A shim is one of the most underrated items in a maintenance fitter's toolkit. Half a millimetre of steel — cut from a roll and slipped under a motor foot — is the difference between a pump that runs reliably for five years and one that consumes bearings every six months. In construction, a plastic packer wedged under a door frame costs almost nothing and saves a door that would never hang correctly. In a precision engine, a valve shim ground to 0.025 mm changes everything about how that engine performs.
Despite their simplicity, shims are widely misunderstood. People confuse them with washers and spacers. They stack too many. They reach for a cedar wedge when the job needs precision steel. They choose the wrong material for the environment — and end up with corroded steel in a food plant or deformed plastic under a two-tonne motor.
This guide covers the full picture: what shims are, how they differ from washers and spacers, every type you will encounter in Australian industry and construction, how to select the right material, how to choose and measure thickness, the rules around stacking, structural and load-bearing considerations, and specific applications from machinery alignment to excavator pins. Written for the Australian trade and industrial market, with products stocked at AIMS Industrial from Champion and Precision Brand.
Shim Materials: Steel, Brass, Stainless & Plastic Compared — Quick Reference
Material selection is where shim choices most often go wrong. The wrong material in the wrong environment corrodes, deforms, or introduces contamination.
| Material | Strength | Corrosion Resistance | Relative Cost | Best Applications |
|---|---|---|---|---|
| Cold-rolled steel | High | Low — will rust | Low | General industrial, dry indoor environments, machinery alignment |
| Stainless steel 304 | High | Excellent | Medium | Food processing, pharmaceutical, washdown environments |
| Stainless steel 316 | High | Excellent (chloride) | Medium–high | Marine, coastal, chemical plant, chlorinated water |
| Brass | Medium | Good (atmospheric) | Medium | Electrical equipment, precision instruments, non-magnetic applications |
| Aluminium | Low–medium | Good | Medium | Aerospace, lightweight applications |
| HDPE / PP plastic | Low | Excellent | Very low | Construction framing, door and window installation |
What Are Shims and What Do They Do?
A shim is a thin, flat piece of material inserted between two surfaces to fill a gap, correct alignment, level a component, or achieve a precise fit. The principle is ancient — craftspeople have been using wedges and spacers to compensate for imperfect dimensions since before recorded engineering. The materials and tolerances are modern; the idea is not.
The core function of a shim is to compensate for dimensional variation that cannot be designed or manufactured out of a system. No surface is perfectly flat. No concrete slab is perfectly level. No motor foot sits at exactly the right height after installation on a real-world base. Shims correct for the imperfection that engineering drawings assume away — they are the bridge between the ideal dimension and the actual one.
In practice, shims perform four distinct functions:
- Gap filling — closing a space between two mating surfaces with precise control over the final gap dimension (e.g., head gasket shims, cylinder head shims, bearing cap shims)
- Alignment correction — raising or lowering one side of a machine to achieve shaft concentricity and angularity within specification (e.g., motor foot shimming, pump alignment, gearbox installation)
- Levelling — bringing a surface to a known datum, typically horizontal, so a machine or structure sits correctly (e.g., levelling a machine tool on a slab, a base plate for a column, a structural beam bearing)
- Preload and clearance adjustment — setting the force applied to a spring, bearing, or valve element (e.g., valve train shims for tappet clearance, differential bearing preload, hydraulic relief valve pressure setting)
The applications span every industrial sector in Australia: manufacturing, food processing, mining, civil construction, marine, agricultural equipment, and automotive. Anywhere two components need to fit precisely — and the precision cannot be machined in after the fact — a shim is the answer.
Shims are cheap. The consequence of getting them wrong is not. A misaligned motor on the wrong shim stack runs hot, vibrates, and fails prematurely. A door frame packed with a timber offcut shifts over time and the door sticks. A base plate shimmed with compressed plastic settles and the column goes out of plumb. Use the right shim for the job.
Shims vs Washers vs Spacers: Key Differences Explained
The confusion between these three items comes from appearance — they all look like flat things that go between surfaces. The function is where they diverge, and understanding the difference matters for selecting the right component.
