Soft seal hydraulic connectors and their applications

Soft seal connectors use an elastomeric object to create a leak-free seal, they can take the form of bonded washers, encapsulated seals and o-rings.

There is a wide range of soft seal connector types, including BSPP, DIN, ORFS and SAE.

BSPP British Standard Pipe Parallel

Country of origin: Britain
Thread angle: 55 degrees (Whitworth)

BSPP threads are also an elastomeric connector.

A soft seal is used to ensure a leak free joint.

RYCO use an O-ring and metal back-up ring or an encapsulated seal to complete the joint.

BSPP Soft Seal Connector O Ring

In port applications, the seal is made by an elastomeric seal at the base of the thread, and not on the machined seat.

BSPP Soft Seal Port Applications

BSPP port threads are very commonly used in Australia, but are becoming less preferred.

BSPP Port Threads Engine

BSPP threads can also be sealed by other methods such as an encapsulated seal or a bonded washer.

When a bonded washer is used, the shoulder of the thread must be machined to a cone to keep the washer away from the edge of the thread.

BSPP Thread Seal Bonded Washer

The photo below shows an encapsulated seal.

BSPP Thread Encapsulated Seal

This sealing method is common on European threads such as Metric and BSPP.

UNO (O-ring Boss)

Unified National
Country of origin: USA
Thread angle: 60 degrees (UNC or I-INF)
Designation: SAE J1926, ISO 11926-2

O-ring Boss threads are port threads, sealed by an O-ring.

They use exactly the same threads as the JIC, but they don’t have the 37 degree sealing face of the JIC.

UNO O Ring Boss Male and Female

This is a very popular port thread. Its principal advantage is that the sealing face is protected when compared to a metric or BSPP port.

Metric – DIN 2353

Deutsche Industrie Normen
Country of origin: Germany
Thread angle: 60 degrees (DIN 3853)
Seat angle: 24 degrees

The elastomeric sealing method uses an O-ring on the sealing face.

Metric DIN Elastomeric Sealing O Ring

DKOL: Light series
DKOS: Heavy series

The “O” in the acronym refers to the O-ring.

Metric ports are like BSPP ports in every respect. They use a bonded washer, and seal on a flat spotface.

Metric DIN Bonded Washer

The only difference is the thread itself.

ORFS O-ring Face Seal

Country of origin: USA
Thread angle: 60 degrees (UNC or I-INF)
Designation: SAE J1453, ISO 8434-3

The ORFS system consists of a male connector with the O-ring in the front face. This seals against a flat face in the female.

ORFS O-Ring Face Seal Thread and Nut

One of the design features of the ORFS connector is that the female swivel nut can be slipped back.

ORFS O-Ring Face Seal Connector Nuts

This is to assist installation in tight spots, and to allow easier inspection and cleaning.

RYCOLOK Staple connector

Country of origin: USA
Designation: SAE J 1467

The staple lock system is primarily used in underground coal mining.

RYCOLOK Staple Connector Male Female

There is no thread, and spanners are not required.

The staple is installed and removed by the use of a hammer, RYCO part number RL250.

SAE Pilot O-ring Male and Female Swivel

Country of origin: USA

Automotive threads, used in air conditioning.

The O-rings used are green in colour, to signify compatibility with refrigerants.

SAE Pilot O-Ring Male and Female Swivel

SAE 4 – Bolt Flange

Country of origin: USA
Designation: SAE J518, IS06162

The SAE flange has gained acceptance universally. It is particularly suited for larger sizes.

SAE Bolt Flange

American equipment manufacturers use UNC bolts, while European manufacturers use metric bolts.

Only high tensile bolts must be used.

There are two principal series of SAE 4 bolt flanges, code 61 and code 62.

Code 61 is the general purpose series. While it is very much a high pressure hydraulic connection, code 61 flanges are also commonly used for suction and return applications.

SAE Bolt Flange Code 61

Code 62 is an “isobaric” series, which means that the pressure rating is consistent, at 420 Bar across the range.

The pressure increase is made possible by increasing the dimensions of the flange diameter and thickness, as well as increasing the bolt size used.

Caterpillar have further modified the code 62 flange for use in their equipment. RYCO manufacture couplings to suit Caterpillar equipment, and call them “Code 62C”.

SAE Bolt Flange Code 62 and Code 62C

The modification is that the flange thickness is increased to 14.2mm in all sizes. The diameter of the flange and the bolt dimensions remain unchanged.

When joining two flanges, at least one of the clamps must be solid.

Never join with two split clamps.

O-rings must never be butted together. One surface must be flat.

SAE Bolt Flange How to Join Correctly

SAE Flange blocks provide a possible solution.

Want more information?

For a quick introduction to the specific types hydraulic connectors check out our series on hydraulic connector thread forms.

Make sure you select the right hydraulic hose for your next project. Download our 15 Keys To Selecting The Right Hose guide today.

Soft Seal Connectors – The ultimate hydraulic connectors

Soft Seal Connectors – The ultimate hydraulic connectors

Soft seal connectors are a modern development in hydraulic connectors, intended to address the most common issues with older designs, including leakage and the effects of vibration.

They’re also designed to work in much higher pressure systems, to meet the growing needs of a wide range of applications across different industries.

Soft Seal Connector Definitions


noun (Chemistry)

An elastic substance occurring naturally, as a natural rubber, or produced synthetically, such as butyl rubber or neoprene.


adjective (Chemistry)

A part that has an elastic quality about it, or one that uses an elastomer.

Soft seal, or elastomeric connectors use a soft seal to complete the joint, resulting in a leak free and vibration resistant joint.

The seal is usually an O-ring.

Advantages of soft seal connectors

Soft seal connectors have a number of advantages, including:

  • They are positive sealing
  • Resistant to vibration
  • Very high pressure capability
  • Easy and quick to assemble, which is important in high-volume production

Disadvantages of soft seal connectors

The O-ring in soft seal connectors may harden with heat or soften with incompatible system fluids, so it is important that the seal material is suitable for the application.

Fluids that can affect the elastic material in soft seal connectors include fuels, oils, solvents and other substances, depending on the composition of the soft seal.

For this reason, selecting the right soft seal connector for the intended application is essential to ensure that the connector continues to perform as expected.

Practical Applications of Soft Seal Connectors

Examine the hydraulic system of this loader.

RYCO Hydraulic Hose Soft Seal Connectors

It has been designed to be leak free. It employs elastomeric sealing with SAE flange, ORFS, UNO ports and O-ring sealing inside the pumps.

It has been in service for a period of time already, and remains clean and leak-free.

Elastomeric couplings are a modern, leak-free alternative to some older design hydraulic couplings.

New hydraulic systems can be designed and sold on the basis of being leak-free.

All it takes is a little understanding of the connection options available.

Soft Seal Connectors In Industry

Soft seal connectors, also called elastomeric connectors, use a soft seal to complete the joint, addressing both leakage and vibration effects.

Types of Soft Seal Connectors

Here are the main types of soft seal connectors available:

  • DKOL (Dicht Kegel O Ring Leicht)
  • DKOS (Dicht Kegel O Ring Schwer)
  • SAE Flange (Society of Automotive Engineers)
  • Quick release couplings
  • BSPP port – British Standard Pipe Parallel
  • UNO port – Unified National O-ring
  • ORFS – O-Ring Face Seal

For an overview of the different hydraulic connector types, check out our introduction to hydraulic connectors.

Want More Information?

Make sure you select the right hydraulic hose for your next project. Download our 15 Keys To Selecting The Right Hose guide today.