A mismatched thread or seal can bring your entire operation to a standstill. These small errors lead to persistent leaks, create serious safety hazards, and result in costly downtime while you hunt for the correct component.

This definitive N-Z glossary decodes the language of hydraulic connections. It explains terms from NPT threads and O-Ring seals to torque specifications, empowering you to build reliable, leak-free systems every time.

NPT to O-Ring Boss?

A pipe thread connection keeps leaking, despite being tightened repeatedly. Overtightening has now cracked the valve body, turning a small leak into a major repair job and extended downtime for the machinery.

NPT is a tapered pipe thread that requires sealant. An O-Ring Boss (ORB) fitting uses a straight thread and a high-durability O-ring, providing a far more reliable seal for modern hydraulic systems.

American Threads and Sealing Methods

In the Americas, tapered pipe threads are a legacy standard that we still supply frequently, but we always advise customers on their limitations compared to modern O-ring seals.

• NPT (National Pipe Taper): This is the most common tapered pipe thread in the United States. The male and female threads are both tapered, and the seal is theoretically made on the flanks of the threads. In practice, a sealant like pipe dope or liquid thread sealant is always required to fill the helical leak path that exists. We strongly advise against using PTFE tape, as slivers can break off and contaminate the hydraulic system.
• NPTF (National Pipe Taper Fuel): Also known as a “Dryseal” thread, NPTF is an improvement on NPT. The threads are designed to crush and deform at the root and crest during assembly, creating a metal-to-metal seal without sealant. While this is the theory, we find that in high-pressure or high-vibration applications, using a liquid sealant is still the best practice to guarantee a leak-free joint.
• O-Ring: An elastomeric loop with a round cross-section, used as a seal. In hydraulic fittings, they are typically made from 90 durometer NBR (Nitrile) or Viton for high-temperature applications. They are the core technology behind the most reliable hydraulic sealing methods.
• O-Ring Boss (ORB): Also called SAE Straight Thread, this fitting (governed by SAE J1926-1) uses a straight, parallel thread. The seal is not made by the threads but by an O-ring that sits in a groove on the male fitting and compresses into a chamfered “boss” on the female port. We consider this the standard for reliable port connections on valves, pumps, and cylinders. It is vastly superior to NPT for high-pressure hydraulics.

ORFS to PSI?

A fitting on a hydraulic excavator persistently leaks under high pressure spikes. The metal-to-metal flare connection can’t handle the system’s intense impulse cycles, causing constant maintenance issues and safety concerns.

O-Ring Face Seal (ORFS) fittings offer superior leak prevention in demanding applications. PSI is the unit of measure for pressure, defining the force a fitting must withstand.

Advanced Sealing and Pressure Metrics

When a customer needs absolute, zero-leak reliability, we almost always guide them to ORFS. The design principle is simple and extremely effective.

• ORFS (O-Ring Face Seal): This fitting (SAE J1453) is our top recommendation for high-vibration and high-pressure applications. As the name suggests, it seals with an O-ring in the face of the male fitting, which is compressed onto a flat face on the female fitting. The threads solely provide clamping force and are not involved in sealing. This soft seal design isolates the connection from vibration and is extremely resistant to leaks caused by pressure impulses.
• Parallel Thread: Any thread form where the male and female threads are parallel to each other (not tapered). These threads cannot create a seal by themselves and always require a separate sealing mechanism, such as a bonded seal (BSPP) or an O-ring (ORB).
• Port: A generic term for the connection point on a hydraulic component where fluid enters or exits. Ports can be found on pumps, motors, cylinders, and valves, and they are machined to accept a specific fitting type, such as NPT, ORB, BSPP, or a four-bolt flange.
• PSI (Pounds per Square Inch): The primary unit of pressure measurement in the United States and some other regions. All fittings have a maximum pressure rating in PSI, which must not be exceeded.

Fitting Type	Sealing Method	Vibration Resistance	Pressure Rating
NPT/NPTF	Metal-to-Metal (Taper)	Poor	Medium
JIC	Metal-to-Metal (Flare)	Fair	High
ORB	O-Ring (in Port)	Good	High
ORFS	O-Ring (in Face)	Excellent	Very High

Quick Disconnect to Reusable Fitting?

Connecting and disconnecting hydraulic lines on a tractor implement is slow and messy. Using standard fittings leads to significant fluid spillage and allows contaminants to enter the open lines.

