Hydraulic Connectors 101

Hydraulic Connectors 101

As the name suggests, hydraulic connectors are used to connect hydraulic circuits by joining two different end terminations together and creating a flow passage to ensure fluid transfer.

Hydraulic connectors can be grouped into three main types:

  • Internal Thread
  • External Thread
  • No Thread

Internal and external threaded connectors are also known as female and male, respectively.

There are many thread standards in the industry primarily based on the country / region of origin. BSP (British Standard Pipe), NPT (National Pipe Tapered), UN (Unified Inch Screw Threads) and Metric to name some of the most common.

To avoid damage to hydraulic systems, selecting the right type of thread is very important.

Common variation in fitting designs are outlined below that is useful in thread identification.

Tapered / Parallel Threads

Some threads have a tapered thread surface, such as BSPT, NPT (denoted by letter T) whereas others are parallel. BSPP, UN are examples for parallel threads.

Tapered threads are designed to seal on the thread surface.

Sealing of Hydraulic Connectors

Along with tapered threads, other sealing methods include:

  • Metal Seal
  • Soft Seal

Metal seal type was developed to maximise sealing of a connector under pressure. Connectors that have this sealing method will include a machined cone seat (convex or concave) that will seal with the corresponding male / female connector.

Common seat angles include:

  • 30° cone seat (BSPP, some Metric fittings)
  • 45° cone seat (SAE fittings)
  • 37° cone seat (JIC fittings)
  • 24° cone seat (Metric fittings)

Some connectors are designed to seal using elastomeric seals, natural or synthetic to prevent leakage. This type of sealing method is common with adaptors intended for port use, where a soft seal on the base of the adaptor will seal with the flat surface of a port.

Soft seal connectors can be seen on the face of male fittings and adaptors (such as ORFS) which will establish a seal with a flat surface on a female fitting of the same size.

Thread Size, Thread Count & Pitch

Thread size is designated by the external major diameter of a thread that can easily be measured using a calliper. The actual measurement is generally slightly lesser than the nominal thread size. Imperial thread sizes are expressed in inches and metric thread sizes in millimetres paired with “M” (e.g. M6 for 6mm).

Thread count (also known as Threads Per Inch – TPI) and pitch are also important factors when identifying threads and are usually stated along with the thread size.

TPI is common with imperial connectors which basically is the number of threads within an inch measured lengthwise.

For metric threads, pitch is used instead which means the distance between two crests of a thread in millimetres.

How to Identify Connectors?
RYCO’s Thread ID Mate application provides an intuitive and simple identification process to help you find detailed information about threads specifications, sizes and more.

Hydraulic threads can be identified using a few simple steps using the Thread ID Kit and RYCO Thread ID Mate application.

  1. Determine the thread type:
    Visually inspect to determine if the connector has and internal or an external thread (if any).
  2. Is the thread tapered or parallel?:
    Tapered threads can be identified by visual inspection or placing a straight surface, such as a ruler, next to the thread to compare the difference in diameter at the shoulder of the fitting to the diameter at the tip.
  3. What type of sealing face / method does the connector have?:
    Check if the connector has a conical sealing face or a flat sealing face, and whether a soft seal is present.
  4. Using Vernier Calipers included within the RYCO Thread ID Kit, measure the outer / inner diameter of the thread:
    Measure the ID / OD of the connector across flats of the Vernier Calipers for better accuracy.
  5. Determine the sealing face seat angle (if any):
    Align the seat gauge of the Thread ID Kit with the seat of the connector and identify the angle that matches with the seat.
  6. Find the pitch / TPI:
    Use the thread pitch gauge to find out the exact pitch / TPI and verify the thread.

RYCO Thread ID Mate offers you a guided, user friendly process to identify connectors quickly and efficiently.

The Thread ID Mate application is available for your Android and iOS devices. That’s thread identification you can keep in your pocket.

Want More Information?

If you would like to know more about hydraulic connectors, contact our RYCO Hydraulics Customer Service team or fill out our contact form.

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.

Tapered pipe thread hydraulic connectors – BSPT and NPTF

Tapered pipe thread hydraulic connectors – BSPT and NPTF

Tapered pipe thread hydraulic connectors have an outside diameter that is cut on a cone.

This results in sides that are not parallel.

Two common types of tapered thread connectors are BSPT and NPTF.

BSPT British Standard Pipe Taper

Country of origin: Britain
Thread angle: 55 degrees (Whitworth)
Seat angle: 30 degrees

BSPT is a tapered thread. This means that the outside diameter of the thread is cut on a cone.

BSPT Thread - Not Parallel

The sides are not parallel.

The taper angle is a rise of 3/4″ for every 12″ in length.

This is approximately 1 degree, 47 minutes.

BSPT Thread - Angles

Male BSPT threads mate with female BSPT threads in adaptors.

BSPT female threads are also used in port applications.

BSPT Thread - Male-Female

As BSPT ports seal directly on the thread, the only machining required is drilling the hole and tapping the thread.

BSPT threads can also be used as a machined seat connector.

BSPT Thread - Nut

The thread used in the female swivel is parallel, BSPP.

This is so that the connector bottoms out onto the sealing face.