What a Washer Does
A washer is a fastener component. Its job is to distribute the clamping load from a bolt head or nut across a larger surface area, preventing the fastener from embedding into soft material or pulling through a large hole. Spring washers (Belleville or helical) add a locking function. Repair washers have an oversized outer diameter for use with damaged holes.
Washers are manufactured to loose dimensional tolerances — a standard flat washer to DIN 125 or AS 1237 has a nominal thickness but that thickness is not a precision measurement. You would never use a standard washer to fill a 0.15 mm gap — you have no reliable idea what thickness you are actually installing. Washers go under fasteners. They do not fill precision gaps.
What a Spacer Does
A spacer maintains a fixed, known distance between two components. Spacers are typically thicker than shims — often a machined cylindrical or tubular component — and their purpose is to hold components at a set distance during assembly. Wheel spacers on a vehicle hub, standoffs in an electronics enclosure, and bearing spacers in a gearbox are all spacers. They are not adjustable. They set a dimension and hold it.
What a Shim Does
A shim is the adjustment tool. It is manufactured to tight thickness tolerances specifically so that you can select — or cut to — the exact dimension you need to fill a measured gap or correct a measured misalignment. The tolerance of quality shim stock is plus or minus 0.003 mm or better. That is the whole point: you measure, you select, you trust the result.
In summary: washer = distributes clamping load under a fastener. Spacer = holds components at a fixed set distance. Shim = fills a measured gap, corrects alignment, achieves a precise fit.
There is one area where the terms overlap: in structural and heavy equipment work, a thick steel plate used under a base plate may be called a shim plate in some documentation even though it functions more like a spacer. What matters is the function — precision gap filling and adjustment — and selecting material manufactured to tight enough tolerances to do it reliably.
Types of Shims: A Complete Overview
The shim category is broader than most people realise. Understanding the different types — and what each is designed for — prevents the wrong type ending up in the wrong application.
Shim Stock (Rolls and Flat Sheets)
Shim stock is precision-rolled metal available in continuous rolls or flat sheets at controlled thicknesses. The user cuts the shim to any shape required — custom footprints, specific slot positions, unusual profiles. This is the most versatile shim format, and it is what most people mean when they refer to "shim stock."
Standard widths for rolls are 150 mm or 300 mm. Sheet sizes vary by supplier — 300 × 300 mm and 300 × 600 mm are common. Thicknesses range from 0.025 mm (1 thou) to 3.0 mm or heavier, with a full range of intermediate gauges. AIMS stocks shim stock in cold-rolled steel, stainless steel 304 and 316, and brass from Precision Brand and Champion.
Slotted Shims (Horseshoe Shims / Alignment Shims)
Slotted shims — called horseshoe shims or U-shims in the trade — have a slot cut from one edge through to a central opening. The slot allows the shim to slide around a bolt or shaft without removing the fastener. You loosen the hold-down bolt, slide the shim stack in or out, then re-torque. This design is the standard for motor and machinery alignment work. The machine does not need to be completely disassembled to adjust the shim stack — a significant time saving on any alignment job.
Slotted alignment shim kits include multiple thicknesses so the technician can build the required correction by stacking. AIMS stocks these kits for standard motor foot sizes.
Tapered Shims
A tapered shim has a wedge profile — thicker at one end, thinner at the other — giving a uniform taper across its length. Tapered shims are used to correct angular misalignment, where one side of a component sits higher than the other and a uniform-thickness shim would not resolve the angular error.
They appear in structural steel work (under base plates on slightly sloped concrete), in some machinery installations, and in automotive applications. Two tapered shims pushed in from opposite ends create an effective shim of adjustable thickness — a useful field technique when standard thicknesses are not available.
Laminated (Peelable) Shims
Laminated shims consist of multiple thin metal layers bonded together into a single assembly. When the total assembled thickness is too much, individual layers are peeled off to reduce thickness — no cutting required. The precision of each remaining layer is maintained because the layers are controlled during manufacture.
Laminated shims are used in production tooling, precision fixtures, and applications where fast, clean adjustment matters without the complexity of managing a loose multi-piece stack. They cost more than plain shim stock but eliminate several practical problems.
