Which O-Ring Durometer (Shore A Hardness) to Pick

Most industrial O-rings are specified at 70 Shore A, and the published guidelines agree that it is the right default: it balances sealing conformity against extrusion resistance. Go harder when pressure or clearance demands it, and treat durometer together with clearance, because extrusion is driven by pressure, clearance and hardness acting together. The Parker® handbook's rule of thumb from its extrusion curves: 70 Shore A to about 800 psi, 80 A to about 1,500 psi, 90 A to about 3,000 psi, at a few thousandths of an inch of diametral clearance. This guide explains the scale, the defaults by material and application, the published pressure-versus-clearance tables, and the trade-offs a harder ring brings.

What Shore A durometer measures

Shore A hardness is the indentation resistance of the rubber on a 0 to 100 scale; O-rings are supplied from about 40 A (soft, for low-pressure and easy conformity) to 90 A (hard, for high pressure). Compound callouts carry the nominal hardness, and Parker's format shows the tolerance: 2-214 N0674-70 means a 70 Shore A nitrile with a plus or minus 5 point tolerance. ASTM D2000 encodes hardness as the first digit of the three-digit block (710 is 70 A with 10 MPa minimum tensile), and the aerospace material specifications state it in the title (AMS7276 is 70 to 80, AMS7259 is 85 to 95).

Default hardness by material and application

Pressure, clearance and hardness

The published extrusion tables give the maximum diametral clearance a plain O-ring can bridge at a given pressure and hardness. Three points from Precision Associates' diametral clearance chart for the 0.139 in (-2xx) section, the most common one (Application Data: Seal Gland Design, read 2026-09-06): a 70 Shore A ring bridges about 0.009 in at 1,000 psi and a 90 Shore A ring about 0.016 in, and at 3,000 psi only the 90 A ring runs without a backup ring, at about 0.007 in. The smaller sections allow less clearance and the larger sections slightly more, so read the maker's chart for your section. All values are diametral; Trelleborg publishes radial figures, and reading one as the other is a factor-of-two error in the direction of failure.

ERIKS' rule sits behind these figures: hardness is not a substitute for a gap. A harder ring resists extrusion better but cannot compensate for an excessive clearance, so the first fix for extrusion is to close the gap, the second is to harden the ring, and the third is a backup ring. Two modifiers matter: for silicone and fluorosilicone, cut every tabulated clearance by 50 percent (ERIKS) or 60 percent (Parker; use the more conservative when the print does not say), and where the parts are not concentric or the metal breathes under pressure, the maximum-eccentricity gap is the one that extrudes, so add the anticipated expansion into the clearance before reading the chart.

When a backup ring is the answer instead of a harder ring

What a harder ring costs you

Harder compounds trade away conformity and need more closure force, so do not specify 90 A by reflex. ERIKS' friction note is the practical consequence: above 70 Shore A, harder means more friction at the same squeeze, not less, because the compressive force is greater; and after ten days standing, breakout friction runs 2 to 5 times the running friction of a lightly loaded seal. A hard ring also seals rough or wavy surfaces worse than a soft one, which is why low-pressure gaskets and lids go softer. Durometer is one axis of material choice; chemistry and temperature are the others, covered in the temperature range guide and the chemical compatibility guide, and the squeeze guide covers how squeeze and hardness interact.

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.

Viton is a trademark of The Chemours Company; Parker is a trademark of Parker Hannifin Corporation. No affiliation or endorsement is implied.

Sources

1. Parker O-Ring Handbook ORD 5700A/US, section 3 (extrusion and gland design) and section 7 (boss gaskets), read 2026-08-12
2. Precision Associates, Application Data: Seal Gland Design (2011), read 2026-09-06
3. ERIKS nv, O-ring Technical Handbook, chapter 12, O-ring Gland Design, read 2026-09-06
4. Trelleborg Sealing Solutions, O-Rings and Back-up Rings catalog (June 2024 edition), read 2026-09-06
5. SAE Mobilus records for AMS 3301, AMS7276, AMS7259 and AS3581, read 2026-09-06; AMS-P-5510 and MS28778 named as callouts
6. ASTM D2000 / SAE J200 line callout letters and suffixes, from published manufacturer guides, read 2026-09-06
7. Conquest Seal's former compound chart (hardness by family)

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.