ORB Fitting Size Chart: Thread Sizes, Dash Sizes & Dimensions

ORB fittings are widely used in hydraulic systems where a reliable, leak-resistant connection is required between a fitting and a threaded port. ORB stands for O-Ring Boss, a connection design that uses a straight thread for mechanical engagement and an O-ring for sealing.

Unlike tapered pipe threads such as NPT, the threads on an ORB fitting do not create the primary seal. Instead, the O-ring is compressed against a machined sealing surface around the port. This design provides excellent resistance to leakage, vibration, and repeated assembly, making ORB connections common in hydraulic pumps, valves, cylinders, manifolds, motors, and mobile equipment.

ORB fittings are commonly associated with SAE hydraulic standards, particularly SAE J1926 for straight-thread O-ring ports and SAE J514 for hydraulic tube fittings and adapters.

An ORB Fitting Size Chart helps identify the correct fitting by comparing several dimensions and designations, including:

• ORB dash size
• Nominal size
• Thread diameter
• Threads per inch (TPI)
• SAE straight-thread designation
• Tube or hose size
• O-ring size
• Port dimensions

Because several hydraulic fitting types can have similar thread diameters, identifying an ORB connection by thread diameter alone can lead to incorrect fitting selection. The presence and position of the O-ring, thread pitch, and port geometry should also be checked.

This guide explains ORB fitting sizes, thread dimensions, dash numbers, port dimensions, O-ring sizes, and practical methods for identifying ORB connections.

1. What Is an ORB Fitting?

An ORB fitting, or O-Ring Boss fitting, is a hydraulic connection that uses a straight-threaded male fitting screwed into a female threaded port. An elastomeric O-ring located near the base of the male thread provides the pressure seal.

The basic connection consists of:

• A male straight-thread fitting
• A female threaded ORB port
• An O-ring
• A machined sealing surface
• A shoulder or hex section used to tighten the fitting

The male threads provide the mechanical force needed to hold the fitting in position. The O-ring performs the actual sealing function.

How an ORB Fitting Seals

When the fitting is tightened into the port, the O-ring is compressed between the fitting and the machined sealing surface of the port.

The sealing principle can be simplified as:

Thread engagement → fitting tightened → O-ring compressed → hydraulic seal created

Because the thread itself does not have to provide the seal, ORB connections generally offer better leak resistance than tapered-thread connections.

The O-ring also compensates for small surface irregularities and dimensional tolerances between the fitting and the port.

ORB Male Fitting

A typical male ORB fitting has:

• SAE straight threads
• An O-ring positioned near the thread shoulder
• A sealing shoulder
• A hex section for tightening

The O-ring normally sits in a groove immediately behind the male thread.

When installed correctly, the O-ring is compressed against the chamfer or spotface of the female port.

ORB Female Port

The female ORB connection is normally a machined port rather than a separate female fitting.

It includes:

• Internal straight threads
• A machined entry chamfer
• A sealing surface
• A defined port depth
• A spotface around the port

The dimensions of the port are important because incorrect machining can cause poor O-ring compression, extrusion, or leakage.

Advantages of ORB Fittings

ORB fittings offer several advantages in hydraulic applications.

Excellent leak resistance

The elastomeric O-ring creates a highly effective pressure seal.

No thread sealant required

Unlike NPT connections, ORB fittings normally do not require PTFE tape or liquid thread sealant.

Reusable connection

The fitting can usually be removed and reinstalled as long as the threads and sealing surfaces remain undamaged and the O-ring is replaced when necessary.

Suitable for high-pressure systems

ORB connections are commonly used in high-pressure hydraulic equipment.

Good vibration resistance

The mechanical straight-thread connection and O-ring seal perform well in applications exposed to vibration and pressure cycling.

Common ORB Applications

ORB fittings are frequently found on:

• Hydraulic pumps
• Hydraulic motors
• Directional control valves
• Pressure control valves
• Hydraulic cylinders
• Manifolds
• Accumulators
• Hydraulic power units
• Construction machinery
• Agricultural equipment
• Mobile hydraulic systems

They are especially common where components have machined SAE hydraulic ports.

2. ORB Thread Standard and Size Terminology

ORB fittings use straight UN or UNF threads rather than tapered pipe threads. The thread provides mechanical retention, while the O-ring provides the hydraulic seal.

ORB port dimensions are commonly standardized under SAE J1926, while many hydraulic adapters and fittings using these ports are covered by SAE J514.

Understanding the terminology used in an ORB fitting size chart is important because a single fitting may be described using several different size designations.

