Gasket Compression Tolerance Stack-Up
Gaskets and seals need the right amount of compression to seal properly. Too little compression causes leaks; too much crushes the gasket and causes extrusion or premature failure. This guide shows how to build a tolerance stack-up that controls gasket compression across all part variations.
Free Height vs. Compressed Thickness
Every gasket has two key dimensions:
- Free height — The uncompressed thickness as manufactured (what you measure before assembly)
- Compressed thickness — The thickness after the lid is bolted down (determined by the metal-to-metal stack)
Compression = Free height − Compressed thickness
The goal of the stack-up is to predict the compressed thickness, then calculate whether the resulting compression falls within the gasket manufacturer's recommended range.
Compression as Percentage
Gasket specs often express compression as a percentage: (Free height − Compressed thickness) / Free height × 100%. A 3.0 mm gasket compressed to 2.4 mm has 20% compression. Most elastomeric gaskets target 15–30% compression.
The Compression Stack
In a typical lid/housing assembly, the gasket sits in a groove or on a flange face. When the lid bolts down, it compresses the gasket until metal surfaces meet (or until the bolts bottom out). The compressed thickness equals the gap left for the gasket.
Stack Contributors
| Component | Dimension | Effect on Gap |
|---|---|---|
| Housing groove depth | Machined pocket for gasket | + Increases gap |
| Lid recess depth | Machined pocket in lid (if present) | + Increases gap |
| Housing flange flatness | Deviation from flat | ± Varies gap locally |
| Lid flatness | Deviation from flat | ± Varies gap locally |
| Hard stop features | Metal-to-metal contact surfaces | Defines final gap |
Two Assembly Types
Metal-to-metal contact: The lid seats against the housing flange. The gasket groove depth alone determines compressed thickness. This is the most common design.
Gasket-controlled gap: No metal-to-metal contact—the gasket itself is the hard stop. Compressed thickness depends on bolt preload and gasket stiffness. This requires a different analysis approach (load-deflection curves).
This guide covers the metal-to-metal case, which is a pure dimensional stack-up.
Gasket Crush Limits
Every gasket material has compression limits:
| Condition | Typical Range | Consequence |
|---|---|---|
| Under-compression | < 10–15% | Insufficient seal, leaks under pressure |
| Target range | 15–30% | Good seal, long life |
| Over-compression | > 30–40% | Extrusion, compression set, early failure |
Check the Datasheet
These ranges vary by material. Silicone rubber tolerates more compression than Buna-N. PTFE has different limits than foam gaskets. Always check the gasket supplier's recommended compression range for your specific material and application.
Building the Signed 1D Loop
For a metal-to-metal lid assembly, the compressed gasket thickness equals the total groove depth (housing + lid recesses) minus any interference from flatness deviations.
Compressed thickness = Groove depth − Flatness stack
Then calculate compression:
Compression = Free height − Compressed thickness
Sign Convention
- + Groove depths (add to available space)
- − Flatness deviations (reduce effective space at worst case)
Flatness is unilateral—surfaces can only deviate outward from the theoretical plane (into the groove space), so it subtracts from available depth.
Worked Example: O-Ring Groove
An electronics enclosure uses a face-seal O-ring in a machined groove. The lid bolts down metal-to-metal against the housing flange.
Given Data
- O-ring cross-section (free height): 3.53 mm ± 0.10 mm
- Target compression: 15–25%
- Housing groove depth: 2.90 mm ± 0.05 mm
- Housing flange flatness: 0.05 mm max
- Lid flatness: 0.05 mm max
(No lid recess—the O-ring groove is entirely in the housing.)
Stack Calculation
| Dimension | Nominal (mm) | Tolerance | Sign |
|---|---|---|---|
| Groove depth | 2.90 | ±0.05 | + |
| Housing flatness | 0 | 0 / −0.05 | − |
| Lid flatness | 0 | 0 / −0.05 | − |
Compressed Thickness (Worst Case)
Nominal: 2.90 − 0 − 0 = 2.90 mm
Maximum: 2.95 − 0 − 0 = 2.95 mm (deep groove, flat surfaces)
Minimum: 2.85 − 0.05 − 0.05 = 2.75 mm (shallow groove, both surfaces bowed)
Compression Analysis
Using nominal O-ring free height (3.53 mm):
Nominal compression: 3.53 − 2.90 = 0.63 mm → 17.8%
Min compression: 3.43 − 2.95 = 0.48 mm → 14.0%
Max compression: 3.63 − 2.75 = 0.88 mm → 24.2%
The worst-case range is 14.0–24.2% compression. The minimum (14.0%) is slightly below the 15% target but within typical O-ring capability. If this margin is insufficient, options include:
- Reduce groove depth nominal (shift compression up)
- Tighten groove depth tolerance
- Tighten flatness requirements
- Select a larger O-ring cross-section
RSS for Production Reality
Worst-case assumes everything goes wrong simultaneously. RSS gives a statistical view:
Dimensional tolerance: ±0.05 mm (groove)
Flatness contributions: treated as 0.025 mm each (half max, one-sided)
RSS = √(0.05² + 0.025² + 0.025²) = √0.00375 = ±0.061 mm
RSS compressed thickness range: 2.90 ± 0.061 → 2.84–2.96 mm
RSS compression range: 16.1–19.5% (well within target)
Common Pitfalls
- Ignoring flatness — On large flanges, flatness can dominate the stack. A 0.1 mm bow on a 300 mm lid is common and eats into your compression margin.
- Wrong free height — Gasket vendors spec nominal ± tolerance. Use the full range, not just nominal.
- Bolt pattern gaps — Compression varies around the perimeter. Areas far from bolts see less clamp load and may under-compress even if the stack is nominal.
- Temperature effects — Thermal expansion of housing vs. lid changes the gap. Elastomers also change stiffness with temperature.
- Compression set over time — Elastomers take a permanent set. Initial compression may be fine, but after years the seal may relax below minimum.
Key Takeaways
- Gasket compression is a 1D stack-up — Groove depth minus flatness deviations
- Check both extremes — Under-compression leaks; over-compression crushes
- Flatness matters on large flanges — Include it in the stack
- Use the gasket supplier's compression range — Don't guess at limits
- RSS shows production reality — Most assemblies land near nominal