A quick disconnect coupling allows for fast, tool-free connections with minimal spillage. Reusable fittings are an older technology allowing for field assembly with hand tools, now largely replaced by crimp fittings.

Functionality and Assembly Methods

Speed of service and ease of use are critical in many applications, especially agriculture and construction. This is where quick disconnects excel.

• Quick Disconnect (or Quick Coupling): This is a two-part fitting (male and female half) that allows for rapid connection and disconnection of a hydraulic line without tools. Internal valves in both halves automatically close when disconnected, preventing fluid loss and minimizing contamination. We supply several types:
  • Poppet Style (ISO 7241-A): The classic, general-purpose “AG” style coupling.
  • Flat Face (ISO 16028): A superior no-drip design that is easy to clean and prevents contamination. It’s the standard for skid steers and other construction equipment.
• Race: This refers to the machined track in a fitting or bearing that contains ball bearings. In a female swivel fitting (like a JIC), the race allows the nut to spin freely for easy assembly.
• Reusable Fitting: A type of fitting that can be attached to a hose using only wrenches. It typically consists of a socket that threads over the hose and a nipple that threads into the socket, compressing the hose to create a seal. We stock these for certain low-pressure or legacy applications, but for any modern, high-pressure system, a crimped fitting is a far safer and more reliable choice. They are useful for emergency field repairs where a crimper is not available.

SAE to Seat Angle?

A customer orders a “3/4 inch SAE fitting” and receives the wrong part. The term “SAE” is too general, as the organization defines many different fitting types, including flare, O-ring, and flange fittings.

SAE is the standards body that defines most hydraulic fittings used in North America. The seat angle is the specific angle of the conical surface where a fitting makes its seal.

Defining Standards and Geometry

“SAE” is not a type of fitting; it’s the organization that writes the standards. Being specific is crucial for ordering parts. When a customer asks for an SAE fitting, we have to ask clarifying questions to determine exactly which standard they need.

• SAE (Society of Automotive Engineers): This professional organization sets the technical standards for countless components in the US, including the most common hydraulic fittings. Important SAE standards include:
  • SAE J514: Covers JIC 37° flare fittings and SAE 45° flare fittings.
  • SAE J1453: Covers O-Ring Face Seal (ORFS) fittings.
  • SAE J1926-1: Covers O-Ring Boss (ORB) fittings.
  • SAE J518: Covers four-bolt flange fittings.
• SAE 45° Flare: This fitting looks similar to JIC but has a 45° seat angle. It is commonly used in lower-pressure applications like automotive brake lines, fuel lines, and refrigeration, but is not suitable for modern high-pressure hydraulic systems. JIC 37° and SAE 45° are not interchangeable.
• Seal: A general term for any device used to prevent the leakage of fluid. In hydraulic fittings, this can be a metal-to-metal seal (like on a JIC fitting) or an elastomeric seal (like an O-ring or bonded seal).
• Seat Angle: The angle of the sealing surface on a cone-style fitting. This is a critical identifying feature. JIC uses a 37° angle, while SAE flare fittings use a 45° angle. Mismatched seat angles will not seal and will damage both fittings.

Skive to Swivel?

A new hose assembly fails because the swivel nut was overtightened during installation. The hose was twisted, putting constant stress on the reinforcement wires and causing a premature rupture near the fitting.

Skiving is the (now mostly obsolete) practice of removing the hose cover before crimping. A swivel allows a fitting’s nut to rotate independently, preventing hose twist during installation.

Assembly Practices and Fitting Features

Proper assembly technique is just as important as selecting the right parts. Understanding features like “no-skive” and “swivel” is key to a fast, reliable, and long-lasting installation.

• Skive: The act of removing the outer rubber cover of a hydraulic hose before crimping a fitting on. This was required for older fitting types to allow the ferrule to grip the wire reinforcement directly. We primarily sell “no-skive” fittings, a modern design with a more aggressive ferrule that can bite through the cover to grip the reinforcement. No-skive is faster, easier, and protects the reinforcement from corrosion.
• Staple-Lok: Also known as a “Steck-O” fitting, this is a unique connection style used heavily in underground mining. It uses a smooth male probe that inserts into a female half, and the two are held together by a large U-shaped staple. It is a robust, compact, and easy-to-assemble connection for tight spaces.
• Stem: The component of a hose fitting that is inserted into the inner tube of the hose. It is also sometimes called the nipple or insert.
• Swivel: A feature of a fitting (usually the female half) that allows the nut to rotate independently of the fitting body. This “live swivel” is crucial because it allows the connection to be tightened without imparting any twist to the hose. A hose installed with even a slight twist will have a dramatically reduced service life.