Plastic Shim Packers (Construction Packers)
Plastic packers — called shim packers in the Australian construction trade, or simply "packers" on site — are non-compressible plastic blocks used to level and align frames, windows, doors, and structural elements. Made from HDPE or polypropylene, they are moisture-resistant, do not rot, do not compress under construction loads, and are UV-stable.
Plastic packers are stackable and come in standard widths (28 mm, 68 mm, 100 mm) and thicknesses from 1 mm to 20 mm. They are a construction-site daily consumable in Australia — every joinery and framing installation uses them.
Valve Shims
Valve shims are precision-ground discs used in overhead cam engines to set valve clearance (tappet clearance). They sit between the cam follower (bucket) and the valve stem end. The clearance is measured with a feeler gauge and the shim thickness is selected from a range — typically in increments of 0.025 mm or 0.05 mm — to bring the clearance within the manufacturer's specification.
Brake Shims
Brake shims are anti-squeal pads bonded to the back of disc brake pads, or inserted between the pad and the caliper piston. They dampen vibration and reduce brake noise. This is a specific automotive application outside AIMS's core industrial range but worth noting as a distinct shim category — a brake shim is not interchangeable with a machinery alignment shim.
Cylinder Head and Gasket Shims
In high-performance engine building, cylinder head shims adjust compression ratio or correct deck height after machining. They sit between the cylinder head and engine block, on top of the head gasket. These are precision components manufactured to very tight flatness and thickness specifications.
Shim Materials: Steel, Brass, Stainless & Plastic Compared
Material selection is where shim choices most often go wrong. The wrong material in the wrong environment corrodes, deforms, or introduces contamination. Here is a clear comparison of each material's properties and the applications they suit.
Cold-Rolled Steel (CRS)
Cold-rolled steel shim stock is the most widely used industrial shim material. It offers high compressive strength, consistent thickness tolerances, excellent formability, and low cost. The manufacturing process — rolling at room temperature — produces a smooth, bright surface finish and tight dimensional control.
The limitation is corrosion: uncoated cold-rolled steel will rust in any environment with moisture, chemicals, or salt. In dry indoor environments, steel shims are the default choice. In outdoor, wet, chemical, or food-processing environments, upgrade to stainless steel.
Stainless Steel 304
Grade 304 stainless steel (18% chromium, 8% nickel) handles water, most dilute acids and alkalis, organic compounds, and general industrial chemical exposure without significant corrosion. It is the standard material for food processing equipment, pharmaceutical plant, and any application requiring regular washdown with detergents or sanitisers. Stainless 304 shim stock costs roughly two to three times more than equivalent carbon steel, but in corrosive environments that cost premium pays back in reliability.
Stainless Steel 316
Grade 316 adds 2–3% molybdenum to the 304 composition, providing superior resistance to chloride-induced pitting corrosion. 316 is the correct choice for marine environments, coastal installations, chlorinated water systems, and chemical plants handling chlorine compounds or strong acids. If the application involves salt water, seawater spray, or aggressive chloride exposure, use 316 — not 304.
Brass
Brass shim stock is non-magnetic, has good thermal and electrical conductivity, and is soft enough not to score or gall precision mating surfaces. These properties make brass the preferred choice in electrical switchgear, precision instruments, and any application where magnetism would cause problems. Brass is softer than steel — do not use brass shims in high-load structural applications where the shim must resist deformation under compressive stress.
Aluminium
Aluminium shim stock is lightweight, corrosion-resistant in most environments, and easy to cut and form. It is used in aerospace, automotive, and applications where weight matters. Its lower compressive strength makes it unsuitable for heavy-load industrial shimming — use steel for machinery.
