Everything you wanted to know about how to shim a guitar neck
This is scary for those who have never done this before, but it's actually really easy and can help fix your guitar - if you do it correctly.
What is a shim?
Any material added to the neck at the pocket where it attaches to the body. I will explain what kind of shim materials you can use in a moment.
What kind of guitars can use a neck shim?
Any guitar that uses a bolt-on neck where the neck can be removed. You need physical access to the neck pocket in order to install a shim. If you have a guitar that uses set-neck construction (like a Gibson Les Paul), then you can't add a shim at the neck pocket since the neck is glued into place.
What is the purpose of a shim?
To raise the neck angle.
Why would you need to raise the neck angle?
If you "run out of saddle", the neck angle needs to be raised.
When setting up your guitar, if you have lowered the string saddles as low as they can possibly go but the string action is still too high, that is when you raise the neck angle.
Example using a Stratocaster or Telecaster style guitar:
If all the grub screws (the little screws that raise/lower the string saddles) are sticking really high up out of the saddles and the action is still too high, add a neck shim. After you do this and reinstall the strings, you will be able to raise your saddles and get the string height that's comfortable for you without having to "slam" your saddles all the way down to the bridge.