For example, an ORB fitting may be identified by:

• Dash size
• Nominal size
• Thread size
• Major thread diameter
• Threads per inch
• Tube or hose size

ORB Dash Size

Hydraulic fittings are commonly identified using a dash number.

The dash number is generally based on sixteenths of an inch.

For example:

• -4 = 4/16 in = 1/4 in
• -6 = 6/16 in = 3/8 in
• -8 = 8/16 in = 1/2 in
• -12 = 12/16 in = 3/4 in
• -16 = 16/16 in = 1 in

However, the dash size does not directly equal the physical outside diameter of the ORB thread.

For example:

ORB -8 uses a 3/4-16 thread.

The -8 designation represents a nominal 1/2-inch hydraulic size, while the actual male thread diameter is approximately 3/4 inch.

This distinction is important when identifying fittings.

Thread Size

ORB thread sizes are usually written in the following format:

Major diameter – Threads per inch

For example:

3/4-16 UNF

This means:

• Nominal thread diameter = 3/4 inch
• Threads per inch = 16
• Thread form = Unified National Fine

Another common example is:

9/16-18 UNF

This thread is commonly associated with an ORB -6 connection.

Major Thread Diameter

The major diameter is the largest diameter of the thread.

For a male fitting, it can be measured across the outside crests of the thread using a vernier caliper.

For example:

A measured male thread OD of approximately 0.75 in combined with 16 TPI usually indicates a 3/4-16 ORB thread, commonly corresponding to size -8.

Thread diameter should always be checked together with thread pitch.

Threads Per Inch

TPI, or threads per inch, indicates how many thread peaks occur within one inch of thread length.

Common ORB thread pitches include:

• 24 TPI
• 20 TPI
• 18 TPI
• 16 TPI
• 14 TPI
• 12 TPI

A thread pitch gauge is normally the easiest tool for determining TPI.

ORB Thread Size Examples

Some commonly encountered ORB sizes include:

ORB Size	Nominal Size	Thread Size	TPI
-4	1/4 in	7/16-20	20
-5	5/16 in	1/2-20	20
-6	3/8 in	9/16-18	18
-8	1/2 in	3/4-16	16
-10	5/8 in	7/8-14	14
-12	3/4 in	1-1/16-12	12
-16	1 in	1-5/16-12	12
-20	1-1/4 in	1-5/8-12	12
-24	1-1/2 in	1-7/8-12	12
-32	2 in	2-1/2-12	12

ORB vs. Thread Diameter

One of the most common mistakes when identifying ORB fittings is assuming that nominal size equals thread diameter.

For example:

ORB -6

Nominal hydraulic size:

3/8 in

Actual thread:

9/16-18

Therefore, measuring a thread OD of approximately 9/16 inch does not mean the connection is a 9/16-inch hydraulic line.

The correct identification is based on the combination of:

Thread diameter + thread pitch + O-ring sealing design

ORB O-Ring Location

The O-ring is normally located in a groove immediately behind the male thread.

This is one of the easiest visual clues for identifying an ORB fitting.

Typical configuration:

Male thread → O-ring groove → O-ring → fitting shoulder

When the fitting is tightened, the O-ring contacts the machined surface at the entrance of the female port.

This is fundamentally different from JIC fittings, where sealing occurs at a 37° metal-to-metal flare surface, and NPT fittings, where sealing occurs primarily through tapered-thread interference.

3. Complete ORB Fitting Size Chart

An ORB Fitting Size Chart allows engineers, technicians, and maintenance personnel to identify a fitting by comparing its dash size, nominal size, thread diameter, and thread pitch.

The table below covers the most common SAE O-Ring Boss fitting sizes used in hydraulic systems.

ORB Dash Size	Nominal Size	Thread Size	TPI	Approx. Male Thread OD
-2	1/8 in	5/16-24	24	0.3125 in
-3	3/16 in	3/8-24	24	0.3750 in
-4	1/4 in	7/16-20	20	0.4375 in
-5	5/16 in	1/2-20	20	0.5000 in
-6	3/8 in	9/16-18	18	0.5625 in
-8	1/2 in	3/4-16	16	0.7500 in
-10	5/8 in	7/8-14	14	0.8750 in
-12	3/4 in	1-1/16-12	12	1.0625 in
-14	7/8 in	1-3/16-12	12	1.1875 in
-16	1 in	1-5/16-12	12	1.3125 in
-20	1-1/4 in	1-5/8-12	12	1.6250 in
-24	1-1/2 in	1-7/8-12	12	1.8750 in
-32	2 in	2-1/2-12	12	2.5000 in

ORB Fitting Size Chart in Metric Dimensions

For users working with metric measuring tools, the approximate major thread diameters can also be expressed in millimeters.