Plastic (HDPE and Polypropylene)
HDPE packers are the construction trade standard for framing and window installation: non-compressible under typical construction loads, moisture-proof, rot-proof, and UV-stable. Polypropylene packers are slightly stiffer and more brittle in cold conditions. Neither is appropriate under heavy industrial equipment — use steel for any machine base shimming application.
| Material | Strength | Corrosion Resistance | Relative Cost | Best Applications |
|---|---|---|---|---|
| Cold-rolled steel | High | Low — will rust | Low | General industrial, dry indoor environments, machinery alignment |
| Stainless steel 304 | High | Excellent | Medium | Food processing, pharmaceutical, washdown environments |
| Stainless steel 316 | High | Excellent (chloride) | Medium–high | Marine, coastal, chemical plant, chlorinated water |
| Brass | Medium | Good (atmospheric) | Medium | Electrical equipment, precision instruments, non-magnetic applications |
| Aluminium | Low–medium | Good | Medium | Aerospace, lightweight applications |
| HDPE / PP plastic | Low | Excellent | Very low | Construction framing, door and window installation |
Shim Stock: What It Is and When to Use It
Shim stock is the raw form of the shim world — precision-rolled metal that you cut to the exact size, shape, and configuration you need. When no standard off-the-shelf shim fits the job, shim stock is the answer.
Why Tolerance Matters
The defining characteristic of quality shim stock is thickness tolerance. Precision Brand shim stock maintains thickness within plus or minus 0.003 mm for fine gauges (0.025 mm to 0.25 mm) and plus or minus 0.005 mm for heavier gauges. This means a shim labelled 0.127 mm (5 thou) is reliably 0.124–0.130 mm — narrow enough that you can trust the measurement when stacking shims to reach a calculated alignment correction.
Low-grade shim material with wide thickness tolerances undermines the whole point of precision shimming. If your 0.1 mm shim is actually anywhere from 0.095–0.108 mm, your alignment calculation is invalid from the start.
Standard Thickness Range
Shim stock is available across a wide range of thicknesses. The Australian trade uses both metric and imperial (thou) designations — both systems are in active use. Common thicknesses:
- 0.025 mm (1 thou) — ultra-fine adjustment, precision instruments, valve shims
- 0.050 mm (2 thou) — fine machinery alignment, bearing preload
- 0.075 mm (3 thou) — general alignment work
- 0.100 mm (4 thou) — general alignment, one of the most used sizes
- 0.125 mm (5 thou) — very common for motor foot shimming
- 0.150 mm (6 thou) — standard alignment thickness
- 0.175 mm (7 thou) — intermediate correction
- 0.250 mm (10 thou) — heavier correction
- 0.500 mm, 0.750 mm, 1.000 mm — structural shimming and base work
- 1.5 mm, 2.0 mm, 3.0 mm+ — heavy structural shimming, excavator pins
Conversion note: 1 thou (thousandth of an inch) = 0.0254 mm. If your alignment software outputs results in thousandths of an inch, convert before selecting shims. Many experienced alignment technicians in Australia work in thou by preference — both units are entirely valid.
Roll vs Sheet
Rolls are better for operations that regularly cut custom shims — continuous supply, easier to handle when cutting strips or long narrow pieces. Flat sheets are more practical for one-off jobs and benchtop cutting — the stock lies flat without the spring-back tendency of a roll. Both formats are available from AIMS across steel, stainless, and brass.
When to Use Shim Stock vs Pre-Cut Shims
Use shim stock when: the required shim shape is non-standard, the slot position does not match standard slotted shims, a continuous strip is needed, or you need a specific material and thickness not available pre-cut. Use pre-cut slotted shims when: doing standard motor alignment, speed matters, or you are working from a kit.
Shimming for Machinery Alignment and Levelling
Machinery alignment is the most consequential application for precision shims in Australian manufacturing, processing, and mining. Motor-to-pump alignment, gearbox installation, compressor mounting, conveyor drive shimming — all depend on shims at the machine feet to achieve shaft concentricity and angularity within the coupling manufacturer's specification.
Why Alignment Matters
A misaligned coupling generates vibration, uneven bearing load distribution, elevated operating temperature, and accelerated seal and coupling wear. Industry data consistently attributes 50% or more of premature rotating machinery failures to misalignment. The bearing that should last 40,000 hours fails in 8,000. The mechanical seal rated for two years goes in six months. The coupling insert that should last years needs quarterly replacement.
Proper shimming and alignment is one of the highest-return maintenance activities in any plant. The cost of a set of alignment shims and an hour of a technician's time is a fraction of the cost of a failed bearing, an emergency motor rewind, or unplanned production downtime.
