Reference guide
What Causes O-Ring Compression Set?
What compression set is and how it is measured (ASTM D395, the D2000 suffix B), the four causes, how to tell it from other failures at teardown, and how the gland is designed to survive it.
Compression set is the permanent flattening an O-ring takes after time under squeeze: the ring comes out of the gland with flat sides instead of a round section and can no longer spring back to seal. The Parker® O-Ring Handbook (ORD 5700) names four causes, heat beyond the compound's rating, a poor or wrongly chosen compound, over-squeeze, and under-cure at molding, and one hard floor: below about 0.005 in of squeeze everything takes close to 100 percent set, which is why 0.007 in is the absolute minimum. This guide covers what set is and how it is measured, how to tell it from the other failure modes at teardown, which materials and specifications control it, and how the gland is designed to survive it.
What compression set is, and how it is measured
Set is measured by compressing a test specimen a fixed amount for a fixed time at a fixed temperature, releasing it, and measuring how much of the deflection it fails to recover; the result is a percentage, and lower is better. ASTM D395 is the standard test method, and in an ASTM D2000 line callout the suffix letter B is the compression-set requirement, with the digits after it naming the test time and temperature code (B14 is 22 hours at 100 C, per ASTM D2000). Aerospace fluorocarbon specifications name the property in the title: SAE AMS7276 is titled for low compression set fluorocarbon rubber for seals. A compound sheet that does not state a set value at the service temperature has not told you how long the seal will hold.
Causes and fixes
| Cause | What happens | Fix | Source |
|---|---|---|---|
| Heat beyond the compound's rating | The elastomer hardens and takes a set at the squeezed shape; set rises steeply near the top of the range | Move up a material class; check the temperature range guide | Parker ORD 5700 section 10 |
| Over-squeeze | Excess strain accelerates set and extrusion; above about 25 percent initial compression the ring over-compresses when hot | Correct the gland per the groove design guide | Parker; ERIKS |
| Under-squeeze | Below about 0.005 in the ring takes near-100 percent set with no force left to seal | Hold the 0.007 in minimum; recompute after stretch | Parker ORD 5700 section 3.6 |
| Poor compound | Commodity compounds set faster than low-set formulations | Specify a low-compression-set compound; ask for the set value at temperature | Parker ORD 5700 |
| Under-cure at molding | Incompletely crosslinked rubber cannot recover | Source from a manufacturer with cure control and test reports | Parker ORD 5700 |
| Wrong material for the medium | Swell softens the ring so it sets under the same squeeze | Re-select per the chemical compatibility guide | Parker ORD 5700 section 10 |
| Long static hold at temperature | Set accumulates with time; the seal that held at commissioning leaks after the first hot season | Start higher in the squeeze band for a poor-set compound | Trelleborg |
Is it compression set, or something else?
A flat-sided ring is diagnostic, but several failures look alike at first glance. The Parker handbook's failure table separates them.
| What the ring looks like | Failure | Likely cause |
|---|---|---|
| Flat-sided oval, no other damage | Compression set | Heat, poor compound, over-squeeze, under-cure |
| Chewed or chipped low-pressure edge | Extrusion and nibbling | Clearance too large for the pressure and hardness |
| One-sided flat wear on a moving seal | Abrasion | Finish too rough, dirty fluid, dry running |
| Corkscrew cuts at about 45 degrees | Spiral failure | Long-stroke reciprocation with the ring rolling |
| Blisters, pits, internal fissures | Explosive decompression | Absorbed gas expanding on blow-down |
| Soft, gummy, enlarged | Chemical swell | Wrong compound for the medium |
| Hard, shrunken, cracked | Chemical extraction or heat aging | Medium extracts plasticizer, or service above the rating |
| Cuts, notches, skives | Installation damage | No chamfer, dry assembly, threads |
If you are replacing the same O-ring on a maintenance cycle and it comes out flat-sided every time, the gland or the compound is wrong, not the ring.
Which materials resist set?
Set resistance is a compound property before it is a family property, but the families rank. On the Parker O-Ring Handbook, FFKM holds its squeeze longer than FKM at high temperature, which is one reason it is chosen for seals that cannot be revisited between overhauls; silicone's set resistance in dry heat is good; peroxide-cured EPDM sets less than sulfur-cured in steam service; and nitrile sets quickly once it runs past its 100 C continuous ceiling (121 C short term). The published temperature ranges are the Parker handbook figures, and every one of them is a continuous-service range, not a set-free range: set rises through the top third of each. The temperature range guide lists them.
