O-ring Shore hardness —
what 70 Shore and 90 Shore
mean and when it matters
For O-rings, 70 Shore A is the standard for most general sealing duties. 90 Shore A is significantly stiffer and is typically selected when higher pressure creates a risk of extrusion. Understanding what the number means — and when it changes the selection — is more useful than simply ordering harder every time something leaks.
What Shore hardness measures
Shore hardness is measured using a durometer — an instrument that presses a standardised indenter into the material surface under defined load and records how far it penetrates. The Shore A scale is used for soft rubbers and elastomers. Shore D covers harder materials such as rigid plastics and is not typically used for standard O-ring compounds.
On the Shore A scale, 0 represents a material that offers no resistance to penetration — completely soft. 100 represents a material that the indenter cannot penetrate at all. In practice, common O-ring elastomers fall in the range of approximately 40 to 90 Shore A, with most general-purpose grades clustered around 70.
Shore A hardness is a material property, not a specification in isolation. The same elastomer family — EPDM, NBR, FKM — can be compounded to different hardness values. A 70 Shore A EPDM and a 90 Shore A EPDM are both EPDM, but they behave differently under compression and under pressure.
For many water and heating static seals, 70 Shore A is the normal starting point. The exact value should still be read from the material data sheet, because hardness belongs to the compound, not to the generic material name alone.
What changes between 70 Shore and 90 Shore
- Softer, more compliant under assembly load
- Conforms more readily to minor surface irregularities
- Lower force needed to achieve correct squeeze
- Better low-pressure sealing on imperfect surfaces
- Starting specification for most static sealing
- More susceptible to extrusion at high pressure
- Stiffer, requires more load to compress
- Less conforming to surface irregularities
- Greater resistance to extrusion through clearance gap
- Better suited to higher system pressure
- More commonly used in dynamic applications
- May not seal as well at very low pressure
The extrusion problem — why hardness matters at high pressure
O-ring extrusion occurs when system pressure forces the O-ring material through the clearance gap between two mating components. At low pressure, the O-ring stays seated in the groove. As pressure increases, the O-ring is pushed against the low-pressure side of the groove and begins to be forced into the gap. If the gap is too large for the compound hardness, the O-ring is progressively nibbled away — a failure mode that leaves a characteristic chipped or ragged appearance on the low-pressure edge of the O-ring.
A harder compound resists this deformation more effectively. The stiffness of the material means it requires more force to be pushed through the gap. This is why 90 Shore A compounds are often specified for higher-pressure hydraulic and pneumatic applications, and why backup rings — rigid rings placed on the low-pressure side of the O-ring — are used in very high-pressure systems regardless of compound hardness.
The important nuance is that extrusion resistance is not only a function of hardness. Clearance gap size, pressure level, and whether the application is static or dynamic all determine whether extrusion is actually the risk. A harder O-ring on a low-pressure static seal with a tight clearance does not improve performance — it simply makes the O-ring harder to compress to the correct squeeze.
Harder is not better — it is different. A 90 Shore O-ring on a low-pressure static heating connection is not a more reliable seal than a 70 Shore O-ring. It is a stiffer seal that requires more assembly load, conforms less to the sealing surfaces, and provides extrusion resistance that the application does not require. Match the hardness to the pressure and clearance geometry, not to a general idea of more robust.
Side-by-side — when each hardness is typically appropriate
| Parameter | 70 Shore A | 90 Shore A |
|---|---|---|
| System pressure | Low to medium — typical heating and plumbing | Higher pressure — hydraulic and pneumatic duties |
| Application type | Static sealing — standard starting choice | Dynamic or high-pressure static where extrusion is a risk |
| Clearance gap | Tight clearance — extrusion not the risk | Larger clearance where extrusion resistance is needed |
| Surface finish | Better conformance to minor irregularities | Less conforming — requires better surface condition |
| Assembly load | Lower — easier to achieve correct squeeze | Higher — needs more force to compress correctly |
| Temperature | Standard range — material dependent | Material dependent — hardness and temperature interact |
Hardness and compression set
Hardness and compression set are related but not the same property. Compression set measures how much permanent deformation an O-ring accumulates under sustained load — how much it fails to recover when the load is removed. A material with high compression set loses sealing force over time as the groove load relaxes.
A harder compound is not automatically lower compression set. Compression set depends primarily on the elastomer type and the cure system. Within EPDM, peroxide-cured compounds generally offer lower compression set than sulphur-cured grades at elevated temperature, regardless of whether the compound is 70 or 90 Shore A.
For heating system O-rings, where the O-ring sits under continuous load at elevated temperature for months at a time, compression set is a more meaningful performance parameter than hardness. The hardness — 70 Shore A — is the standard starting point. The compression set data is what indicates how long the seal remains effective.
Reading a hardness specification — tolerance and variation
A Shore A value on a data sheet is a nominal value with tolerance. A compound specified as 70 Shore A typically has a tolerance around five points, so a tested value in the mid-60s to mid-70s can still be inside the stated grade, depending on the specification and test method.
This means that two O-rings both specified as 70 Shore A from different compounds or suppliers may behave slightly differently in practice. The Shore A value is a useful reference for selection and comparison, but it should be read as a range rather than an exact single value.
70 Shore A is the standard starting specification for most O-ring sealing duties. 90 Shore A is selected when higher pressure or larger clearance gaps create a real risk of extrusion.
Harder is not more reliable in general. It is more resistant to extrusion under specific pressure and clearance conditions. For heating system static sealing at standard pressures, 70 Shore A with low compression set — confirmed by published compound data — is the relevant specification to look for.
FAQ
What does Shore hardness mean for O-rings?
Shore hardness measures the resistance of the O-ring material to indentation by a standardised probe under defined load. For O-rings, the Shore A scale is used — a higher number means a harder, stiffer rubber. A 70 Shore A O-ring is a common starting point for general sealing duties. A 90 Shore A O-ring is significantly stiffer and is often used where higher pressure creates a risk of the O-ring extruding through the clearance gap, or where the application requires greater resistance to deformation.
When should I use 90 Shore instead of 70 Shore O-rings?
90 Shore A O-rings are often selected when higher system pressure creates a risk of extrusion — where the O-ring material is forced through the clearance gap between mating components under pressure. The harder compound resists this deformation. 90 Shore is also used in some dynamic applications where softer compounds wear more rapidly. For many standard static seals at typical pressures, 70 Shore A is the usual starting specification.
Does a harder O-ring seal better?
Not necessarily. A harder O-ring requires more assembly load to achieve the same compression, and may not conform as well to minor surface irregularities in the groove. For low-pressure static sealing on smooth surfaces, a softer 70 Shore compound often provides adequate sealing with less risk of installation problems. A harder compound can provide better extrusion resistance at higher pressure, but this advantage is only relevant when extrusion is the actual failure risk.