A shim is a thin piece of material used to fill small gaps between objects, typically to level a surface, improve alignment, or adjust spacing. Shims range from rough-cut wood wedges used in home construction to precision metal strips thinner than a sheet of paper, machined to tolerances of a thousandth of an inch. While the concept is simple, shims show up in a surprisingly wide range of fields, from installing a kitchen window to tuning a high-performance engine to improving the clarity of an MRI scan.
How Shims Work
At its core, a shim compensates for an imperfect fit. Two surfaces that should be flush, parallel, or evenly spaced rarely are in practice. Manufacturing tolerances, wear over time, uneven cuts, and natural variation all create tiny gaps. A shim fills that gap so the assembled parts behave as though the fit were perfect.
Shims come in two basic shapes. Flat shims raise one surface relative to another without changing the angle between them. Tapered (wedge-shaped) shims change the angle, which is useful when you need to tilt a component into alignment. Choosing the wrong type can create new problems. In guitar repair, for example, using a flat shim where a tapered one is needed can create a gap under the neck that distorts the joint.
Common Shim Materials
The material a shim is made from depends on where it will live and what forces it needs to handle.
- Wood: Usually cedar or pine, wood shims are the go-to for residential construction. They’re soft enough to trim with a utility knife and work well for squaring window frames and doors in their rough openings.
- Metal: Stainless steel, aluminum, brass, and copper shims handle industrial applications. Stainless steel resists corrosion in wet environments. Brass is impervious to saltwater, making it a standard choice in marine settings. High-carbon steel tolerates extreme heat.
- Plastic: Made from materials like polypropylene and ABS, plastic shims are the most versatile for construction. They won’t rot, compress, or corrode, and they work for both interior restoration and outdoor projects like precast concrete installation.
Industrial shim stock is manufactured in rolls and sheets, with thicknesses starting as thin as 0.001 inches (roughly the thickness of a human hair) and going up to 0.031 inches. Tolerances are tight: a shim in the 0.001 to 0.003 inch range is held to within 10% of its stated thickness.
Window and Door Installation
One of the most common places people encounter shims is during window or door installation. Rough openings in framed walls are deliberately cut slightly oversized to allow adjustment, and shims bridge the gap between the frame and the rough opening to get everything square and level.
The process starts at the bottom of the window, where shims raise the lower edge until it’s level. From there, shims go on the sides of the window, placed about six inches from the top and six inches from the bottom on each side. If diagonal measurements reveal the frame is slightly racked (out of square), gently shimming one upper corner toward the framing can pull it back into alignment. Without proper shimming, windows bind when you try to open them, and gaps around the trim become uneven.
Industrial Machinery Alignment
In factories, shims solve a problem called “soft foot,” which is one of the most common issues when aligning rotating equipment like motors, pumps, and gearboxes. Soft foot occurs when a machine’s mounting feet don’t sit flat on the baseplate, similar to a chair with one short leg that rocks when you sit in it.
When you bolt down a machine with soft foot, the casing bends slightly to conform to the uneven surface. That distortion shifts the shaft position, causes vibration, shortens the life of electric motors, and creates clearance problems inside gearboxes and pumps. The fix is placing flat, clean shims under the short foot to eliminate the gap. Technicians check each foot with a feeler gauge as thin as 0.002 inches, shimming as needed until all four feet sit solidly on the base. The goal is to use the fewest shims possible, since stacking too many can introduce its own instability.
Automotive Valve Adjustment
In many overhead-cam engines, including those in vintage sports cars, several Toyota models, and most motorcycles, shims set the precise clearance between the camshaft lobes and the valve stems. This clearance matters because it determines how fully and how precisely the engine’s valves open and close.
The shims sit between bucket-shaped tappets and the tops of the valve stems. A typical clearance spec might be 0.006 to 0.008 inches, measured by sliding a feeler gauge between the cam lobe and the tappet. If the gap is wrong, the mechanic has to remove the camshafts and timing chain, pull out the tappets, measure the existing shim, calculate what thickness the replacement needs to be, install the new shim, reassemble everything, and re-measure. It’s one of the more labor-intensive maintenance tasks on these engines, but the mechanical simplicity of this design (it eliminates the need for rocker arms) makes it popular in performance applications.
MRI Scanner Calibration
The concept of shimming extends well beyond physical spacers. In MRI machines, “shimming” refers to fine-tuning the magnetic field so it’s perfectly uniform across the scanning area. Even small variations in field strength distort the image, so every MRI scanner relies on two types of shimming.
Passive shimming uses small iron plates placed at calculated positions inside the magnet bore. These plates become magnetized and generate tiny corrective fields that counteract imperfections in the main magnet. The arrangement is calculated uniquely for each scanner and stays fixed once installed.
Active shimming uses adjustable electromagnetic coils that can be independently controlled during each scan. This is necessary because every patient’s body creates its own magnetic field distortions based on their size, shape, and anatomy. Active shimming adapts in real time to compensate for these patient-specific variations, something fixed iron plates can’t do. Together, the two systems ensure the magnetic field is homogeneous enough to produce clear, reliable images.
Joint Replacement Surgery
Surgeons use shims during joint replacement procedures to correct imperfect bone cuts or compensate for bone loss. A shim is inserted between the cut surface of the bone and the prosthetic component, effectively building up the bone side of the joint to balance the gap between the upper and lower bones.
This balancing act matters for two reasons. First, the gap when the joint is bent needs to match the gap when it’s straight, or the joint will feel loose in one position and tight in another. Second, the joint’s side-to-side alignment needs to be correct so weight distributes evenly across the implant. Shims allow the surgeon to fine-tune both of these without relying entirely on releasing ligaments, which was the traditional approach and often required more aggressive soft-tissue work. The technique applies to knees, hips, shoulders, ankles, and virtually any joint where a prosthetic component meets a bone surface.
Why Shim Selection Matters
Choosing the wrong shim material or thickness can create worse problems than the original gap. Wood shims compress over time, which makes them a poor choice for load-bearing industrial applications. Metal shims can corrode if the alloy doesn’t match the environment. Stacking too many thin shims introduces flex and instability, which is why machinists aim for the fewest shims possible to reach the target thickness.
The shape matters too. Using a flat shim where a tapered one is needed leaves a void on one side of the gap, and the part may pivot rather than sit firmly. Using a tapered shim where a flat one is needed tilts the component when all you wanted was a parallel lift. Matching the shim to the specific geometry of the gap is what separates a solid repair from one that causes new problems down the line.