The 6-step process used by hydraulic specialists. Using the wrong fitting type is the leading cause of hydraulic leaks and system failures. Learn to identify any fitting with confidence before you order.
Why this matters: Connecting the wrong fitting type, even if it appears to thread in, creates a leak path that will fail under pressure. A misidentified fitting on a 3,000 PSI system can result in a hose blowout, equipment damage, or serious injury. Take the time to identify correctly.
Work through these steps in order. Each step eliminates possibilities and narrows down the fitting type.
Use digital calipers to measure the outside diameter (OD) of male threads, or the inside diameter (ID) of female threads. Record the measurement in both inches and millimeters. This single measurement eliminates most of the field, metric threads will have ODs like 12mm, 14mm, 16mm; imperial threads will measure close to fractions like 0.375" (3/8"), 0.500" (1/2"), etc.
Field Tips
Use a thread pitch gauge, a set of metal combs with calibrated teeth that match specific thread pitches. Place each comb against the thread until you find a perfect fit. For imperial threads, the number on the matching comb is the TPI. For metric threads, the number is the pitch in mm (e.g., 1.5 = 1.5mm between threads).
Field Tips
Thread angle is the single most important distinction for hydraulic threads. BSP threads use a 55° angle; NPT, JIC, SAE, and most metric threads use 60°. A thread gauge set or close visual inspection with a protractor can reveal this. Never assume, mixing 55° and 60° threads causes leaks even if they appear to partially engage.
Field Tips
Look at the end face and thread body of the fitting. The seal method is the most important functional characteristic. A flat face with a circular machined groove = ORFS. A 37° inner cone = JIC. Tapered threads that get narrower = NPT or BSPT. An O-ring sitting around the thread body on the male end = SAE ORB. A flat face designed for a bonded seal = BSPP.
Field Tips
Run your fingertip along the thread from tip to base. If the diameter clearly decreases toward the free end (tip), the thread is tapered (NPT, BSPT). If the diameter remains constant, it is parallel (BSPP, JIC, ORFS, SAE ORB, metric). You can verify with calipers: measure the OD at the tip vs. the base. Tapered threads change by 1/16" per inch of thread length.
Field Tips
Port fittings thread into a machined port on a pump, motor, valve, or manifold body. Inline fittings connect two hoses or tubes together. This distinction matters because port threads are usually SAE ORB, BSPP, or metric DIN, while inline threads are JIC, ORFS, or NPT. If you're pulling a fitting out of a port, the thread type on the body side is almost certainly ORB or metric.
Field Tips
Six fitting types you'll encounter in the field, with the key visual identifier for each.
National Pipe Tapered
Key Visual Identifier
Tapered threads, no O-ring, no flare, threads seal the connection
How to Spot It in 5 Seconds
Threads get noticeably narrower toward the tip. No groove, no cone, no O-ring.
British Standard Pipe
Key Visual Identifier
55° thread angle, either parallel (BSPP) or tapered (BSPT)
How to Spot It in 5 Seconds
Looks like NPT but slightly different thread angle. BSPP is parallel, OD stays constant. Often on European or Asian-made equipment.
Joint Industry Council, 37° Flare
Key Visual Identifier
37° inner cone visible inside the female fitting; straight UNF thread
How to Spot It in 5 Seconds
Look inside the female fitting, you'll see a 37° cone. The male has a matching flared end. No O-ring visible.
O-Ring Face Seal
Key Visual Identifier
Flat face with a machined circular groove on the male end; straight UNF thread
How to Spot It in 5 Seconds
Flat face on the male end with a rubber O-ring (or empty groove) visible. Completely flat, no cone, no taper.
SAE O-Ring Boss
Key Visual Identifier
O-ring sits in a groove around the outside of the male thread body
How to Spot It in 5 Seconds
O-ring is on the outside of the male thread, not the face. The fitting screws into a port and the O-ring seals against the port face.
Metric DIN / ISO 6149
Key Visual Identifier
Metric thread pitch (e.g., M14×1.5), often with O-ring on face or bonded seal
How to Spot It in 5 Seconds
Thread OD measures in whole millimeters (12, 14, 16, 18...). Threads are fine and even. Common on European and Asian hydraulic equipment.
These three pairs fool even experienced hydraulic technicians. Here's how to tell them apart definitively.
They look almost identical, but they are NOT interchangeable.
Both NPT and BSPT are tapered pipe threads. Both are available in similar sizes. The difference is the thread angle: NPT uses 60° and BSPT uses 55°. They will partially engage, sometimes appearing to tighten, but will never seal. The definitive test is a thread angle gauge. Alternatively, knowing your equipment origin helps: North American equipment uses NPT; British, European, and Australian equipment uses BSP. Japanese and Korean equipment can use either.
Both have flare seats, but the angles are different and they will not seal each other.
JIC fittings use a 37° flare. AN (Army-Navy) and SAE 45° flare fittings use a 45° flare. The thread sizes are often similar or identical, and you can sometimes thread them together, but the seat angles will not match and the resulting connection will leak. AN fittings are common in aerospace, automotive, and military applications; JIC is standard in industrial hydraulics. The 37° JIC is far more common in heavy equipment.
Both use O-rings. The critical difference is WHERE the O-ring seals.
SAE ORB (O-Ring Boss) has the O-ring on the outside of the male thread body, the O-ring seals against the port face when the fitting is threaded in. ORFS (O-Ring Face Seal) has the O-ring recessed into a groove on the flat face of the male fitting, the O-ring seals against the female flat face. ORB is a port fitting; ORFS is an inline fitting. They use different thread types and are not interchangeable, though the O-ring appearance can cause confusion.
Every hydraulic service technician should carry these three tools.
The single most important fitting identification tool. Measures thread OD, ID, and depth with 0.001" precision. A $30 digital caliper eliminates most fitting identification guesswork. Get one with both inch and mm display.
Primary Use
Measure male OD or female ID to determine nominal thread size.
A set of thin metal combs with calibrated teeth. Each comb is labeled with its TPI or mm pitch. Match the comb to the thread profile to identify pitch instantly. Available in imperial and metric sets.
Primary Use
Confirms TPI after you have the OD. Essential for distinguishing fine vs. coarse threads of the same nominal size.
Known-standard fittings in common sizes that you use to physically test-fit against an unknown fitting. If a -8 ORFS male threads smoothly into your unknown female, you know the female is -8 ORFS. Hydraulic Hunter supplies thread checker kits.
Primary Use
Definitive confirmation, physical engagement test. Faster than measurement for experienced technicians.
Don't guess when the stakes are a hydraulic blowout. Bring your fitting, hose, or port measurement to our Cleveland, TN location. Our specialists identify fittings for free, and we stock virtually every common size and type.
We carry every major fitting type and can match virtually any hydraulic fitting. Request a quote or call our specialists in Cleveland, TN.
Detailed comparison of the two most common fitting types.
Dash sizes, flow rates, and pressure ratings explained.
What causes leaks and how to fix them.
Practical thread ID tips for the field.