Types of Misalignment Shims Correct
- Parallel (offset) misalignment — shaft centrelines are parallel but offset from each other. Corrected by moving the motor sideways (horizontal) or shimming feet (vertical).
- Angular misalignment — shaft centrelines meet at an angle. Corrected by shimming the front or rear feet of the motor by different amounts to change the shaft angle.
Most alignment jobs involve both types simultaneously. Laser alignment equipment measures both and calculates the exact shim thickness required at each of the four feet.
The Alignment Shimming Process
- Soft foot check first — Loosen each hold-down bolt in turn and measure whether the machine lifts. Soft foot creates measurement errors that make alignment impossible to achieve cleanly. Correct it by shimming the lifting foot until all four feet sit solidly.
- Measure misalignment — Laser alignment equipment or dial indicators measure offset and angularity. Laser systems calculate the required shim corrections at each foot automatically.
- Select shims — Choose slotted shims in the required thickness, or stack to achieve the total correction. Keep stacks to three or fewer shims where possible.
- Insert and torque — Slacken the hold-down bolt, slide the shim in, re-torque to specification, re-measure. Repeat until within coupling tolerance.
- Document the result — Record the final shim stack at each foot, pre- and post-alignment readings, and date. This is the baseline for the next alignment check.
Levelling a Machine Base
For new machine installations on a concrete slab, steel shims bring the base plate to level before the void is grouted. Place shim stacks at each support point, level with a precision spirit level or laser level to within 0.05 mm/m or better, then fill the void with non-shrink epoxy grout. The shims become a permanent load-carrying component embedded in the grout.
Shim Packers in Construction: Doors, Windows and Frames
In the Australian construction trade, "shim packers" or simply "packers" are a daily site consumable on any framing, joinery, or window installation job. The term is distinctly Australian — in the UK they are called packing pieces; in the US, shims or shim wedges. In Australia, ask for packers or shim packers.
Why Frames Need Shimming
No wall opening or floor surface is perfect. Concrete slabs have surface variation. Wall studs bow slightly. Masonry openings are rarely square. To install a door or window correctly — plumb, level, and square — the frame must be adjusted to compensate for the imperfection of the opening it sits in. Packers fill the gap between the perfect frame and the imperfect opening, allowing precise control of position without modifying either.
Getting this right matters: a door frame that is not plumb creates a door that swings open or closed on its own, or binds in the frame. A window sill that is not level causes water pooling. Two minutes spent correctly packing a frame saves significant remediation later.
Door Frame Installation
Place packers at hinge locations (every hinge position must be backed by a packer so the fixing screw goes into solid material behind the frame), at the strike plate location, and at the head. Start at the bottom: set the first packer to bring the bottom of the hinge jamb to plumb and level, then work upward. Check plumb on both jambs and level on the head before fixing permanently.
Window Frame Installation
Window sills must be level across their full width — check with a long level and shim up the low end. Jambs must be plumb — shim at the top and bottom of each jamb as needed. Use the same stackable approach: measure the gap at each packer position and select the combination of thicknesses that fills it without gaps or forcing.
Standard Packer Sizes
- Widths: 28 mm, 68 mm, 100 mm — matching common stud and frame widths
- Thicknesses: 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 5 mm, 6 mm, 8 mm, 10 mm, 12 mm, 15 mm, 20 mm
- Length: typically 100 mm
A practical site kit carries 1 mm, 2 mm, 3 mm, 5 mm, and 10 mm packers — which combine to hit any required thickness from 1 mm to 20 mm+ without needing every possible size. Colour-coding by thickness (common in quality packer ranges) makes grabbing the right packer fast without measuring each piece.
Precision Shims for Engineering Applications
Beyond construction and routine machinery alignment, shims perform critical functions in precision engineering — applications where tolerances are tight and errors have direct mechanical consequences.
Valve Train Shimming
In overhead cam engines — common in modern diesel and petrol equipment — valve clearance (often called "tappet clearance" in the Australian trade) is set by selecting a shim disc of the correct thickness between the cam follower and the valve stem end. The clearance is measured with a feeler gauge at the specified temperature (usually cold), the existing shim is measured with a micrometer, and the correct replacement is selected from a range covering typically 2.5 mm to 3.5 mm in 0.025 mm steps.