ORB Size	Thread Size	Approx. Thread OD
-2	5/16-24	7.94 mm
-3	3/8-24	9.53 mm
-4	7/16-20	11.11 mm
-5	1/2-20	12.70 mm
-6	9/16-18	14.29 mm
-8	3/4-16	19.05 mm
-10	7/8-14	22.23 mm
-12	1-1/16-12	26.99 mm
-14	1-3/16-12	30.16 mm
-16	1-5/16-12	33.34 mm
-20	1-5/8-12	41.28 mm
-24	1-7/8-12	47.63 mm
-32	2-1/2-12	63.50 mm

These values represent the nominal major diameter of the thread. Actual measured dimensions may be slightly smaller because of manufacturing tolerances and thread geometry.

How to Use the ORB Size Chart

Suppose the outside diameter of a male thread measures approximately:

19.0 mm

and the thread pitch corresponds to:

16 TPI

The closest match in the chart is:

3/4-16 thread

This corresponds to:

ORB -8

Another example:

Measured thread OD:

14.2 mm

Measured pitch:

18 TPI

The fitting is likely:

9/16-18 ORB -6

It is important to confirm that the fitting also has an O-ring located behind the male thread before identifying it as ORB.

Important Note About Nominal Size

The nominal ORB size does not equal the actual thread diameter.

For example:

ORB -8 = nominal 1/2 in size

but the thread is:

3/4-16

Similarly:

ORB -12 = nominal 3/4 in size

but the thread is:

1-1/16-12

For this reason, always use both the thread diameter and thread pitch when identifying ORB fittings.

4. ORB Thread Size and Dash Size Chart

The dash size system is commonly used throughout the hydraulic industry to simplify the identification of fittings, hoses, tubes, and ports.

For ORB fittings, the dash number generally represents the nominal hydraulic size in sixteenths of an inch.

For example:

-4 = 4/16 in = 1/4 in

-6 = 6/16 in = 3/8 in

-8 = 8/16 in = 1/2 in

-12 = 12/16 in = 3/4 in

-16 = 16/16 in = 1 in

However, the ORB thread itself is larger than the nominal dash size.

ORB Dash Size to Thread Size Chart

Dash Size	Nominal Size	ORB Thread Size
-2	1/8 in	5/16-24
-3	3/16 in	3/8-24
-4	1/4 in	7/16-20
-5	5/16 in	1/2-20
-6	3/8 in	9/16-18
-8	1/2 in	3/4-16
-10	5/8 in	7/8-14
-12	3/4 in	1-1/16-12
-14	7/8 in	1-3/16-12
-16	1 in	1-5/16-12
-20	1-1/4 in	1-5/8-12
-24	1-1/2 in	1-7/8-12
-32	2 in	2-1/2-12

Common ORB Sizes

ORB -4

ORB -4 is one of the smaller common hydraulic port sizes.

Nominal size:

1/4 in

Thread:

7/16-20

It is frequently used in:

• Pressure gauge connections
• Small hydraulic valves
• Instrumentation lines
• Pilot circuits

ORB -6

ORB -6 is commonly used in medium-flow hydraulic systems.

Nominal size:

3/8 in

Thread:

9/16-18

Typical applications include:

• Hydraulic control valves
• Small cylinders
• Mobile equipment
• Hydraulic manifolds

ORB -8

ORB -8 is one of the most widely encountered ORB sizes.

Nominal size:

1/2 in

Thread:

3/4-16

It is commonly found on:

• Hydraulic pumps
• Motors
• Cylinders
• Directional control valves
• Manifold blocks

ORB -10

Nominal size:

5/8 in

Thread:

7/8-14

This size is commonly used where slightly higher flow rates are required.

ORB -12

Nominal size:

3/4 in

Thread:

1-1/16-12

ORB -12 is commonly used on medium-to-large hydraulic equipment.

ORB -16

Nominal size:

1 in

Thread:

1-5/16-12

This size is commonly found on larger pumps, motors, cylinders, and manifolds.

ORB -20, -24, and -32

These larger sizes are typically used in high-flow hydraulic circuits and heavy-duty equipment.

