O-rings: a guide to sizes, cross-sections and materials

An O-ring is an elastomer ring with a circular cross-section: simple, inexpensive and incredibly versatile. You find it everywhere, from hydraulic fittings to cylinders, from pumps to taps, from food machinery to pneumatic systems. Precisely because it is so common, its selection tends to be underestimated. In reality, diameter, cross-section, material and hardness must all be right, otherwise the seal lasts little or does not work at all.

How an O-ring works

The O-ring is fitted in a groove and compressed between two surfaces. This initial compression, called squeeze, creates the seal even at zero pressure. When fluid pressure arrives, the ring is pushed against the opposite side of the groove and the sealing force increases. It can work statically (between two parts that do not move, such as a flange) or dynamically (on a moving rod or shaft), although demanding dynamic applications often call for dedicated profiles.

How to measure it

An O-ring is identified by two dimensions:

• d1 – inner diameter;
• d2 – cross-section (the thickness of the cord).

For example, an O-ring 20 x 2.62 has an inner diameter of 20 mm and a cross-section of 2.62 mm. When measuring a used O-ring, remember that it may have stretched, flattened or swollen: if possible, measure the groove, or check several points of the cross-section with a caliper without squeezing it.

The most common cross-sections

Many O-rings follow international standard series with recurring cross-sections such as 1.78 – 2.62 – 3.53 – 5.33 – 6.99 mm, as well as metric series (1 – 1.5 – 2 – 2.5 – 3 – 4 – 5 mm and so on). In our catalogue you will find individual O-rings in a huge range of sizes, as well as assortment boxes with the most used cross-sections: very handy in the workshop to always have the right spare part to hand.

Choosing the material

• NBR (nitrile rubber): the most common choice for mineral oils, greases and fuels at medium temperatures. Good value for money.
• FKM (Viton): for higher temperatures and many aggressive fluids, synthetic oils and fuels.
• Silicone (VMQ): wide temperature range, also suitable for food contact in FDA-compliant grades; not suited to heavy dynamic use or to mineral oils.
• EPDM: excellent for water, steam and many detergents, but not compatible with mineral oils.
• HNBR, fluorosilicone, FFKM: special materials for particular temperature and chemical compatibility requirements.

On our website each O-ring family shows its material and temperature indications. If you are unsure about compatibility with a specific fluid, ask us before ordering.

Hardness

Hardness is expressed in Shore A. The most common value is 70 Shore A, suitable for most applications. Higher hardness (for example 90 Shore A) resists extrusion better at high pressure, while lower hardness seals better with low clamping forces.

The most frequent mistakes

• Wrong cross-section: an O-ring that is too thin is not compressed enough and leaks; one that is too thick fills the groove and gets damaged.
• Extrusion: at high pressures, if the clearance is too large, the O-ring is pushed into the gap and nibbled. Back-up rings are used in these cases.
• Dry assembly: lubricating the O-ring with a compatible product prevents twisting and cuts.
• Sharp edges: threads and unchamfered edges can cut the ring during assembly.

With the search by dimensions on our website you can enter inner diameter and cross-section and immediately find the O-rings available in the different materials.

Frequently asked questions

Are metric and inch O-rings interchangeable?

Only if inner diameter and cross-section match or are very close. Even a slightly different cross-section changes the squeeze in the groove: always check the groove dimensions.

Can I join cord to make a custom O-ring?

For large-diameter static seals, joined cord is a practical solution. For dynamic or high-pressure applications a moulded O-ring is preferable.

Why does a new O-ring swell after a few days?

It is almost always a chemical compatibility problem with the fluid: check the material before replacing it with an identical one.