Designing the gland so set does not end the seal
Trelleborg lists compensating for compression set as one of the five reasons initial compression exists, and the practical rule follows: a compound that will take set has to start higher in the published squeeze band, so that after the set is taken there is still squeeze left. The bands are in the squeeze guide. Gland fill matters too: a ring with room to expand takes less strain than one packed at 90 percent fill, so the 60 to 85 percent band (Parker) is checked at maximum ring against minimum groove. And the operating temperature, not the specification title, sets the expectation: the Parker handbook warns that specification temperature limits do not necessarily represent operating limits.
The figures in this guide are the published guidelines of the manufacturers named beside them. They are not Conquest Seal engineering recommendations: Conquest distributes seals and cuts gaskets, and does not design parts. If a drawing calls out SAE AS4716, SAE AS5857, ISO 3601-2 or MIL-G-5514, the standard governs and these figures are context.
Parker is a trademark of Parker Hannifin Corporation. No affiliation or endorsement is implied.
Sources
- Parker O-Ring Handbook ORD 5700A/US, sections 3.6 and 3.7 (squeeze and gland fill) and section 10 (failure analysis), read 2026-08-12
- Parker O-Ring Handbook ORD 5700, section 2.2 (elastomer heat and cold figures), read 2026-09-14
- ERIKS nv, O-ring Technical Handbook, chapter 12, O-ring Gland Design, read 2026-09-06
- Trelleborg Sealing Solutions, O-Rings and Back-up Rings catalog (June 2024 edition), read 2026-09-06
- Parker compound E3609-70 datasheet for ISO 10993-5 and -10 compliant EPDM (ORD 5772), read 2026-08-13
- SAE Mobilus records for AMS7276 and AMS7259, read 2026-09-06
- ASTM D395 (compression set test) and ASTM D2000 suffix codes, from published manufacturer guides, read 2026-09-06
Reviewed by Grant Midstokke, Conquest Seal Corporation. Last updated September 2026. Spec values in this guide are drawn from the sources listed above; the compound datasheet and your application conditions govern the final choice.
Frequently asked questions
What is a good compression set value?
Lower is better, and it is only meaningful with the test time and temperature attached. Compare compounds on the same ASTM D395 condition, and read the D2000 suffix B code to see which condition the compound was qualified at.
Can a set O-ring be reused?
No. A flat-sided ring has already lost the recovery it needs to seal; replace it and fix the cause.
Does a harder O-ring take less set?
Not as a rule. Hardness controls extrusion resistance; set is controlled by the compound's chemistry and cure. See the durometer guide.
Why did a new seal leak after the first hot cycle?
Set taken at temperature: the ring expanded into the gland, took its set in the expanded state, and had no squeeze left when it cooled and contracted. Starting higher in the squeeze band, or a lower-set compound, is the usual answer.
Which specification promises low set?
SAE AMS7276 (fluorocarbon, "low compression set") and the AMS7259 companion for 85 to 95 A; see the AMS7276 guide. For industrial compounds, the D2000 suffix B requirement on the compound sheet.
Which D2000 suffix sets the compression-set test?
The B suffix, tested to ASTM D395 Method B for 22 hours: B14 is at 100 C, B16 at 150 C and B37 at 175 C, and the datasheet reports the percent set after the test. The digit is a temperature code that follows a different ladder for the F low-temperature suffix, so B37 and F17 are different temperatures.
How does the groove protect against set?
By leaving room. Published guidelines hold gland fill between 60 and 85 percent of the groove volume, with 75 percent as the target, so thermal expansion and swell do not drive the ring into over-squeeze. Check the fill at the maximum ring section against the minimum groove, because a gland that reads 80 percent at nominal can read 90 at the tolerance extremes.
Which compound sheet lists low set?
Parker's E3609-70 sheet for ISO 10993-5 and -10 compliant EPDM (ORD 5772, read 2026-08-13) lists a 70 Shore A compound rated -54 C to 121 C (-65 F to 250 F), with low compression set and resistance to repeated steam, gamma and ethylene oxide sterilization. Low set is a compound property; ask for the datasheet figure at the temperature the seal will see.
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