They may be found on:

• Large hydraulic power units
• Heavy construction equipment
• Industrial presses
• Large hydraulic cylinders
• High-flow pump and motor connections

Dash Size vs. Hose Size

Although ORB dash sizes often correspond to nominal hose or tube sizes, the connection size and hose size should still be checked independently.

For example, an adapter may have:

ORB -8 male × JIC -6 male

In this case, the ORB port side and the connected hydraulic line do not have the same dash size.

Therefore, when selecting an adapter, always verify both ends separately.

5. ORB Port Dimensions and O-Ring Sizes

The performance of an ORB connection depends not only on the thread but also on the geometry of the female port and the correct O-ring.

A typical ORB port includes several important dimensions:

• Internal thread diameter
• Thread depth
• Port depth
• Entry chamfer
• Spotface diameter
• Spotface flatness
• O-ring sealing area

These features determine how the fitting is positioned and how the O-ring is compressed during assembly.

ORB Port Geometry

A typical SAE straight-thread O-ring port consists of:

Spotface → chamfer → straight thread → internal passage

The male fitting is threaded into the port until the O-ring contacts the port chamfer and sealing surface.

As tightening continues, the O-ring is compressed to create a pressure-tight seal.

The spotface around the port also provides a flat surface for the fitting shoulder and helps control sealing geometry.

Why the Port Chamfer Matters

The entry chamfer guides the O-ring into position during installation.

If the chamfer is:

• Too sharp
• Damaged
• Scratched
• Burred
• Incorrectly machined

the O-ring can be cut or pinched during assembly.

A damaged O-ring may initially appear to seal correctly but can fail after the system is pressurized.

ORB O-Ring Sizes

Each ORB fitting size requires an O-ring with the correct inside diameter and cross-section.

The O-ring must fit correctly around the fitting and provide sufficient compression when installed in the port.

Typical ORB O-rings are selected according to the applicable SAE fitting or port specification.

The correct O-ring should always be verified using:

• Fitting manufacturer data
• SAE dimensional requirements
• Equipment manufacturer specifications

Using an O-ring that is too small may overstretch it during installation.

Using an O-ring that is too large may prevent correct seating or cause extrusion.

Common O-Ring Materials

Several elastomer materials are used with ORB fittings.

NBR

Nitrile rubber (NBR) is one of the most common materials used in standard hydraulic systems.

It generally offers good compatibility with:

• Mineral-based hydraulic oils
• Petroleum oils
• Many industrial hydraulic fluids

NBR is commonly used for general-purpose hydraulic service.

FKM

Fluorocarbon rubber (FKM) is often selected where higher temperatures or more aggressive fluids are involved.

Typical applications include:

• High-temperature hydraulic systems
• Synthetic fluids
• Chemical processing equipment

EPDM

EPDM may be suitable for certain water-based fluids and other compatible media.

However, EPDM is generally not suitable for many petroleum-based hydraulic oils.

Fluid compatibility must therefore be checked before selecting the O-ring material.

O-Ring Condition

Before installing an ORB fitting, inspect the O-ring for:

• Cuts
• Cracks
• Flattening
• Hardening
• Swelling
• Abrasion
• Permanent deformation

A damaged O-ring should be replaced before the fitting is reassembled.

Lubricating the O-Ring

A light film of compatible lubricant or system fluid can help reduce friction during assembly and decrease the risk of cutting or twisting the O-ring.

The lubricant must be compatible with:

• The hydraulic fluid
• The O-ring material
• The system cleanliness requirements

For sensitive systems, such as semiconductor, oxygen, or high-purity applications, only approved lubricants should be used.

Why Correct O-Ring Selection Is Important

The O-ring is the primary sealing component in an ORB connection.

Even if the thread size is correct, the connection can still leak if the O-ring has:

• Incorrect dimensions
• Incorrect material
• Excessive compression
• Insufficient compression
• Surface damage
• Chemical incompatibility

For this reason, ORB fitting selection should always consider both the thread size and the O-ring specification.

6. How to Identify and Measure an ORB Fitting

Correctly identifying an ORB fitting requires more than simply measuring the thread diameter. Several hydraulic connection types can have similar straight-thread dimensions, so the thread size, thread pitch, sealing method, and O-ring location should all be checked.

A typical ORB fitting can be identified by three main features:

• Straight SAE threads
• An O-ring located behind the male thread
• A female port with a machined sealing chamfer

The most reliable identification method is to measure the thread diameter and pitch, then compare the results with an ORB fitting size chart.

Step 1: Check for an O-Ring

Start by inspecting the male fitting.

An ORB fitting normally has an O-ring positioned in a groove directly behind the thread.

