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.
Where should a shim be installed?
If using a full-size wedge shaped shim, all you have to do is lay the shim down in the neck pocket with the thin side facing the headstock and the thicker side facing the body. Done and done.
If installing a shim using another material to save some money, you can basically use anything you want, but some materials work better than others.
Recommended materials for homemade shims and where to place them in the neck pocket
The materials I mention here will be specifically to create a wedge shape similar to the StewMac shims mentioned above.
Wedges are agreed upon by luthiers and techs to be the best way to go about shimming a neck for two reasons. First, to avoid neck damage. Second, to prevent weird noises (as in rattling) from happening during play.
Creating a wedge
The goal here is to make a wedge the same as the StewMac one, meaning the raised end facing the body and the lower end facing the headstock.
The easiest materials to work with for creating a wedge are masking tape or aluminum foil.
Either of these materials are easy to source, easy to cut, easy to shape, and most importantly easy to layer to create that "ramp" you're looking for to get the wedge shape.
Luthiers and techs won't tell you to use tape or foil but rather wood and wood alone. However this is on the assumption you have a belt sander, all the proper sanding paper at the ready, a fully equipped work area, and expert knowledge on even how to use a belt sander properly.
Using foil or tape is more than good enough to create your wedge shim with. No wood required. No belt sander required. No expensive work area required. Use what works and just get on with life.
How much shim do you need to add?
Luthiers have measurement tools and years of experience to determine how much angle a neck needs to be raised to achieve proper string height.
Non-luthiers (which is everyone else, and that most likely includes you) just guess and hope for the best.
If you have absolutely no idea how thick your shim should be, start thinner and then move up to thicker if need be. You don't need much and shouldn't have to go any thicker than a credit card's thickness.
Yes, this may mean you have to take off and on the neck a few times before you get the neck angle you're looking for. You can be comforted by the fact that almost nobody gets it right on the first try. This means you can safely expect to have to take the neck off at least twice or possibly three times to get that shim height just right.
How to deal with factory shims?
This is the last thing I'll mention, and it's important to talk about.
If you've determined that yes, you need a shim, but you've never taken the neck off your guitar before, you may be in for a surprise or two and need to prepare for it.
For any guitar that sold new for under $1,000 USD, take that neck off slowly the first time you do it, because you may find weirdness.
The weirdness I'm referring to is paint chips and possibly a makeshift shim in there "installed" by the factory who made the guitar.
Before I get into that, most guitars with bolt-on necks that sell for over $1,000 USD will have a properly made neck pocket where you won't find any paint chips. And if you do find a shim in there, it will have been made properly.
For the sub-$1,000 stuff however, what may happen when you take off that neck is see a few pieces of very thin wood (similar to balsa wood) fall out of the pocket.
Those pieces you find, if any, were put there by the factory. On the production line, the guy who put together the guitar noticed that the saddles were too low. He then quickly took off the neck, grabbed a few slivers of scrap wood, threw them in the neck pocket, put the neck back on, set up the guitar again, cleaned it up and then sent it on down the line to be shipped.
This happens a lot on sub-$1,000 guitars. And if it's on yours, that's totally normal.
Does this mean the shim put in there by the factory failed? No. The shim was put in there to get the guitar "good enough" for sale. When you bought the guitar later and went to lower the action, the existing shim simply wasn't thick enough.
In the end, if you find what seems like random pieces of wood in the pocket on first neck removal, don't be surprised by this. It's not random. It was just a guy on the production line that put in the absolute least amount of effort to get the guitar ready for sale.
Don't reuse those garbage pieces if you find any, by the way. Create a proper wedge shim either by buying one or making it yourself. Anything is better than what was in that pocket originally.
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Published 2020 Feb 27