What's a shim and when do I need one?

A shim is a thin piece of material — wood, plastic, or metal depending on the application — inserted to correct an angle or fill a gap that's throwing off geometry somewhere in the guitar. It sounds unglamorous because it is. It's also one of the most cost-effective fixes in the repair world when it's the right tool for the job.

Neck angle shims

The most common use on bolt-on guitars (hello, Taylor). Instead of a full neck reset, a shim placed in the neck pocket changes the neck's angle relative to the body, which raises or lowers the string height at the bridge. This is exactly why bolt-on necks are so much cheaper and faster to correct than glued dovetail joints — you're adjusting geometry with a shim instead of steaming apart a decades-old glue joint.

Nut shims

Occasionally used to raise string height at the nut slightly, though in most cases a proper nut replacement or reslotting is the better long-term fix. A shim under the nut is sometimes a quick, less-invasive stopgap.

When you actually need one

If your action is high at the bridge, your saddle is already low or maxed out, and the neck angle itself is the issue — not a warped top or bridge problem — a shim (on bolt-on necks especially) can solve it without the cost or risk of a full neck reset.

The catch: shimming the wrong thing, or shimming to mask a different underlying problem, just kicks the real issue down the road. Get an actual measurement of where the problem lives before assuming a shim is the answer.