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 a set specification. This is particularly true of wood, the precise dimensions of which can change over time with changes in temperature, and humidity in particular. To account for this we allow a small range of measurements for the manufactured dimensions of the neck and the neck pocket. We are talking about a few thousands of an 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.
A visually obvious example (in guitars with a bolt-on neck) is the gap between the bass side of the neck pocket and the body. Take a look at many guitars coming off the same production line and you’ll see a range of widths of that gap. If the width of the neck neel and neck pocket happen to closely match, there will be almost no gap and you won’t fit a thin sheet of paper in there. If there are small differences between pocket and heel widths we’ll have a small gap and, occasionally, the gap can be big enough to take a credit card.
Manufacturing tolerances are applied 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 neck heel which is within specified tolerances but slightly thin. That neck will sit in a guitar body with a neck pocket that is slightly deep, though also within tolerance. The deep pocket will seat the neck low relative to the top of the body/bridge. The thin neck puts the fingerboard slightly lower again. This means the bridge will be a little high relative to the top of the frets. The saddles won’t need much height off the bridge plate to set correct action. It is possible that the saddles cannot be adjusted low enough to give low action over the frets.
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 a slightly thick neck will raise the fingerboard slightly more. We would have to raise the saddles to maintain the correct clearance over the frets. Again, it may not be possible to raise the saddles enough to achieve the required string height.
In situations where it’s not possible to correctly adjust string height by saddle adjustments we can use a shim – a thin piece of wood or card at one end or the other of the neck pocket, to very slightly angle the end of the neck relative to the body. At the distance of the bridge from the neck pocket, even a thin shim can make a difference – a piece of business card (0.25 mm) at one end of the neck pocket can have the same effect as giving us more than 1 mm (3/64″) extra adjustment 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 necks and bodies that do not measure absolutely dead-on perfect. You won’t find a shim in every neck pocket, perhaps not even most of them, but they are a perfectly normal and useful part of the setup process when required. They have been used regularly by the top manufacturers of bolt-on guitar necks for over sixty years. Shims are not evidence of sloppy manufacturing. Nor do they affect tone or sustain of your guitar in any way.
Final 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. As mentiomned above, all the major bolt-on neck guitar manufacturers have been using shims for over half a century and I know of no evidence that this kind of warping is a problem. I have nothing against full pocket shims but I do think they are an unnecessary complication when a smaller short shim will do the same job perfectly well.
Revised: March 14, 2018