the trem posts have been screwed down to it's maximum. Meaning, I can't lower the trem posts anymore. So the only option that is left in this case, is to shim the neck at the neck pocket.
I proceed to cut a piece of cardboard (I used a name card) at about 1.5 cm, remove the neck from the neck pocket and placed it towards the very end of the neck pocket as shown in the picture. If you are using a piece of cardboard as a shim, please keep in mind that there is no real need for you to make the holes for the screw. Since the screws are already sharp, you can actually screw through the cardboard pretty easily.
Proceed to string up the guitar after re-attaching the neck back to the neck pocket. Make sure all 4 screws are tighten. We don't want the neck shifting around in the neck pocket after you have strung up the guitar. Some of the tuning instability can be attributed to this. After that, proceed to adjust the action of your guitar again and make sure that the neck is still straight and the spring of the bridge is balanced. Test every note on the neck and make sure that no notes are chocking or buzzing. Especially around the upper fret area. Notice in the picture on the super low action I was able to set for my friend. There weren't any buzzing or whatsoever. The guitar now plays like a dream. At least a shredder's dream.