Thread Pitch to Zinc Plating?

A metric fitting won’t thread into a port, even though the diameter seems correct. The thread pitch is wrong, a subtle but critical detail that prevents the connection from being made.

Thread pitch is the distance between threads and is a critical dimension for identifying fittings. Zinc plating is the most common protective coating used to prevent corrosion on steel hydraulic fittings.

Identification Details and Material Protection

The final details of identification and material science are what separate a professional from an amateur. Using tools like calipers and pitch gauges is a daily activity in our business.

• Thread Pitch: A critical measurement for identifying a thread. For metric threads, it’s the distance in millimeters between two adjacent thread crests (e.g., 1.5mm). For imperial threads (like UNF), it’s measured in Threads Per Inch, or TPI (e.g., 12 TPI). A fitting with the correct diameter but the wrong pitch will not engage properly.
• Torque: The rotational force applied to a fitting when tightening it. Every fitting type has a recommended assembly torque.
  • Under-torquing: The fitting will leak or blow off.
  • Over-torquing: The fitting can be damaged, flares can crack, O-rings can be extruded, and threads can be stripped. Following the manufacturer’s torque specifications is essential for a reliable seal.
• Viton® (FKM): A brand name for a Fluoroelastomer. We supply O-rings and seals made from Viton for applications involving high temperatures or aggressive chemicals that would destroy standard NBR (Nitrile).
• Working Pressure: The maximum pressure a fitting is rated to handle continuously in service. This should never be exceeded. The working pressure of a full hose assembly is always limited by its lowest-rated component (either the hose or one of the fittings).
• Zinc Plating: Most steel hydraulic fittings are electroplated with a layer of zinc to protect them from rust. Standard plating offers moderate corrosion resistance. We also supply fittings with higher-performance plating (often called Zinc-Nickel or similar) that can withstand over 720 hours of salt spray testing, making them ideal for marine or other highly corrosive environments.

Conclusion

This N-Z glossary completes our ultimate guide. Mastering these terms is vital for anyone who specifies, builds, or maintains hydraulic systems, ensuring optimal performance and safety.

Using the wrong hydraulic fitting causes leaks and system failure. This creates dangerous work environments, leads to expensive equipment downtime, and wastes valuable hydraulic fluid.

This glossary defines crucial hydraulic fitting terms from A to M. It clarifies the function and application of everything from Adapters and AN fittings to JIC and Metric threads, ensuring you select the right component every time.

The world of hydraulic fittings is filled with acronyms and standards that can be confusing. A JIC fitting looks similar to an AN fitting, but they are not always interchangeable. A BSPP thread requires a seal, while a BSPT thread does not. Making the wrong choice can be the difference between a reliable, leak-free connection and a constant maintenance headache.

Adapter to BSPP?

A pump has a metric port, but the hose assembly has a JIC fitting. This mismatch stops the assembly dead in its tracks, wasting time and requiring an urgent search for the right part.

An adapter connects two components with different thread sizes or types. BSPP is a common parallel thread standard that requires a bonded seal or O-ring to create a leak-proof connection against a smooth face.

Connecting Dissimilar Threads

In a perfect world, every component in a hydraulic system would have the same port type. In reality, this is never the case. We frequently see pumps from Europe with metric or BSPP ports that need to connect to hoses using American JIC or ORFS fittings. This is where adapters are essential.

• Adapter: A fitting that has different connection types on each end. Its sole purpose is to “adapt” one thread style or size to another. For example, an adapter could have a male BSPP thread on one end and a female JIC flare on the other. They are problem-solvers that allow otherwise incompatible components to be connected.
• AN Fitting: This stands for Army-Navy. It is a flare fitting with a 37° seating angle, originally designed for military and aerospace applications. AN fittings are made to extremely high tolerances for maximum reliability. While they look identical to industrial JIC fittings, their thread class and manufacturing standards are much more stringent. You can use a high-quality AN fitting in a JIC application, but you should never use a standard JIC fitting in a true AN application.
• BSPP (British StandardPipe Parallel): This is a parallel thread form, also known as a “G” thread under ISO 228-1. Because the male and female threads are parallel, they do not create a seal by themselves. The seal is made by compressing a soft seal between the male fitting and a smooth face on the female port. The most common method we supply is a “bonded seal,” which is a metal washer with a vulcanized rubber ring on its inner diameter. This provides an excellent, reliable seal against the port face.

