No two airframes come together exactly alike. Machined fittings, formed skins, and composite panels each carry their own tolerances, and when parts meet at an interface those tolerances stack. The result is a gap, somewhere, on nearly every assembly. Shims exist to fill those gaps deliberately, so that fasteners clamp structure against structure instead of bending it into contact. Laminated shims, built from thin bonded foils that peel to thickness, are the most flexible tool in that kit. Here is how they work, how they compare with solid and edge-bonded types, and what to watch when you buy them.
Key takeaways
- ✓Shims control fit-up gaps so fasteners clamp parts without bending them; pulling a gap closed with bolt preload builds stress the joint was never sized for.
- ✓Laminated (peelable) shims are stacks of thin foils, typically 0.002 in or 0.003 in per layer, peeled to the required thickness at assembly.
- ✓Solid shims suit gaps that are known and repeatable; edge-bonded laminated stock keeps adhesive out of the peeled surfaces.
- ✓Common materials include aluminum, brass, 300-series stainless, and titanium; composite interfaces raise galvanic compatibility questions.
- ✓A shim in primary structure is a structural part: its stock needs the same lot traceability and certification as any other aerospace hardware.
Why Shimming Exists
Aircraft structure is assembled to drawings that define not just part dimensions but interface gaps. When two parts meet with a gap beyond the allowed limit, simply torquing the fasteners until the gap closes is not an option. Pulling parts together with bolt preload locks a bending stress into the structure, often called pull-up or pre-load stress, that the joint was never sized to carry. In metal structure it invites fatigue problems; in composite structure it can cause delamination, which is one reason composite-heavy airframes are famously shim-intensive.
Assembly documents therefore take the opposite approach: measure the gap, then fill it with a shim of the right material and thickness so the fastener clamps a solid stack. Shims are not a repair, and they are not an admission of error. They are a designed-in feature of how real structure, built from real parts with real tolerances, goes together.
Laminated, Solid, and Edge-Bonded Shims
A laminated, or peelable, shim is a stack of thin metal foils bonded into a single sheet that can be machined and handled as one piece. Its trick is adjustability: at assembly, layers peel away one at a time until the shim matches the measured gap. One part number covers a whole range of final thicknesses, which is exactly what you want when the gap is not known until the parts meet.
A solid shim is a single piece of material at a fixed thickness, machined or ground to suit. It offers maximum stiffness with no layers to separate, and it makes sense where the gap is known in advance and repeatable from unit to unit. Edge-bonded laminated stock is a variant in which the foils are typically bonded only along the edges rather than across their full faces, which keeps adhesive out of the peeled surfaces. Which of the three an interface gets is usually spelled out by the drawing or the applicable process specification.
Foil Thickness and Build
Peelable shim stock is typically built from foils of 0.002 in or 0.003 in per layer, laminated up to a range of overall sheet thicknesses. The foil thickness sets the resolution of adjustment: with 0.002 in laminations, the assembler can match the measured gap to within about a thousandth either way. Drawings commonly bound both the maximum total shim thickness at an interface and the minimum that may remain after peeling, since a shim peeled down to its last layer or two behaves more like foil than structure. As always, the applicable drawing or specification, not habit, sets those limits.
Material Choices
Shim stock comes in most of the materials the surrounding structure does. Aluminum alloys are light and easy to peel and trim. Brass peels cleanly and machines well. Corrosion-resistant steels, such as the 300-series stainless grades, are the default in many structural applications for strength and corrosion behavior. Titanium appears where weight, temperature, or galvanic compatibility with carbon fiber composites matters, since aluminum against carbon composite creates a galvanic couple that designers typically avoid. Non-metallic laminated stock, such as polyester, exists as well for lighter duty applications. The drawing normally dictates the material; where it offers options, match the shim to the structure it touches.
Traceability: Shim Stock Is Not a Shop Supply
Because shim stock ships as sheets and gets trimmed at assembly, it is tempting to treat it like consumables. It is not. A shim installed in primary structure is a structural part, and its material needs the same traceability as the fitting it sits under: manufacturer identity, lot control, and certification passed through intact. This is exactly the discipline AS9120 audits in a distributor, including the rules for splitting a lot without breaking its link to the original certificates. We covered how that chain works for Boeing-standard hardware in our BACB28 bushing guide, and the logic for shim stock is the same. On receipt, the certificate of conformance should tie the sheet in your hands to a manufacturer and a lot, not just to a material description.
Choosing and Buying Shims
- Start from the drawing: allowed shim materials, maximum total thickness, minimum remaining thickness, and whether peelable stock is permitted at that interface.
- Match the type to the workflow: peelable stock when gaps are measured at assembly, solid shims when the thickness is fixed and known in advance.
- Browse the shims category or start from our aerospace shims overview, and quote against the full callout including material and thickness.
- Check the paperwork before you order: every Instaparts shipment includes a certificate of conformance with full traceability.