Incorrect valve clearance causes noisy valve operation (too much clearance) or poor valve closing and potential burning (too little). This is not a task where close is good enough — which is why valve shims are manufactured to tolerances of plus or minus 0.01 mm or better.
Bearing Preload
Tapered roller bearings in differentials, wheel hubs, and gearboxes require a specific preload — a controlled compressive force applied during assembly. Shims or collapsible spacers set this preload during build. Too little preload and the bearing runs loose, generating noise and heat. Too much and it overloads and overheats. Setting bearing preload requires proper measurement (rolling torque method) and correct shim selection — not a feel-based approximation.
Hydraulic Relief Valve Pressure Setting
Pressure relief valves in hydraulic circuits use a spring-loaded element set by shims between the spring end and the valve body. Adding shims raises the relief pressure; removing shims lowers it. Adjustments of 0.1 mm per shim can change the relief pressure by several bar — making this a precision shim application despite its straightforward appearance.
Machine Tool Calibration and Fixturing
In CNC and manual machining, shims adjust cutting tool heights, align workholding fixtures to a known datum, and compensate for tool variation in production jigs. Required adjustments are often in the 0.01–0.1 mm range — achievable with quality shim stock and proper measurement. Shimming is the standard production-floor method for fine calibration adjustments without the cost and time of machining.
How to Choose the Right Shim Thickness
Choosing the right shim thickness starts with measurement — not estimation, and not by trying shims until one fits. Here is the process for getting it right.
Step 1: Measure the Gap
For gaps under 1 mm: Use a feeler gauge (thickness gauge). A feeler gauge set provides blades from 0.05 mm to 1.0 mm or more. Insert blades until the correct thickness is found — the blade should slide through with light, consistent drag. Intermediate gaps are bridged by stacking two blades.
For gaps over 1 mm: Use a digital vernier caliper for direct measurement, or a dial test indicator against a known datum.
For machinery alignment: Laser alignment equipment measures offset and angularity at the coupling and calculates the exact correction required at each machine foot. Shim selection follows from this calculation — no manual gap measurement is needed in modern laser alignment work.
Step 2: Select or Build the Thickness
If a single shim at the measured thickness is available, use it. If not, stack shims to achieve the total. Keep the number of pieces to three or fewer. For example, a 0.375 mm gap can be filled with three 0.125 mm shims, or with one 0.25 mm plus one 0.125 mm — the two-piece stack is more stable and easier to handle.
Step 3: Test Fit Before Final Assembly
Fit the shim or stack into the gap before final torquing. The shim should slide in with slight resistance — not fall in freely (under-size) and not require force (over-size). A shim that must be hammered in is deforming the gap it is supposed to fill precisely. Once the fit is confirmed, torque to specification and re-check the measurement after torquing, as bolting can shift the shim position slightly.
Common Thickness Sets to Stock
For a typical industrial maintenance situation, stocking 0.025, 0.050, 0.075, 0.100, 0.125, 0.150, 0.200, 0.250, 0.500, and 1.000 mm gives the flexibility to hit almost any required thickness within 0.025 mm by stacking. A slotted alignment shim kit from AIMS covers the range needed for motor foot shimming in ready-to-use horseshoe form.
Can You Stack Shims? (and How Many Is Too Many)
Yes — stacking shims is entirely acceptable and is standard practice in alignment and gap-filling work. The question is where the practical limit lies and how to do it correctly.
Why Stacking Works
Quality shim stock is rolled to a known thickness within a tight tolerance. Stacking three 0.125 mm shims gives a total of 0.375 mm, and because each individual shim is accurate to plus or minus 0.003 mm, the cumulative error of the stack is plus or minus 0.009 mm — well within the tolerance of most alignment applications. The dimensional accuracy of a properly stacked shim assembly is entirely adequate for the tasks shims are used for.