Bulkhead to Compression Fitting?

A hose needs to pass through a machine panel, but the connection is flimsy. The vibration from the machine causes the fitting to loosen, creating a dangerous leak point inside the enclosure.

A bulkhead fitting is designed to mount securely through a panel, providing a fixed connection point. A compression fitting creates a seal by squeezing a ferrule onto a tube, a common method in instrumentation.

!A bulkhead fitting securely mounted through a steel plate, showing the locknut on the backside.

Mounting and Sealing Methods

Proper mounting and sealing are at the core of a reliable hydraulic system. The choice of fitting often depends on the physical layout of the machine and the type of conduit being used, whether it’s flexible hose or rigid tubing.

• Bulkhead Fitting: This is not a thread type, but a style of fitting body. A bulkhead fitting has a longer body with a locknut. This design allows it to pass through a hole in a panel or bracket, with the locknut tightened on the other side to create a rigid, fixed-point connection. It’s the standard solution for routing a fluid line through a machine wall or operator’s cab, preventing chaffing and stress on the hose. We see them used extensively on mobile equipment.
• Captive Seal: This is a feature of some fittings, particularly BSPP, where the O-ring or bonded seal is held in a special groove on the male fitting. This is a significant advantage for maintenance and assembly because the seal cannot be dropped or lost. It stays “captive” on the fitting, making installation in tight spaces much easier and more reliable.
• Code 61 / Code 62: These are standards for four-bolt flange fittings (defined by SAE J518). They are a very common choice for connecting larger hose sizes (typically 1/2″ and up) directly to pumps and motors.
  • Code 61: Standard duty series, rated for pressures up to 3000-5000 PSI depending on size.
  • Code 62: Heavy-duty series with a larger flange head and bolt pattern, rated for 6000 PSI across all sizes.
• Compression Fitting: This type of fitting seals by compressing a ring (called a ferrule or “olive”) onto the outside of a metal tube. It’s a non-welded solution often used in lower-pressure hydraulic systems, pneumatics, and instrumentation lines. The most robust version of this concept is the “bite-type” fitting.

DIN Fitting to Elastomeric Seal?

A connection on equipment from Germany is leaking. The thread looks metric, but a standard metric fitting does not seat correctly, leading to continued frustration and a search for the right component.

DIN fittings are a German industrial standard for metric bite-type and flareless fittings. An elastomeric seal is any seal made from a rubber-like material, such as an O-ring or bonded seal.

European Standards and Sealing Materials

Many of our international clients, especially those with European machinery, rely on our expertise with DIN standards. Understanding these is key to providing the right parts.

• DIN Fitting: Stands for Deutsches Institut für Normung (German Institute for Standardization). This refers to a whole family of metric fittings. The most common is the DIN 2353 / ISO 8434-1 bite-type tube fitting. These are a global standard for high-pressure tubing and are specified by their tube OD and a series (LL for Extra Light, L for Light, or S for Heavy) which determines their pressure rating.
• Dryseal Thread: This is a term for certain tapered pipe threads, primarily NPTF (National Pipe Taper Fuel). Unlike standard NPT, NPTF threads are designed to crush and deform at the root and crest during tightening. This creates a metal-to-metal seal without the theoretical need for sealant. In our experience, we still recommend using a liquid sealant (not tape) to aid assembly and ensure a 100% leak-free connection, especially in high-vibration environments.
• Durometer: This is the measurement of a material’s hardness. For the elastomeric seals in hydraulics (like O-rings), it is measured on the Shore A scale. A typical hydraulic O-ring is made from 90 durometer Nitrile. This hardness is required to resist extrusion under high pressure. Using a softer O-ring, like one intended for plumbing, would result in immediate failure in a hydraulic application.
• Elastomeric Seal: This is the technical term for any seal made from an elastic polymer (an elastomer). In hydraulic fittings, this refers to O-rings, bonded seals, and other custom-molded seals. The seal’s material must be compatible with the system’s fluid and temperature range. The most common material we supply is NBR (Nitrile) for standard petroleum-based fluids.