Typical arrangement:

Thread → O-ring groove → O-ring → fitting shoulder

If there is no O-ring and the fitting seals on a cone or flare surface, it may be another connection type such as JIC.

If the thread itself is tapered, the fitting may be NPT rather than ORB.

Step 2: Check Whether the Thread Is Straight or Tapered

ORB fittings use straight threads.

The thread diameter remains essentially constant along the threaded length.

NPT threads are tapered, which means the thread diameter becomes progressively larger toward the fitting body.

A quick visual inspection can often identify the difference, but a caliper provides better confirmation.

Measure the thread diameter at two positions:

• Near the end of the thread
• Near the fitting shoulder

If both measurements are approximately the same, the thread is likely straight.

If the diameter changes noticeably, the connection may be tapered.

Step 3: Measure the Male Thread Outside Diameter

Use a vernier caliper or digital caliper to measure across the outside of the male thread.

Measure across the thread crests rather than the thread roots.

For example:

If the measured OD is approximately:

0.750 in or 19.05 mm

the fitting may use a:

3/4 in thread

If the thread pitch is 16 TPI, this corresponds to:

3/4-16 ORB -8

Another example:

Measured thread OD:

0.562 in or approximately 14.3 mm

Thread pitch:

18 TPI

Likely connection:

9/16-18 ORB -6

Step 4: Measure the Female Thread

For a female ORB port, measure the approximate inside diameter across the thread.

Female thread measurements are less direct than male thread measurements because the caliper contacts the internal thread geometry rather than the nominal major diameter.

For this reason, it is normally easier to identify a fitting using the mating male thread whenever possible.

Step 5: Determine the Thread Pitch

Use a thread pitch gauge to determine the number of threads per inch.

Common ORB pitches include:

• 24 TPI
• 20 TPI
• 18 TPI
• 16 TPI
• 14 TPI
• 12 TPI

Place the thread gauge against the thread until the teeth align completely with the thread profile.

For example:

9/16 in OD + 18 TPI = 9/16-18

This normally corresponds to:

ORB -6

Quick ORB Identification Chart

Approx. Male Thread OD	TPI	Thread Size	ORB Size
11.1 mm	20	7/16-20	-4
12.7 mm	20	1/2-20	-5
14.3 mm	18	9/16-18	-6
19.1 mm	16	3/4-16	-8
22.2 mm	14	7/8-14	-10
27.0 mm	12	1-1/16-12	-12
33.3 mm	12	1-5/16-12	-16
41.3 mm	12	1-5/8-12	-20
47.6 mm	12	1-7/8-12	-24
63.5 mm	12	2-1/2-12	-32

Do Not Identify ORB by Thread Diameter Alone

A thread diameter can sometimes match more than one fitting family.

For example, a straight SAE thread may also appear on other hydraulic connections.

Always confirm:

Thread diameter + thread pitch + straight thread + O-ring sealing method

This combination provides a much more reliable identification.

Common Identification Mistakes

Common errors include:

• Confusing ORB with JIC
• Confusing straight threads with NPT
• Assuming dash size equals thread diameter
• Measuring thread pitch incorrectly
• Ignoring the sealing surface
• Reusing a damaged O-ring
• Selecting a fitting based only on hose size

A correct ORB fitting must match both the mechanical thread and the sealing geometry.

7. ORB vs. JIC, NPT, and Other Hydraulic Fittings

ORB fittings are commonly confused with JIC, NPT, and BSPP connections because all of these fitting types are widely used in hydraulic and industrial fluid systems.

The major difference is the way each connection creates a seal.

ORB vs. JIC

ORB and JIC fittings may both use SAE straight threads, but their sealing methods are completely different.

An ORB connection seals using:

An elastomeric O-ring

A JIC connection seals using:

A 37-degree metal-to-metal flare

Therefore, matching thread sizes do not necessarily mean the fittings are interchangeable.

Feature	ORB	JIC
Thread Type	Straight	Straight
Primary Seal	O-ring	37° flare
Thread Sealant	Not normally required	Not required
Typical Port	SAE O-ring boss port	JIC flare connection
Reusability	High	High
Vibration Resistance	Very good	Very good
Common Use	Component ports	Hose and tube connections

Example: ORB -8 vs. JIC -8

An ORB -8 fitting uses:

3/4-16 thread

A JIC -8 connection also commonly uses:

3/4-16 thread

However, they are not the same connection.

The ORB fitting seals using an O-ring at the port.

