FKM Sealing Rings & Cord

What are FKM sealing rings and cord?

FKM sealing rings are O-rings made from FKM, a fluoroelastomer rubber, and FKM cord is the same material supplied as a continuous length for making custom rings. FKM is a premium sealing material, chosen when a seal has to cope with high temperatures, oils, fuels, and aggressive chemicals all at once. Where nitrile would harden or break down from heat, FKM keeps sealing.

A standard FKM sealing ring or cord has these main features:

• A solid, round cross section, measured as its section diameter
• FKM fluoroelastomer rubber, which resists heat, oils, fuels, and many chemicals
• A hardness rating on the Shore A scale, most commonly 75
• For O-rings, an inside diameter that sets the size of the ring.

FKM’s main strength is its high temperature and chemical resistance.

• It can be used continuously at temperatures up to around +200°C, far higher than nitrile or EPDM.
• It resists mineral oils, synthetic oils, fuels, and many acids and solvents, which makes it ideal for engines, fuel systems, and chemical equipment.

FKM cord is supplied as a continuous extruded length, which can be cut and joined to make a seal of any size, or used on its own in a groove as a gasket or sealing strip.

Why are FKM sealing rings and cord essential?

In hot engines, gearboxes, hydraulic systems, and chemical plants, ordinary rubber seals can harden, crack, or swell quickly. FKM is designed for these demanding jobs, giving a long-lasting seal where heat, oil, fuel, and chemicals are all present. It also resists ageing, ozone, and weathering, and has low gas permeability.

Although FKM costs more than nitrile, it often lasts much longer in hot or chemical environments, reducing leaks, downtime, and replacement costs. Cord makes it easy to produce large or non-standard FKM seals without waiting for a special part.

Typical applications include:

• Engines, turbochargers, and exhaust systems
• Fuel systems and fuel injection equipment
• Hydraulic and pneumatic systems running hot
• Gearboxes and transmissions
• Chemical processing and pumping equipment
• Valves and pumps handling aggressive fluids
• Ovens, dryers, and high-temperature machinery
• Automotive and motorsport applications

The main types of FKM sealing rings and cord

• Metric FKM O-ring cord - Supplied in a wide range of metric section sizes, from around 1mm up to 20mm and more, sold by the metre.
• Imperial FKM O-ring cord - Supplied in inch section sizes, such as 1/16, 1/8, and 3/32 inch, for imperial grooves and equipment.
• Standard size FKM O-rings - Ready-made FKM rings in set inside diameters and section sizes.
• FKM V-rings - Rubber shaft seals made from FKM, which fit on the shaft and seal against a face. They give high-temperature and chemical resistance for shaft sealing.
• 75 Shore FKM - The most common hardness, which gives a good balance of sealing, durability, and resistance to being squeezed out of the groove.

How to choose the right FKM sealing ring or cord?

• Temperature - Choose FKM where temperatures are too high for nitrile. FKM suits temperatures from around -25°C up to +200°C, but becomes stiff at very low temperatures.
• Fluid compatibility - FKM resists oils, fuels, and many chemicals. It is not suitable for hot water and steam, glycol-based brake fluids, or some solvents such as ketones, where EPDM or another material may be better.
• Section size - Choose a section diameter that suits the groove depth and width, so the seal is compressed by the right amount.
• Inside diameter or length - For an O-ring, choose the inside diameter to suit the groove. For cord, work out the length needed to make the ring.
• Cost and service life - FKM costs more than nitrile, so use it where its heat and chemical resistance are needed, and nitrile where they are not.

Essential references and specialised ranges

How the numbering works:

FKM cord is referenced by its section diameter, material, and hardness:

3mm FKM 75 = 3mm section FKM cord, 75 Shore hardness

5.33mm FKM 75 = 5.33mm section FKM cord, 75 Shore hardness

FKM O-rings are referenced by their inside diameter and section diameter:

153 x 4 FKM = 153mm inside diameter, 4mm section

To work out how much cord is needed for a ring, add the inside diameter to the section diameter, then multiply by 3.14. For example, for a 150mm inside diameter ring in 4mm cord: (150 + 4) x 3.14 = about 484mm of cord.

Suffixes:

FKM = fluoroelastomer rubber

75 = hardness on the Shore A scale

Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.

FAQ - All you need to know about FKM sealing rings and cord

What is FKM?

FKM is a type of fluoroelastomer, a synthetic rubber containing fluorine. The fluorine gives it excellent resistance to heat, oils, fuels, and chemicals, which is why it is used for demanding sealing jobs.

What is the difference between FKM and nitrile?

Both resist oils and fuels, but FKM handles much higher temperatures and a wider range of chemicals. Nitrile is lower in cost and better at low temperatures, so it suits general applications, while FKM suits hot or chemically aggressive ones.

Can FKM be used with water or steam?

FKM is not the best choice for hot water or steam, which can make it swell and lose strength over time. EPDM is usually better for water and steam.

How do I join FKM cord?

Cut both ends squarely with a sharp blade, then join them with a suitable instant adhesive or a splicing kit. Check the joint is suitable for the temperature the seal will run at, as some adhesives cannot cope with high heat.

How do I fit an FKM O-ring?

Clean the groove and surfaces and remove any burrs. Lightly lubricate the ring with a compatible oil or grease, then fit it into the groove without twisting or overstretching it. FKM is less stretchy than nitrile, so take care when fitting it over larger diameters.

How do I identify the size I need?

For an O-ring, measure the inside diameter and section diameter of the old ring, or measure the groove. For cord, measure the section diameter with callipers and work out the length from the groove size.

How can I tell if an FKM seal needs replacing?

Warning signs include:

• Leaks around the seal
• A flattened ring that has lost its round shape
• Hardened, cracked, or brittle rubber
• Swelling or softening from an incompatible fluid
• Cuts or nicks from fitting
• A failed or split joint on a cord ring