Reference guide
Which O-Ring Durometer (Shore A Hardness) to Pick
Most O-rings are specified at 70 Shore A, the default hardness because it balances conforming to the gland surfaces against resisting extrusion. A harder ring is chosen as pressure and clearance rise, and makers' clearance charts, such as Precision Associates' Design Chart 6, give the gap each hardness can bridge at a given pressure. A backup ring is the other route, letting a softer ring hold a higher pressure.
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. 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
| Material or application | Usual hardness | Note |
|---|---|---|
| Nitrile, EPDM, silicone, general industrial | 70 to 80 Shore A | The usual range for most applications (Parker handbook, page 2-10); Parker's stock nitrile kit is N0674-70 (page 3-6) |
| Fluorocarbon (Viton®) and FFKM | 75 Shore A | The aerospace specifications are 70 to 80 A documents |
| Boss gaskets on straight-thread fittings | 90 Shore A | Conventional (Parker handbook Table 8-3; MS28778 to AMS-P-5510) |
| High pressure, rapid gas decompression | 90 Shore A | M83248/2 and AS3581 (AMS7259) in fluorocarbon; 90 A FFKM grades |
| Polyurethane hydraulic seals | By compound | Parker's handbook names a 90 A polyurethane compound (P0648-90) and credits the family with high wear resistance and tensile strength |
| Low-pressure, easy-conformity seals, soft gaskets | 40 to 60 Shore A | AMS 3301 to 3303 silicone grades; softer nitrile and EPDM compounds in Parker's Table 8-5 (N0545-40, N0299-50, N0525-60; E0529-60) |
| Vacuum and dovetail face seals | 70 Shore A | Conformity over extrusion resistance |
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 Design Chart 6 of Precision Associates' Application Data: Seal Gland Design, for the 0.139 in (-2xx) section: 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 the 90 A ring bridges about 0.007 in, a pressure for which the chart gives no 70 A figure. 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.
Two modifiers matter: Parker halves its chart clearances for silicone; 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
| Published threshold | Guidance | Source |
|---|---|---|
| By ring size | backup above about 725 psi for rings over 50 mm inside diameter, above about 1,450 psi under 50 mm | Trelleborg |
| Two rings | over 1,500 psi, two backup rings | Precision Associates |
| Which side | one ring on the low-pressure side for one-way pressure; one on each side for alternating pressure | Parker |
What a harder ring costs you
Harder compounds trade away conformity and need more closure force, so do not specify 90 A by reflex. 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
- Parker O-Ring Handbook ORD 5700 (2021 printing): sections 4.2 (back-up rings; silicone clearances) and 4.5 (boss seals), and section 8.1.1 and Table 8-3 (compound numbering; MIL-P-5510 boss gasket), read 2026-09-24; page 2-10 (hardness guidance) and page 3-6 (stock kits), read 2026-09-25; section 2.2.12 and compound P0648-90 (polyurethane), read 2026-09-27
- Precision Associates, Application Data: Seal Gland Design (2011), read 2026-09-06 and 2026-09-24; Design Chart 6, diametral clearance vs. durometer hardness, read 2026-09-29
- Trelleborg Sealing Solutions, O-Rings and Back-up Rings catalog (June 2024 edition), read 2026-09-06
- SAE Mobilus records for AMS 3301, AMS7276, AMS7259 and AS3581, read 2026-09-06; AMS-P-5510 and MS28778 named as callouts
- ASTM D2000 / SAE J200 line callout letters and suffixes; Parker O-Ring Handbook ORD 5700 (2021 printing), Table 8-5, pages 8-6 and 8-7 (ASTM D2000 callout format and suffix callouts); ASTM D2000-18(2024)e1 store listing
- Parker O-Ring Handbook ORD 5700 (2021 printing), page 2-10 (70 to 80 Shore A the most suitable compromise for most applications), page 3-6 (stock kit N0674-70, NBR 70) and Table 8-5, pages 8-6 and 8-7 (N0545-40, N0299-50, N0525-60; E0529-60), read 2026-09-25
Frequently asked questions
Is 70 A or 90 A better?
Neither; they answer different pressures. 70 A seals better on imperfect surfaces at low pressure; 90 A resists extrusion at high pressure and is the boss-gasket convention.
Can I fix extrusion by going from 70 A to 75 A?
Rarely. Five points of hardness barely move the clearance table; close the gap, go to 90 A, or add a backup ring.
Does hardness change with temperature?
Yes. Rubber hardens as it cools and softens as it warms, and fluid swell softens it; the durometer on the sheet is measured at room temperature.
What tolerance does a durometer callout carry?
Typically plus or minus 5 points on a compound callout (Parker's 70 A is 65 to 75); the aerospace specifications state the band in the title (AMS7276: 70 to 80).
Which durometer for a metric ISO 3601 gland?
The same logic; ISO 3601-2 housings are drawn for the standard hardness, and the pressure and clearance decide whether to go harder. See the metric sizes guide.
How much clearance can a 70 A ring bridge at a given pressure?
On Precision Associates' Design Chart 6 for a 0.139 in (2xx) ring, a 70 A ring bridges about 0.009 in of diametral clearance at 1,000 psi and a 90 A ring about 0.016 in; at 3,000 psi the 90 A ring bridges about 0.007 in, and the chart gives no 70 A figure at that pressure. Those are diametral figures, the full bore-minus-piston gap, and they assume the parts are concentric.
Which hardness do the compound families come in?
By compound more than by family: makers mold most families across a spread of hardness, and the datasheet for the compound you buy states its nominal Shore A and its tolerance. This guide lists the usual hardness by material and application, with 70 A the common default.
Does a softer ring need less squeeze?
No, the squeeze bands are set by seal type and cross-section, not by hardness. Hardness changes the force that squeeze generates and the friction it causes, which is why a harder ring is not a free upgrade in dynamic service.
Related guides
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