The JIC fitting seals at the 37° flare seat.

This is why thread size alone cannot be used to identify the fitting type.

ORB vs. NPT

NPT connections use tapered pipe threads.

The threads become tighter as the male and female components are screwed together.

The seal is created primarily through:

• Thread interference
• Thread deformation
• Thread sealant

ORB connections use straight threads and an O-ring.

Feature	ORB	NPT
Thread Form	Straight	Tapered
Seal Method	O-ring	Thread interference
Sealant Required	Normally no	Normally yes
Repeat Assembly	Good	More limited
Leak Resistance	Excellent	Depends heavily on assembly
Port Damage Risk	Lower	Higher if overtightened

ORB connections are generally preferred in hydraulic systems where repeated servicing and reliable leak control are important.

ORB vs. BSPP O-Ring Connections

BSPP connections also use parallel threads and may use elastomeric seals.

However, the thread form is different.

ORB commonly uses Unified threads such as:

3/4-16 UNF

BSPP uses British Standard Pipe Parallel threads such as:

G 1/2

The thread angle is also different:

• Unified thread: 60°
• BSP thread: 55°

Because of these differences, ORB and BSPP threads should not be mixed.

ORB vs. Metric O-Ring Ports

Metric hydraulic ports may look very similar to ORB connections because they can also use:

• Straight threads
• O-ring sealing
• Machined port faces

However, metric ports use metric thread designations such as:

M18 × 1.5

rather than SAE Unified thread designations such as:

3/4-16

A caliper and thread pitch gauge should be used to distinguish between SAE and metric threads.

Comparison of Common Hydraulic Connections

Connection	Thread Type	Sealing Method	Typical Example
ORB	Straight SAE	O-ring	3/4-16 ORB
JIC	Straight SAE	37° flare	3/4-16 JIC
NPT	Tapered pipe	Thread interference	1/2-14 NPT
BSPP	Parallel BSP	Washer/O-ring/cone	G 1/2
BSPT	Tapered BSP	Thread interference	R 1/2
Metric O-ring	Straight metric	O-ring	M18 × 1.5

Which Connection Provides Better Leak Resistance?

There is no single fitting type that is best for every application.

However, ORB connections offer several advantages where reliable port sealing is required because the O-ring provides a controlled sealing interface.

JIC connections are particularly useful for hose and tubing connections because the metal flare creates a reusable mechanical seal.

NPT is still widely used but can require more installation care because sealing depends heavily on thread engagement and sealant.

8. ORB Fitting Pressure Ratings and Applications

ORB fittings are commonly used in medium- and high-pressure hydraulic systems because the O-ring sealing design provides reliable leak resistance under pressure cycling and vibration.

However, there is no single pressure rating that applies to every ORB fitting.

The allowable working pressure depends on several factors.

These include:

• Fitting size
• Fitting material
• Port material
• Fitting design
• O-ring material
• Fluid type
• Operating temperature
• Manufacturer specifications
• Applicable hydraulic standards

For this reason, the final pressure rating should always be taken from the fitting or equipment manufacturer’s technical data.

Effect of Fitting Size on Pressure Rating

In many fitting systems, smaller fitting sizes can tolerate higher pressures than very large sizes because the force generated by internal pressure increases with the effective pressure area.

However, actual pressure ratings depend heavily on fitting design and material.

Do not assume that every ORB -6 fitting, for example, has the same pressure rating across all manufacturers.

Fitting Material

Common ORB fitting materials include:

• Carbon steel
• Stainless steel
• Brass

Carbon steel is widely used in industrial and mobile hydraulic systems.

Stainless steel may be selected where corrosion resistance or fluid compatibility is important.

Brass is generally used in lower-pressure applications and with compatible fluids.

Material strength directly affects the fitting’s pressure capability.

Port Material

The material of the component containing the female ORB port is also important.

Common port materials include:

• Carbon steel
• Stainless steel
• Aluminum
• Cast iron

A steel fitting threaded into an aluminum manifold may be limited by the strength of the aluminum threads rather than the fitting itself.

Overtightening can damage softer port materials.

O-Ring Material

The O-ring must withstand both the operating pressure and the system fluid.

Common materials include:

NBR

Commonly used for mineral oil hydraulic systems.

FKM

Often selected for higher temperatures and certain aggressive fluids.

EPDM

Used with selected water-based or chemically compatible fluids.

The wrong elastomer can swell, harden, crack, or lose sealing capability.

Temperature Effects

Temperature affects both the fitting and the elastomer seal.