Face Seal to Ferrule?

A fitting in a high-vibration area keeps weeping fluid, no matter how much it’s tightened. The metal-to-metal flare connection cannot maintain its seal under the constant shaking of the machine.

A face seal fitting uses a soft O-ring seal for superior leak resistance, especially under vibration. A ferrule is the component in a bite-type or compression fitting that cuts into or squeezes the tube.

Sealing Methods for Demanding Applications

When a client tells us they have a leak they just can’t fix, especially on mobile equipment like excavators or tractors, our first question is often about the fitting type. This is where the difference between a flare fitting and a face seal fitting becomes critical.

• Face Seal Fitting (O-Ring Face Seal or ORFS): This is widely considered one of the best fitting types for leak prevention. Governed by SAE J1453, the ORFS connection creates a seal by compressing an O-ring in the face of the male fitting directly against a flat face on the female fitting. The threads are only used to provide clamping force. We strongly recommend ORFS for high-pressure, high-vibration applications. The soft seal absorbs vibration and can tolerate minor imperfections, providing a seal that is far more reliable than any metal-to-metal connection.
• Ferrule: This is a critical component in any compression or bite-type fitting. It is the ring or sleeve that gets compressed onto the tube.
  • In a standard *compression fitting*, the ferrule is soft and simply squeezes the tube.
  • In a bite-type fitting (like a DIN fitting), the ferrule is a hardened, precision-engineered cutting ring that physically bites into the tube, providing immense holding power and a positive metal-to-metal seal. This “bite” makes it extremely resistant to pressure and vibration.

Feature	JIC (Flare Seal)	ORFS (Face Seal)
Sealing Mechanism	37° Metal-to-Metal Cone	O-Ring in Face
Vibration Resistance	Fair	Excellent
Torque Sensitivity	High (can crack flare)	Low (less risk of damage)
Reusability	Good	Excellent (just replace O-ring)
Cost	Low	Higher
Best Application	General Industrial, Static	High Pressure, High Vibration

Flange Fitting to Four-Bolt Flange?

Connecting a large 2-inch hose to a main pump is difficult with a standard screw-in fitting. The torque required to tighten it is massive, and there is no clearance for a large wrench.

A flange fitting provides an easy-to-install, high-pressure connection for larger hoses using bolts instead of high torque. They are commonly known as four-bolt flanges, governed by the SAE J518 standard.

Solutions for Large Diameter Connections

As hose sizes increase, so does the difficulty of installation. A threaded fitting for a hose over 1 inch in diameter would require enormous torque to create a secure seal. This is impractical and often impossible in the tight confines of machinery. The industry’s solution is the flange fitting.

• Flange Fitting: This system uses a flanged head on the fitting, which has an O-ring to create a face seal against a port. A separate clamp, either split or solid, is used to bolt the flange head to the port. The bolts provide the clamping force, requiring much less torque than a giant threaded fitting. This makes installation and maintenance of large hoses dramatically simpler and safer.
• Flare Fitting: A type of fitting that creates a metal-to-metal seal on a conical surface. The end of the tube is flared (widened into a cone shape), and this flare is then compressed against a matching cone on the fitting. The most common example in our industry is the JIC 37° flare fitting.
• Flat Face Fitting: This term can be confusing. It most often refers to O-Ring Face Seal (ORFS) fittings, which seal against a flat face. It is also used to describe ISO 16028 quick-connect couplings, which have a flat face to minimize spillage and air inclusion when connecting or disconnecting.
• Four-Bolt Flange: This is the common name for the SAE J518 flange fitting system. It is the go-to standard for port connections on pumps, motors, and cylinders in sizes from 1/2″ to 3″ and larger. As mentioned before, they come in two pressure series: Code 61 (standard pressure) and Code 62 (high pressure). They are not interchangeable. A technician must measure the bolt-hole spacing or the flange diameter to correctly identify which series they have.

JIC to Male Fitting?

A customer insists a JIC and an AN fitting are the same, causing confusion during ordering. While they look alike, their specifications and intended applications are crucially different, which can impact performance.

JIC is the industrial standard for 37° flare fittings, versatile and widely used. A male fitting has external threads, designed to screw into a female fitting which has internal threads.

