‘70 Shore A’ appears on almost every O-ring drawing and specification sheet, but few buyers can say why. Hardness — formally called durometer — measures how easily the elastomer resists indentation, and it directly controls how the ring behaves under pressure, in the groove, and during installation. It is the most important specification after size and material.
Shore A (defined by ASTM D2240 / ISO 48-4) presses a spring-loaded needle into the rubber and reads the resistance on a 0–100 scale. A higher number means a harder compound. Most sealing compounds sit between 50A and 90A; above 90A the industry switches to the Shore D scale — note that 90 Shore A is roughly equivalent to 40 Shore D.
Two practical details matter for buyers. First, durometer is measured at a defined temperature and read after a short dwell time — field readings with a pocket durometer are indicative, not precision values. Second, molded O-rings carry a typical hardness tolerance of ±5 points, so ‘70A’ means the batch may legitimately arrive at 65–75A.
| Hardness | Characteristics | Typical applications |
|---|---|---|
| 50A | Very soft; conforms easily, low installation force | Low-pressure seals, delicate surfaces, low-temp service, small grooves |
| 60A | Soft; good conformability with reasonable strength | Low-pressure pneumatic, vacuum, face seals on uneven flanges |
| 70A | The universal balance of sealing force, extrusion resistance and installability | Default for general industrial, automotive and hydraulic O-rings |
| 80A | Firm; better extrusion and wear resistance | Higher-pressure hydraulics, dynamic service, wider clearance gaps |
| 90A | Hard; behaves almost like an engineering plastic | High-pressure systems, anti-extrusion duty, heavy dynamic loads |
Seventy became the global default because it sits at the intersection of three competing demands. It is soft enough to flow into the microscopic roughness of metal surfaces and generate contact pressure at modest squeeze, hard enough to resist extrusion into clearance gaps at normal industrial pressures, and firm enough to install over sharp gland edges without rolling or tearing. Most catalog compounds — NBR, FKM, EPDM alike — are therefore stocked in 70A as the baseline.
The temptation is to specify 90A for ‘extra reliability’, but harder compounds need more squeeze force to seal, which means higher assembly torque, greater friction in dynamic service, and poorer sealing of low-pressure fluids where contact pressure is already marginal. Go harder only when pressure and clearance demand it — typically when the ring shows nibbling or extrusion in service. Conversely, soft compounds seal low pressure beautifully but extrude early as pressure rises.
Elastomers are viscoelastic: every compound gets noticeably harder in the cold and softer when hot. A 70A NBR ring behaves like 80A+ at −30°C and like 60A at 100°C. In cold-start applications the ring may briefly leak until it warms up, while at high temperature a softening compound is more extrusion-prone — which is why high-temperature designs often combine FKM with 80–90A hardness.
Hardness changes the numbers in your groove calculation but not the method: squeeze still targets 5–30% on radial seals and 10–35% on face seals, and the extrusion-gap limits shift with pressure and durometer, as covered in our extrusion and groove design article. When pressure exceeds the compound's gap limits, the standard remedies are a harder compound or a back-up ring — not more squeeze, which only accelerates compression set.
Specify 70A unless your pressure, clearance or temperature gives you a documented reason to move. Then move one grade at a time and verify by test.
All standard compounds in our O-ring range are available in multiple hardness grades, and we can compound custom durometers for OEM projects. For hardness alongside the full material comparison, see our elastomer materials guide.