Shim Thickness Selection Guide | How to Choose the Right Shim Stock Thickness
Shim Thickness Selection Guide
How to choose the right shim stock thickness for precision alignment, spacing, and tolerance control.
Selecting shim thickness is not just a “pick one number” decision. In real assemblies, you’ll balance measured gap, load conditions, adjustment flexibility, and material behaviour to achieve stable, repeatable results. This guide outlines a practical, engineering-led approach.
Why shim thickness matters
Shims are used to control spacing, compensate for manufacturing tolerances, and fine-tune alignment. Incorrect thickness selection can contribute to vibration, uneven load distribution, accelerated wear, and loss of dimensional accuracy.
Step 1 — Measure the gap or adjustment required
Start with accurate measurement. Common methods include:
- Feeler gauges for quick gap checks
- Dial indicators for alignment and runout verification
- Laser alignment tools for motor/drive assemblies
- Callipers/micrometers where access allows
Step 2 — Understand load and operating conditions
High-load applications
- Machine base alignment
- Motor mounts and structural supports
- Heavy equipment maintenance
Thicker individual shims can improve rigidity and reduce risk of compression or movement under load.
Precision adjustment applications
- Tooling setups and fixtures
- Calibration and fine spacing
- Assemblies with tight tolerance targets
Thinner shims enable finer tuning, especially where incremental adjustment is required.
Step 3 — Single shim vs stacked shims
Using a single shim
- More rigid
- Lower chance of interlayer movement
- Cleaner installation
Using a stack (layered shims)
- Better fine adjustment on-site
- Easier tuning during install
- Flexible combinations to hit exact values
Step 4 — Consider material behaviour
Material selection influences how a given thickness performs under load and in different environments:
- Stainless steel (304): strong, corrosion-resistant, stable for industrial environments
- Brass: good machinability, common in general engineering and fitting work
- Copper: high electrical/thermal conductivity for grounding and heat transfer applications
Step 5 — Account for tolerances and repeatability
When thickness control is critical, plan for tolerances and repeatability:
- Confirm the tolerance band for the shim stock thickness
- Use layered combinations to hit target values with adjustability
- Record stack configurations for maintenance and future servicing
Typical shim thickness ranges
| Thickness range | Typical use |
|---|---|
| 0.05 mm – 0.10 mm | Fine adjustment, calibration, precision spacing |
| 0.20 mm – 0.50 mm | General alignment, spacing, fit correction |
| 1.0 mm+ | Structural packing, heavy-duty support, larger corrections |
These ranges are general guidance. Final selection should reflect load, vibration, environment, and the need for adjustment flexibility.
Common mistakes to avoid
- Choosing a single thickness without allowing for final tuning
- Over-stacking thin shims in high-load or high-vibration assemblies
- Ignoring material stiffness and the effect of compression under load
- Not considering thermal expansion in temperature-variable environments
- Installing shims on dirty, damaged, or uneven mating surfaces
Engineering best practices
- Prioritise measurement accuracy before selecting shim thickness
- Use layered combinations where fine adjustment is required
- Keep shim surfaces clean and flat during installation
- Minimise stack height where rigidity matters
- Document final shim thickness/stack for maintenance records
Summary
The right shim thickness is achieved by combining accurate measurement with sensible selection strategy: match thickness to the correction required, choose stacking only where it helps, and align material choice with the operating environment. Done properly, shimming improves alignment, reduces wear, and supports long-term reliability.