Common Standards and Gender Terminology

JIC is arguably the most common hydraulic fitting we sell for general industrial and mobile applications in North America and many other parts of the world. Understanding its characteristics and how to identify it is a fundamental skill.

• JIC (Joint Industry Council): Now governed by SAE J514, this fitting features a 37° flare seating surface. The seal is made when the 37° flare on the male fitting mates with the 37° cone on the female fitting. It is popular because it is easy to identify, widely available, and relatively inexpensive. However, its metal-to-metal seal makes it more prone to leaking in high-vibration applications compared to an ORFS fitting.
• Komatsu Fitting: Equipment manufacturer Komatsu often uses a metric flare fitting that looks very similar to JIC but has 30° flare angle and metric threads. It is not interchangeable with JIC and requires special attention when servicing Komatsu machinery.
• Live Swivel: This is a feature on some female fittings that allows them to rotate or “swivel” even when the connection is fully tightened. This prevents hose twisting during installation and allows the hose to relax into its natural position, significantly extending its service life. Female JIC fittings, for example, are almost always swivel fittings.
• Male / Female: This is the standard terminology for fitting “gender.”
  • Male Fitting: Has external threads.
  • Female Fitting: Has internal threads. It’s that simple. A male fitting screws into a female fitting. In the case of flare fittings like JIC, the male has the flared cone on the outside, and the female has it on the inside, and typically includes a swivel nut.

Conclusion

This A-M glossary covers the foundational terms in hydraulics. Understanding these concepts is the first step toward building, servicing, and maintaining reliable, leak-free fluid power systems for a huge range of machinery.

You’re holding a hydraulic fitting, the production line is down, and nobody knows what thread it is. Ordering the wrong replacement means more delays, wasted money, and mounting frustration.

Identifying a hydraulic thread is a simple, methodical process. By using a caliper to determine if the thread is parallel or tapered, measure its diameter, and find its pitch or TPI, you can accurately cross-reference it with a standard chart and find the exact match every time.

Why Is Identifying the Wrong Thread So Dangerous?

You think two threads look similar enough to fit, so you force them together. It seems to tighten, but you’ve just created a ticking time bomb that is guaranteed to leak or fail.

Mixing incompatible thread types, like NPT and BSPT, creates a high-risk connection. The mismatched thread profiles and angles prevent a proper seal and can strip the threads, leading to dangerous high-pressure leaks, equipment damage, and costly, unscheduled downtime.

What’s the First Step in Distinguishing the Thread Type?

You’re looking at a fitting, but you can’t tell if it’s supposed to seal on the threads or with a gasket. This single distinction determines the entire identification process.

The first crucial step is to determine if your thread is parallel or **tapered**. Use your caliper to measure the diameter at the beginning and end of the threads. If the diameters are the same, it’s parallel; if they get smaller, it’s tapered.

This initial check is the most important fork in the road. It immediately cuts your potential options in half and tells you how the fitting is designed to seal. Parallel threads (like Metric Parallel or BSPP) almost always seal using a gasket, O-ring, or bonded seal on a surface. Tapered threads (like NPT or BSPT) are designed to seal by the threads themselves wedging together, usually with the help of a sealant tape or paste. A procurement manager like Tony knows that getting this first step right prevents a whole category of ordering errors. This simple measurement provides the first and most critical piece of the puzzle.

The Caliper Technique for Parallel vs. Tapered

This is a simple but precise test. Take your digital caliper and follow these steps carefully:

First Measurement: On a male (external) thread, carefully place the caliper jaws over the crests of the second or third thread from the end. Note the measurement. Avoid the very first thread as it’s often damaged or chamfered.

Second Measurement: Move the caliper down the fitting and measure the diameter again, perhaps over the eighth or ninth thread.

Compare:

• Parallel: If the two measurements are nearly identical (a difference of less than 0.1mm), you have a parallel thread.
• Tapered: If the second measurement is noticeably smaller than the first, you have a tapered thread.

You can also use your eyes as a quick check. A parallel thread will have a consistent profile, like a cylinder. A tapered thread will look like a very subtle cone.

