People often wonder why two seemingly identical electric guitars sometimes require noticeably different bridge or saddle heights to achieve a similar string action and feel. The answer has to do with manufacturing tolerances when making the neck and body. Let’s take a look at the issue and explain why shims are often used in guitar construction.
Nothing can be made or reproduced absolutely perfectly. To account for this we allow a range that’s acceptable, in this case we allow a small range of measurements for the dimensions of the neck and the neck pocket. We are only talking about a few thousands of a inch (the lightest guitar strings are about 10 thousands of an inch) but when you put neck and body together, those tolerances can add up to noticeable differences.
An obvious example (in guitars with a bolt-on neck) is the gap between the bass side of the neck pocket and the body. If the width of the neck and neck pocket closely match, you won’t fit a thin sheet of paper in there. If there are small differences we’ll have a gap and, occasionally, the gap can be big enough to take a credit card.
Tolerances apply to every dimension of the neck and body but let’s focus on the thickness of the neck heel and the depth of the neck pocket.
First, let’s imagine a a slightly thin neck in a slightly deep pocket. The deep pocket will seat the neck lower in the body, a thin neck puts the fingerboard slightly lower again. The strings will be a little low to the guitar top and so the saddles won’t need much height off the bridge plate to set correct action.
Now let’s think of the reverse situation – a slightly shallow neck pocket will raise the end of the neck relative to the body and slightly thick neck will raise the fingerboard slightly more. We would have to raise the saddles to maintain the correct clearance over the last fret.
Sometimes, it may not be possible to get acceptable string height by adjusting the saddle or bridge height alone. In those situations we can use a shim – a thin piece of wood or card at one end of the neck pocket, to very slightly angle the end of the neck, raising or lowering the strings over the body of the guitar. At the distance of the bridge from the neck pocket, even a very thin shim can make a difference – a piece of business card (0.25 mm) at one end of the neck pocket can raise or lower the strings more than 1 mm (3/64″) at the saddles.
Shims are a quick and neat technique to assure an acceptable range of height adjustment at the saddles, avoiding the expense of rejecting too many out-of-tolerance necks and bodies. You won’t find a shim in every neck pocket but they are a perfectly normal and useful part of the setup process when required. Shims are not evidence of sloppy manufacturing. Nor do they do not affect tone or sustain of your guitar.
Finally note – Some luthiers and guitar techs say that only “full pocket” shims should be used. This line of thought says that a small shim leaves gap in the neck pocket between the body and the neck. that will eventually cause the neck to warp. I don’t think this is correct and I fail to see how we warp such a short length of neck right at the point where it is thickest. All the major bolt-on neck guitar manufacturers have been using small shims for decades and I know of no evidence that this kind of warping is a real problem. I have nothing against neck poet shims as such but I do not think they are unnecessary complication (to say nothing of time and money) when a smaller short shim will do the same job without problems.