Free Engineering Tool
O-Ring Size Selection
Find the right standard O-ring size per ISO 3601 for piston, rod, or face seal applications. Get groove dimensions, squeeze percentage, and stretch — instantly.
Results
ISO 3601 Standard Cross-Sections
| CS (mm) | AS568 Series | CS Tolerance | Typical ID Range | Groove Depth (20% sq.) | Groove Width |
|---|---|---|---|---|---|
| 1.78 | -0XX | ±0.08 | 1–25 mm | 1.42 | 2.50 |
| 2.62 | -1XX | ±0.09 | 9–76 mm | 2.10 | 3.60 |
| 3.53 | -2XX | ±0.10 | 15–133 mm | 2.82 | 4.80 |
| 5.33 | -3XX | ±0.13 | 19–267 mm | 4.26 | 7.15 |
| 6.99 | -4XX | ±0.15 | 50–292 mm | 5.59 | 9.40 |
O-Ring Squeeze
Squeeze is the diametral compression of the O-ring cross-section when installed in the groove. It creates the initial seal contact stress.
- Dynamic seals (piston/rod): 10–20% squeeze recommended
- Static seals (face/flange): 15–30% squeeze recommended
Groove Dimensions
The groove width must allow the O-ring to expand laterally when squeezed, plus accommodate thermal expansion and swell.
Stretch & Compression
For piston seals, the O-ring is stretched over the piston. For rod seals, it is compressed into the bore groove.
- Max stretch: 5% (standard elastomers), 3% (PTFE, hard compounds)
- Max compression: 3% of ID
Target: O-ring OD ≈ bore = 50 mm
Required ID = 50 − 2 × 3.53 = 42.94 mm
Nearest standard ID: 42.86 mm → OD = 42.86 + 2 × 3.53 = 49.92 mm
Groove depth (20% squeeze) = 3.53 × 0.80 = 2.82 mm
Groove width = 3.53 × 1.35 = 4.77 mm
Groove bottom diameter* = 50 − 2 × 2.82 = 44.36 mm
Stretch = (44.36 − 42.86) / 42.86 × 100 ≈ 3.5% ✓ (within the 5% limit)
*assuming piston OD ≈ bore; subtract the piston-to-bore clearance from the piston OD for the exact groove bottom diameter
⚠️ Note: Always verify groove fill ratio (O-ring volume vs groove volume). The O-ring should fill 70–85% of the groove. Under-filling causes instability; over-filling prevents thermal expansion and causes extrusion.
ℹ️ Tip: For pressures above 3.5 MPa (static) or 7 MPa (dynamic), add a backup ring on the low-pressure side to prevent O-ring extrusion into the gap.
Follow these steps:
- Identify application: piston seal, rod seal, or face seal
- Determine diameter: bore diameter (piston) or shaft diameter (rod)
- Choose cross-section: larger CS for higher pressures and larger diameters. CS 3.53 mm is the most common general-purpose size
- Find standard ID: for piston seals, ID = bore − 2×CS. For rod seals, ID ≈ rod diameter. Select nearest standard size from ISO 3601
- Verify squeeze: aim for 15–25% (static) or 10–20% (dynamic)
- Check stretch: should be ≤5% of ID
Squeeze is the percentage compression of the O-ring cross-section in the groove:
Squeeze % = (CS − compressed height) / CS × 100
It creates the initial contact stress needed for sealing. Guidelines:
- 10–20% for dynamic seals (piston/rod) — balances sealing and friction
- 15–30% for static seals (face/flange) — higher squeeze for reliability
- Too little squeeze → leakage, poor seal
- Too much squeeze → excessive friction, heat, spiral failure, short life
Stretch occurs when the O-ring is installed on a groove diameter larger than its free-state ID. Effects:
- Reduces cross-section — the CS becomes thinner as the O-ring stretches, reducing effective squeeze
- Increases stress — higher internal stress accelerates aging
- Max 5% for most elastomers (NBR, FKM)
- Max 3% for hard/brittle materials (PTFE-encapsulated, FFKM)
Conversely, compression (negative stretch, ID larger than groove) can cause the O-ring to buckle or twist in the groove.
Common O-ring materials and their applications:
- NBR (Nitrile) — −40 to +120°C, excellent oil resistance, most common
- FKM/Viton — −20 to +200°C, excellent chemical and heat resistance
- EPDM — −50 to +150°C, water/steam/brake fluid, NOT for oils
- Silicone (VMQ) — −60 to +200°C, food-grade, medical
- FFKM (Kalrez) — −25 to +325°C, universal chemical resistance, expensive
- PTFE — −200 to +260°C, chemically inert, used as encapsulation or backup rings
Backup rings prevent O-ring extrusion into the gap between mating surfaces under pressure. Use backup rings when:
- Static seal: pressure > 3.5 MPa (500 psi)
- Dynamic seal: pressure > 7 MPa (1000 psi) with 70 Shore A O-ring
- Large gaps: diametral clearance > 0.1 mm at low pressure, less at high pressure
Place one backup ring on the low-pressure side. For bi-directional pressure, use two backup rings (one on each side). PTFE is the standard backup ring material.
The procedures described on this page were developed and field-tested by the Vibromera team using the Balanset-1A, a portable dual-channel balancer and vibration analyzer for on-site rotor balancing.
Professional field balancing instruments and software. Used in 50+ countries.
Nikolai Shelkovenko
Nikolai Shelkovenko is a vibration analysis engineer and the founder and CEO of Vibromera. For more than 15 years he has balanced rotating equipment in the field rather than on a test bench: mulchers, industrial fans, crushers, centrifuges, shafts and spindles. That work is what the Balanset instruments grew out of — they were designed as a tool a specialist can carry to the machine and use alone, on site, not as laboratory equipment. Vibromera was founded in 2017 and has been based in Porto, Portugal, since 2023. Development, assembly and support of the Balanset line all happen here. The flagship instrument is the Balanset-1A, a portable analyser for single- and two-plane balancing and for vibration diagnostics. Nikolai is personally involved in customer support, in working through difficult balancing cases and in the development of the software. He works with customers worldwide, in any language.