Feature	Parallel Thread	Tapered Thread
Visual Look	Straight, like a cylinder	Conical, gets smaller toward the end
Caliper Test	Diameters at start and end are the same	Diameter at start is larger than at the end
Sealing Method	O-ring, washer, bonded seal, or metal face seal	The threads themselves wedge together (with sealant)
Common Examples	BSPP (G), Metric Parallel, UN/UNF (JIC, ORB)	NPT, BSPT (R/Rc), Metric Tapered

How Do You Accurately Measure the Thread Diameter?

You’ve identified your thread as parallel, but when you measure it, the number doesn’t match any standard you can find in a chart. A tiny error in measurement leads to a dead end.

For a male (external) thread, you must measure the major diameter (the widest part, from crest to crest). For a female (internal) thread, you measure the minor diameter (the narrowest part, from root to root). This measurement must be precise and perpendicular to the fitting.

This step seems basic, but it’s where small mistakes happen. I once had a customer who was measuring the diameter of his male fitting at a slight angle. This small tilt was adding 0.5mm to his measurement, causing him to look at the wrong section of the thread chart and order the wrong part twice. I advised him to gently rock the caliper as he closes it on the threads; the largest reading he sees before it starts to decrease is the true major diameter. For tapered threads, consistency is key—always record which thread you are measuring (e.g., “diameter at the 4th thread”). Precision here is what makes the final identification possible.

Measuring Male (External) Threads

The most important dimension on a male thread is its outside diameter, technically called the **major diameter**.

• Technique: Place the caliper jaws over the crests (the peaks) of the threads.
• Be Perpendicular: Ensure the caliper is perfectly straight across the fitting, not at an angle.
• Find the True Diameter: Gently close the caliper and give it a slight rock back and forth to ensure you are capturing the widest possible dimension.
• For Tapered Threads: Because the diameter changes, you need to measure at a consistent point. Good practice is to measure across the 4th or 5th thread crest from the end of the fitting. This avoids the often-damaged first thread and gives a representative measurement.

Measuring Female (Internal) Threads

For a female thread, you need to measure the **minor diameter**.

• Technique: Use the small, internal measuring jaws on the top of your caliper.
• Go Deep: Insert the jaws into the threaded hole, letting them expand until they make firm contact with the thread roots (the valleys).
• Find the True Diameter: Again, a slight wiggle as you expand the jaws will help you find the true, smallest diameter of the hole. This measurement is often more difficult to get perfectly and can be less reliable than a male thread measurement. Whenever possible, try to find a matching male fitting to measure instead.

Crucial Point: Remember that the measured diameter will rarely be the exact nominal size. For example, a 1/2″ NPT thread has a nominal major diameter of 0.840 inches (21.34 mm), not 0.5 inches. This is why a thread chart is essential. You are not looking for an exact match to the name, but an exact match to the dimensions in the chart.

How Do You Determine the Thread Pitch or TPI?

You have the diameter, but is it a coarse metric thread or a fine imperial one? This single detail separates two completely different and incompatible standards.

You must determine the distance between threads. For Metric threads, you measure the distance from one crest to the next in millimeters; this is the **pitch**. For Imperial threads (like BSP or NPT), you count the number of threads in one inch; this is the **TPI**.

Measuring Metric Pitch

Metric systems are beautifully simple. The pitch is a direct measurement.

• Method: Carefully place the sharp points of your caliper jaws on the crest of two adjacent threads.
• Measurement: The reading on your caliper (e.g., 1.5mm or 2.0mm) is the pitch.
• Increase Accuracy: To reduce error, you can measure across several threads (e.g., 5 threads) and then divide the total distance by the number of threads. For example, if the distance over 5 threads is 7.5mm, your pitch is 7.5 ÷ 5 = 1.5mm.

Measuring Imperial TPI (Threads Per Inch)

Imperial systems are based on a count, not a direct measurement.

• The Best Caliper Method: Set your caliper’s jaws to exactly 25.4mm (which is 1 inch). Place one jaw on the crest of a thread and then count how many thread crests fall within that 1-inch span. This count is your TPI.
• The Calculation Method: If the threaded section is too short for a full inch, you can use a calculation. Measure the distance over a smaller number of threads (e.g., 5 or 10). Then use this formula: **TPI = 25.4 / (Total Length / Number of Threads)**. For example, if 5 threads cover a distance of 9.07mm, the average pitch is 1.814mm. The TPI would be 25.4 / 1.814 = 14 TPI.