Where Stacking Causes Problems
The limitation of stacking is physical, not dimensional. As the stack grows:
- The stack becomes less stable under vibration and can shift, particularly if individual shims are not held firmly by the clamping load
- Slotted shims become harder to insert cleanly as the stack thickness increases
- In corrosive environments, individual shims can corrode together, making future removal difficult
- The total number of loose pieces increases — more opportunities for pieces to fall, be mislabelled, or end up in the wrong position during reassembly
The Practical Rule
Three to four shims maximum in a single stack for alignment and precision work. For corrections exceeding 3–4 mm, use a machined spacer plate or a single thick steel shim rather than a tall stack of thin ones. For corrections under 0.3 mm, a single shim is always better than two if one is available at the right thickness.
Stacking Best Practices
- Place thicker shims at the bottom and thinner shims on top — stable base, fine adjustment at the top
- Use the same alloy throughout the stack — mixing carbon steel and stainless can lead to galvanic corrosion bonding them together in wet environments
- In outdoor or corrosive environments, apply a thin coat of anti-seize between shims to prevent bonding
- Mark the thickness of each shim with a permanent marker before assembly — you will need that information at the next service
- Consider laminated shims as an alternative to loose stacks for applications requiring fine, repeatable adjustment
Are Shims Structural? Load-Bearing Considerations
Steel shims carrying structural loads is not unusual — it is the designed intent in many applications. Column base plates, machine mounting pads, and structural steel connections all routinely use steel shims as permanent load-carrying components. The question is whether the right material is selected and whether the application is within its limits.
Steel Shims in Structural Applications
Cold-rolled steel and stainless steel shims have high compressive strength — well above the bearing stresses typically encountered in structural base plate connections or machinery mounting. A stack of steel shims under a bolted base plate, properly installed and grouted, is a permanent structural element that carries the full column or machine load.
For structural steel work in Australia, AS 4100 (Steel Structures) governs base plate connections. Where shims are specified, they should be structural-grade steel, sized to fully cover the bearing area, and grouted in position after the structure is aligned. Check with the structural engineer for specific shim size and material requirements — these will be in the drawings or engineer's notes.
Machinery Mounting Loads
Under an industrial motor or pump, the machine foot bears the combined static weight of the machine plus dynamic loads from vibration and torque reaction. For a properly installed, bolted-down machine, these loads are largely compressive — and steel shims handle compressive loads well. The shim stack should cover the full area of the machine foot where possible, distributing the load evenly rather than concentrating it.
What Cannot Carry Structural Load
Timber (cedar, pine, hardwood): Wood under sustained compressive load compresses, creeps, and deforms over time — meaning a machine that is correctly aligned today will be out of specification in six to twelve months. Timber also rots, swells with moisture, and provides no predictable compressive performance. Cedar shims are a legitimate tool for temporary positioning during installation; they are not a permanent solution in any structural or machinery application.
Plastic packers under heavy machinery: HDPE construction packers are rated for construction-level loads in frame and window installation. They are not rated for the sustained compressive loads of industrial machinery. Do not substitute construction plastic packers for steel shims under motor feet, pump bases, or any heavy industrial equipment.
Shims for Excavators and Heavy Equipment
Heavy earthmoving equipment — excavators, loaders, bulldozers, cranes — uses shims in several critical locations. These are high-load, high-vibration, outdoor environments with mud, water, and aggressive conditions. The shims used here are thick, high-strength steel — nothing like the thin alignment shims used on electric motors.
Excavator Pin Shimming
Excavator buckets, arms, and booms connect via large-diameter steel pins running through bronze or steel bushes. As the bushes wear — under the constant loading and cycling of digging — lateral play develops at the pin joint. The bucket wiggles side-to-side in the boss rather than tracking straight, reducing dig accuracy, increasing loading on the pin and boss faces, and accelerating further wear in a self-worsening cycle.
Steel shims take up this lateral play. The pin is removed, a shim of the appropriate thickness is fitted between the boss face and the machine structure on one or both sides, and the pin is refitted. The shim reduces total lateral clearance to within OEM specification — typically less than 1–2 mm for most excavators. Pin shims for this application are thick (typically 3–6 mm) and manufactured from high-strength steel to handle the side loads in the joint.