The female swivel is made as a two part assembly.

The majority of these connections use a crimped nut.

BSPT Thread - Nut

Larger sizes use a wire locked nut.

This increases the pressure capability of the connection.

BSPT Thread - Wire Lock

NPTF National Pipe Taper (Fuel)

Country of origin: USA
Thread angle: 60 degrees (ANSI)
Seat angle: 30 degrees

NPT is the standard American pipe thread.

NPTF is a derivative used in hydraulics. The “F” means that it is modified for fuel use.

NPTF Thread Diagram

NPTF threads have controlled truncation which means that the roots and crests of NPT threads fit more closely with their mating parts than most other threads.

For this reason, NPT threads feel “sharper” than their BSPT counterparts.

NPTF Thread Diagram

NPT threads are also used in port applications.

NPT port threads will look identical to BSPT ports, but it is important that NPT threads are not interchanged with BSP threads.

NPTF Thread Diagram

When NPT threads are used as machined seat fittings, a parallel female thread is used.

This is to ensure that the threads bottom out on the machined seat.

This thread is called NPSM, National Pipe, Straight Mechanical.

All RYCO NPT parts are extemally grooved or notched so that they can be easily identified.

NPTF Thread Nut

A notch results when a groove is machined in a hex.

Larger sizes use a wire locked nut.

NPTF Thread Wire Lock

This increases the pressure capability of the connection.

NPT threads are the preferred thread for very high pressure systems such as jacking systems.

Working Pressure 700 Bar – 10,000

Pipe threads are mechanically very strong because the thread length is generally twice as long as that of an equivalent sized thread such as JIC.

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.

Hydraulic connector thread forms and their applications

Hydraulic connector thread forms and their applications

If you’re new to the world of hydraulics, it can seem difficult to gain an understanding of the many hydraulic connectors available today.

In this series of articles, you’ll learn about the different types of hydraulic connectors and their key differences, which will assist in making some sense of this complex subject.

Factors affecting development of hydraulic connectors

Two factors have influenced the development of hydraulic connectors:

1: A reluctance to adopt industrial standards between countries
Many of these connector types were developed during the 20th century; a time of fierce industrial competition.

Co-operation between the major industrialised countries was limited for many years due to conflict.

2: The evolution of hydraulic systems
Hydraulic systems have changed a great deal in a short period of time.

Driving this change is the gradual increase of the desired machine output force, which is seen as a pressure increase.

In the previous series of articles introducing the three types of hydraulic connectors we discussed three principle types of connectors:

  1. Tapered threads
  2. Metal seal connectors
  3. Soft seal connectors

Each major industrial region has developed threads and connectors which fall into these categories.

How a thread is defined

There are three main elements that define a thread.

  1. The thread angle
  2. The thread pitch
  3. The thread size, or outside diameter

Other dimensions such as the depth of the thread and the minor diameter are calculated from the above sizes.

The following sections describe some basic fundamentals of threads.

Standards are used in engineering to ensure that parts from two manufacturers will fit together.

Most threads are made to a standard, and have set engineering dimensions.


The thread angle of a BSPP thread is 55 degrees.

Different threads have different thread angles.

For example NPT is 60 degrees.

Thread Anatomy Angle


The pitch of a thread is the length between two crests or two roots.

A ⅜” BSPP thread has a pitch of 19 threads per inch (TPI).

1” divided by 19 is 0.0526”.

Thread Anatomy Pitch

Outside Diameter

The outside diameter of a thread is nominated in the standard.

BSPP is a NOMINAL thread size.

(The diameter is the not the size as stated, but instead, suits the walls of a pipe of that bore size.)

Thread Anatomy Outside Diameter


The depth of a thread is based on a ratio of the pitch. (d = 0.61 x P).

In this case: 0.61 x 0.526 = 0.032”

Thread Anatomy Depth

Minor Diameter

The minor diameter of a thread is calculated as follows:
md = OD – (2 x depth)
md = 0.64” – (2 x 0.032”)
md = 0.64” – 0.064”
md = 0.576”

Thread Anatomy Minor Diameter

Tapered Pipe Threads

Tapered pipe threads have an outside diameter that is cut on a cone, and the sides are not parallel.

Tapered pipe thread types include BSPT, NPT, UNS, JIS, Metric Taper and NPTF.

Metal Seal Connectors

Metal seal hydraulic connectors use a profiled seat to form a seal.

Common types of metal seal connectors include JIC, DIN, BSPP, BSPP Flat Face, JIS, NSPM, 30 Degree Flare, Komatsu, Metric French GAZ, Metric French Millimetric and SAE (Automotive).

Soft Seal Connectors

Soft seal connectors use an elastomeric soft seal to ensure a leak-free joint, and may use O-rings, encapsulated seals and bonded washers.

Soft seal connector types include BSPP, UNO, DIN, ORFS, RYCOLOK, SAE Pilot O-Ring and SAE Flange.


The available hydraulic connectors can be defined by two sub groups: country of origin and sealing method.

The table below shows how each region has developed solutions for each of the sealing methods.

Thread Forms Sealing Methods by Country

For a quick introduction to hydraulic connectors check out our series on 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, or read about BSP Thread here.