This calculation is more prone to error, so the direct 1-inch counting method is always preferred when possible.

How Do You Put It All Together and Find the Standard?

You have three key pieces of data: thread type, diameter, and pitch/TPI. Now what? You are standing at the finish line but need to know which race you’ve won.

The final step is to take your measurements and compare them against a standard hydraulic thread chart. By cross-referencing your data, you can definitively identify the thread standard, such as NPT, BSPP, or Metric.

This is the “aha!” moment. It’s where all the careful measurements pay off. It feels like being a detective. You’ve gathered all the clues, and now you are comparing them to your list of suspects in the thread chart. Let’s walk through a few real-world examples from my experience helping customers.

Example 1: The Tapered Fitting

A customer in Chile sent me his measurements from a fitting on his mining equipment.

1. Type: Tapered (diameter was smaller at the end).
2. Diameter: ~33.2mm (measured at the 4th thread).
3. Pitch/TPI: He counted 11.5 threads over a 1-inch span. So, **11.5 TPI**.

We opened our thread chart. We looked for a tapered thread with a diameter around 33.2mm and **11.5 TPI**. There was only one match: **1″ NPT**. Case closed.

Example 2: The Parallel Fitting

A farm owner in New Zealand needed to replace a hose on his European tractor.

1. Type: Parallel (diameters were identical).
2. Diameter: ~26.4mm (male major diameter).
3. Pitch/TPI: He counted **14 TPI**.

We went to the chart. We looked for a parallel thread, ~**26.4mm** OD, and **14 TPI**. The perfect match was **BSPP (G) 3/4″**. The G stands for parallel.

Example 3: The Metric Fitting

An engineer from a factory in Romania had a fitting from a German press.

1. Type: Parallel.
2. Diameter: ~29.8mm (male major diameter).
3. Pitch/TPI: He measured the distance between two crests as **1.5mm**.

Chart time. We searched for a parallel thread, ~**29.8mm** OD, with a **1.5mm pitch**. This pointed directly to a Metric M30 x 1.5 thread.

This systematic process removes all guesswork. You just need a good-quality caliper and a reliable thread chart. We provide these charts to all our customers to empower them to identify parts correctly on their own.

What Other Clues Can Confirm Your Identification?

Your measurements point to two very similar thread standards. How can you be 100% certain you’ve chosen the right one without any doubt?

Look beyond the threads. The sealing method and sealing angles are definitive clues. A thread that seals with a 37° flared cone is JIC, while one that uses a bonded washer on a flat face is BSPP. These physical features confirm your caliper measurements.

This is advanced detective work. Sometimes, two threads, like a Metric and an Imperial one, can have very similar diameters and pitches. The sealing method is the tie-breaker. It tells you how the fitting *functions*, which is unique to each standard. This is how a true professional confirms their choice. It’s not just about what the thread *is*, but what it *does*.

Identifying by Sealing Method

Once you have a likely candidate from your measurements, check the sealing surface for confirmation.

• Tapered Threads (NPT, BSPT): The seal is made by the threads themselves. You will almost always find old thread sealant tape or paste on a male fitting. The port it connects to will be simple and unadorned.
• BSPP (G) Threads: This parallel thread seals with a soft washer or a “bonded seal” (a metal washer with a rubber ring vulcanized to the inside). The male fitting will have a flat face behind the threads to compress this seal against a flat surface on the port.
• JIC / SAE 37° Flare: This is incredibly common in North America. It’s a parallel thread (UN/UNF), but the sealing is done by a distinctive 37° flared cone on the male fitting mating against a matching 37° cone on the female fitting.
• O-Ring Boss (ORB): This also uses a parallel UN/UNF thread. The seal is made by an O-ring that sits at the base of the male thread. It’s designed to fit into a special chamfered “boss” or groove at the top of the female port.
• 60° Cone Seat (BSPP): Some BSPP fittings don’t use a washer. Instead, they have a 60° cone inside the female fitting that mates with a matching cone on the male end. This is common but can be confused with JIC, so measuring the angle is key (if you have a protractor).

By combining your three caliper measurements with a visual confirmation of the sealing method, you can identify virtually any hydraulic fitting with an extremely high degree of confidence.

Conclusion

Mastering this caliper technique transforms uncertainty into expertise. It ensures every hydraulic connection you make is the correct one, guaranteeing system safety, reliability, and efficiency.