Shims and shim stock are used to fill in a space between two mating surfaces or components. Shim stock consists of thin pieces of metal, plastic or other material used to fill space between objects for support, leveling, or adjustment of fit. Shims and shim stock can also act as electrical insulation and seal joints. Valve shims are used to adjust valve spring pressure. Laminated shims allow layers can be peeled away to produce a shim of required thickness. A carpenter or cabinet-maker can use small pieces of wood (shims) to fill gaps between larger pieces. Bearing shims are used for bearing adjustment. A shim kit contains shims of varying thicknesses and sizes arranged in a carrying case with labels for easy selection of the right size shim.
Shims and shim stock can be purchased in sheets and/or rolls depending on thickness, color, and tolerance. Metric shims are specified in units such as millimeters (mm). Metal shims can be 304 stainless steel, carbon steel, aluminum, copper, and brass. Slotted shims are used on assembled machine components. The slot aligns the shim to be installed without disassembling the rotating shaft components or motor mounts. Plastic shims are color-coded for quick, accurate recognition of thickness at time of use. Custom shims can be fabricated to required specifications, including material, size, and finish. Shims and shim stock can be thin or thick, tapered or wedged.
Standard Shims for Aircraft
Standard shims for aircraft can be identified by manufacturer and part number, for example Bell Helicopter 120-062 and 120-098, or Boeing Airplane Company BACF3F or BACF3T. Shims and shim stock meet standards set by ASTM International (formerly called the American Society for Testing and Materials (ASTM), for example, AS5371 for notched shims for magnetic particle inspection and C1115, which describes products composed of dense elastomeric silicone rubber that are fabricated into gaskets and accessories (such as setting blocks, spacers, and shims). Shims and shim stock can be designed to meet specific industry requirements, such as the Society of Automotive Engineers (SAE) specification for filled-resin compound shims (SAE AMS3726C).