Always check the OEM service manual for the specific machine and joint: maximum allowable play and the correct shimming procedure vary by machine model.
Undercarriage Components
Track tension on crawler equipment is adjusted via a hydraulic tensioner, but shims may be used during track reassembly and component replacement to set initial dimensions and compensate for worn components. Undercarriage shimming is a specialist task requiring knowledge of OEM service specifications.
Structural Base Plates and Outrigger Support
On mobile cranes, elevated work platforms, and other outrigger-supported equipment, base plate shimming may be used to level the machine on uneven ground before operation. These applications use thick steel shims or machined steel plates — not standard alignment shims. Load capacities are high, and correct support is critical for operational safety.
How to Measure and Cut Shim Stock
The ability to cut your own shim from stock is one of the most useful capabilities in a workshop. The process is simple, but the details matter for a result that is accurate, burr-free, and safe to handle.
Measuring and Marking
Mark the shim profile on the stock material using a fine-tip permanent marker or a scriber. For straight-edged shims, use a steel rule and scriber. For complex shapes, make a paper or cardboard template first, trace around it, then cut. For slotted shims, mark both the outer profile and the slot position carefully — the slot must align with the bolt centre. Measure twice, cut once.
Cutting Methods by Thickness
- 0.025–0.100 mm (1–4 thou): Sharp scissors or shim-cutting scissors. At these thicknesses, the material cuts like thin metal foil. Handle carefully — the edges are sharp.
- 0.100–0.500 mm (4–20 thou): Aviation snips (compound action tin snips) for straight cuts, curves, and complex shapes. Left-hand and right-hand snips are available. Keep blades sharp — dull snips fold and buckle the edge rather than cutting clean.
- 0.500–1.500 mm (20–60 thou): Aviation snips for shorter cuts; a metal-cutting bandsaw for long straight cuts. Stainless steel in this range work-hardens quickly — a bandsaw is cleaner than snips.
- Over 1.5 mm: Metal-cutting bandsaw, angle grinder with cutting disc, or guillotine shear. Mark the cut line clearly, clamp the stock securely, and use eye and hand protection.
Cutting the Slot in a Horseshoe Shim
To cut the slot from flat stock for a horseshoe shim, use the drill-and-snip method: drill a clearance hole at the inner end of the slot (matching or slightly larger than the bolt diameter), then cut down both sides of the slot from the outer edge to the drilled hole using aviation snips. The drilled hole gives a clean radius at the inner end of the slot rather than a sharp corner, which can become a stress riser under repeated loading.
Deburring
Any cut edge on metal shim stock will have a burr. Deburr all cut edges before fitting — a burred edge will damage mating surfaces, prevent the shim from sitting flat, and is a laceration hazard during handling. Use a fine file, a deburring tool, or fine abrasive paper on a flat surface. For thin shim stock, draw a flat file lightly across the edge — one or two strokes is enough. Do not over-file.
Marking Shims Before Assembly
If the shim is going into an installation that will be disturbed in future — a motor that will need re-alignment, a base plate that may be lifted — mark the shim thickness with a permanent marker before assembly. When the machine comes apart at the next service, you know immediately what is in the stack without having to micrometer every piece. It takes ten seconds and saves significant time later.
Where to Buy Shims in Australia
AIMS Industrial stocks a comprehensive range of precision shims and shim stock for Australian industrial, construction, and engineering applications. The range includes shim stock rolls and flat sheets in cold-rolled steel, stainless steel 304 and 316, and brass across a full range of thicknesses from 0.025 mm upward; slotted alignment shim kits for motor and machinery alignment work; plastic HDPE shim packers for construction framing, door, and window installation; and specialty shim products from Champion and Precision Brand.
All products are available online with delivery to anywhere in Australia. For technical advice on material selection, thickness specification, or choosing the right shim format for a specific application, contact the AIMS Industrial team.
Browse Shims & Shim Stock at AIMS Industrial →
For GD&T symbols and their meanings under Australian and international standards, see our GD&T Symbols Guide.
For dry and lubricated torque values across all common metric bolt grades, see our Metric Bolt Torque Chart.