At elevated temperatures:

• O-rings may harden faster
• Elastomer life may decrease
• Fluid viscosity may change
• Pressure capability may need to be derated

At very low temperatures:

• Elastomers can lose flexibility
• Sealing performance may decrease

Always check the approved temperature range for the selected O-ring material.

Common ORB Applications

ORB fittings are widely used on hydraulic components where a compact and reliable port connection is required.

Typical applications include:

Hydraulic Pumps

ORB ports are frequently found on hydraulic pump:

• Inlet ports
• Outlet ports
• Drain ports
• Control ports

Hydraulic Motors

Hydraulic motors may use ORB ports for:

• Pressure connections
• Return connections
• Case drain connections

Hydraulic Cylinders

ORB fittings are often installed directly into cylinder ports.

The O-ring sealing design provides good resistance to pressure cycling.

Hydraulic Manifolds

Machined hydraulic manifolds commonly use ORB ports because they are easy to manufacture and provide reliable sealing.

Typical manifold components include:

• Directional valves
• Relief valves
• Flow controls
• Pressure switches
• Sensors
• Cartridge valves

Mobile Hydraulic Equipment

ORB fittings are frequently used in:

• Excavators
• Loaders
• Agricultural machinery
• Cranes
• Forestry equipment
• Material handling equipment

The good resistance to vibration makes ORB suitable for mobile hydraulic systems.

Hydraulic Power Units

Hydraulic power units may use ORB connections on:

• Pumps
• Filters
• Accumulators
• Manifolds
• Control valves
• Pressure gauges

Why ORB Is Common in Hydraulic Systems

ORB fittings provide several characteristics that make them well suited for hydraulic applications:

• Reliable elastomeric sealing
• No tapered-thread sealant normally required
• Straight-thread installation
• Good vibration resistance
• Good reusability
• Compact port design
• Easy component replacement

Pressure Rating Safety

Never determine the safe working pressure from thread size alone.

Two fittings with the same ORB thread can have different ratings because of differences in:

• Material
• Wall thickness
• Manufacturing process
• Adapter geometry
• Temperature
• Port construction

The system working pressure should not exceed the lowest-rated component in the connection.

For critical hydraulic applications, verify the pressure rating of the complete assembly, including:

Port + fitting + O-ring + hose/tube + adapter + connected component

9. How to Select the Correct ORB Fitting Size

Selecting the correct ORB fitting requires matching more than just the thread diameter. The fitting must be compatible with the port, hydraulic line, working pressure, fluid, and required connection style.

A practical selection process is outlined below.

Identify the Existing Port

Start by confirming that the component actually uses an ORB port.

Check for:

• Straight SAE threads
• A machined sealing chamfer
• An O-ring sealing interface
• No tapered thread form

If possible, refer to the equipment drawing, valve datasheet, manifold specification, or manufacturer’s port designation.

A port labeled, for example:

SAE -8 ORB

typically corresponds to:

3/4-16 straight thread

Measure the Thread Diameter

Use a vernier or digital caliper to measure the male thread outside diameter.

Typical examples include:

Measured Thread OD	Likely ORB Thread	ORB Size
Approx. 11.1 mm	7/16-20	-4
Approx. 14.3 mm	9/16-18	-6
Approx. 19.1 mm	3/4-16	-8
Approx. 22.2 mm	7/8-14	-10
Approx. 27.0 mm	1-1/16-12	-12
Approx. 33.3 mm	1-5/16-12	-16

The measured diameter should then be confirmed using the thread pitch.

Confirm the Thread Pitch

Use a thread pitch gauge to determine the threads per inch.

For example:

19.05 mm OD + 16 TPI

indicates:

3/4-16 thread

This corresponds to:

ORB -8

Checking both diameter and pitch reduces the risk of confusing ORB with other straight-thread connections.

Match the ORB Dash Size

Once the thread size has been identified, use the ORB fitting size chart to determine the dash size.

For example:

9/16-18 = ORB -6

3/4-16 = ORB -8

7/8-14 = ORB -10

1-1/16-12 = ORB -12

The dash size is especially useful when ordering hydraulic fittings and adapters.

Match the Hose or Tube Size

The port size and line size are not always identical.

For example, an adapter may be specified as:

ORB -8 × JIC -6

This means:

• Port side = ORB -8
• Hose or tube side = JIC -6

The fitting should therefore be selected according to both connection ends.

Check the Required Flow Rate

The fitting should also be large enough for the required hydraulic flow.

A fitting that is too small may increase:

• Fluid velocity
• Pressure drop
• Heat generation
• Turbulence
• Energy loss

Therefore, line size should not be selected only according to the available port.

For high-flow systems, verify the recommended hose or tube size based on acceptable fluid velocity and pressure drop.

Verify the Working Pressure

The fitting must have a pressure rating equal to or higher than the system’s maximum operating pressure.

Also consider temporary pressure spikes.

The allowable pressure may depend on:

• ORB size
• Material
• Fitting geometry
• Port material
• Temperature
• Manufacturer

Always use the lowest-rated component as the system limit.

Select the Correct O-Ring Material

The O-ring material must be compatible with the hydraulic fluid and operating temperature.

Typical options include:

O-Ring Material	Typical Use
NBR	Mineral-based hydraulic oils
FKM	Higher temperature and selected aggressive fluids
EPDM	Selected water-based and compatible fluids

Do not select an O-ring based only on pressure rating.

Chemical compatibility is equally important.

Select the Fitting Configuration

ORB fittings are available in many configurations.

Common options include:

• Straight adapters
• 45° elbows
• 90° elbows
• Tees
• Bulkhead adapters
• Reducers
• Plugs
• Adjustable elbows
• Swivel adapters

The fitting geometry should provide enough clearance for installation and maintenance.

Adjustable ORB Fittings

Adjustable ORB fittings are useful when an elbow or tee must be oriented in a specific direction.

These fittings typically use:

• A locknut
• A backup washer
• An O-ring

The fitting can be positioned correctly before the locknut is tightened.

This is particularly useful on:

• Manifolds
• Hydraulic cylinders
• Pumps
• Valve blocks

Common ORB Adapter Combinations

ORB ports are often connected to other hydraulic fitting systems.

Common combinations include:

• ORB to JIC
• ORB to NPT
• ORB to BSPP
• ORB to metric
• ORB to hose barb
• ORB to tube fitting

For example:

ORB -8 × JIC -8

may be used to connect a hydraulic component with an SAE ORB port to a JIC hose assembly.

ORB Fitting Selection Example

Suppose a hydraulic valve has a male fitting removed from its port.

The measured thread outside diameter is approximately:

19.0 mm

The thread gauge shows:

16 TPI

The fitting also has an O-ring located directly behind the male thread.

From the ORB size chart:

19.05 mm ≈ 3/4 in

and:

3/4-16 = ORB -8

Therefore, the port is most likely an:

ORB -8 connection

If the new hose assembly uses a JIC -8 female swivel, an appropriate adapter could be:

ORB -8 male × JIC -8 male

The final adapter should still be checked for:

• Working pressure
• Material compatibility
• O-ring material
• Manufacturer specification

ORB Fitting Selection Checklist

Before ordering an ORB fitting, confirm:

• ORB dash size
• Thread diameter
• Thread pitch
• Male or female connection
• Port sealing style
• Hose or tube size
• Required flow
• Working pressure
• Temperature
• Hydraulic fluid
• O-ring material
• Fitting material
• Straight or angled configuration
• Adapter type

This approach helps prevent incorrect fitting selection and reduces the risk of leakage or port damage.

Conclusion

An ORB Fitting Size Chart is a practical reference for identifying SAE O-Ring Boss fittings used throughout hydraulic systems.

ORB fittings use straight threads for mechanical engagement and an elastomeric O-ring for sealing. This design provides reliable leak resistance and makes ORB connections especially suitable for hydraulic pumps, valves, cylinders, manifolds, motors, and other high-pressure equipment.

The most important point when identifying an ORB fitting is that the dash size is not the same as the actual thread diameter.

For example:

• ORB -6 uses a 9/16-18 thread
• ORB -8 uses a 3/4-16 thread
• ORB -10 uses a 7/8-14 thread
• ORB -12 uses a 1-1/16-12 thread
• ORB -16 uses a 1-5/16-12 thread

The most reliable identification method is to check:

Thread diameter + thread pitch + straight thread form + O-ring sealing design

When selecting a replacement fitting, also verify the pressure rating, fitting material, port material, hydraulic fluid compatibility, and O-ring material.

For critical applications, the dimensional and pressure requirements should always be confirmed against the relevant SAE specification and the fitting manufacturer’s technical data rather than relying only on nominal size.

Used correctly, an ORB fitting size chart makes it much easier to identify fittings, select adapters, troubleshoot hydraulic connections, and avoid mismatched threads or sealing methods.

Pipe Wall Thickness Chart

NPS to DN Conversion Chart