Hydraulic fitting threads are one of those topics where everyone has a strong opinion and most opinions are partially wrong. There are at least eight thread families in common industrial use, four of them look nearly identical, and the wrong combination will either leak immediately or — worse — hold long enough to convince you it works and then let go at 3,000 psi. This is the field guide we wish we had when starting out.
For an interactive lookup, drop your measurements into the Thread Identifier and it will return the family, dash size, and mating thread.
The Four Big Families
JIC (SAE J514) — 37° Flare
The dominant fluid power fitting in North American hydraulics. The seal is metal-to-metal at a 37° flare cone. Threads are straight UN/UNF and provide mechanical retention only — they do not seal.
- How to spot it: 37° cone seat, straight threads, hex nut with a flat tip on the male side
- Common sizes: 7/16″-20 (-04), 9/16″-18 (-06), 3/4″-16 (-08), 7/8″-14 (-10), 1-1/16″-12 (-12)
- Pressure rating: up to 5,000 psi at smaller sizes
- Re-makeability: good, but each cycle work-hardens the cone — replace fittings showing visible scoring
ORFS (SAE J1453) — O-Ring Face Seal
The modern high-vibration alternative to JIC. Seals on an O-ring captured in a face groove on the male side. Threads are straight UN/UNF, similar pitch to JIC but the flat face is the giveaway.
- How to spot it: flat machined face with an O-ring groove on the male; no flare cone
- Common sizes: 9/16″-18 (-04), 11/16″-16 (-06), 13/16″-16 (-08), 1″-14 (-10), 1-3/16″-12 (-12)
- Pressure rating: up to 6,000 psi sustained, excellent dynamic resistance
- Where it shines: mobile equipment, anything that vibrates, anywhere a JIC connection has been leaking and the customer is tired of retorquing
NPT / NPTF — Tapered Pipe Thread
The fitting type most pneumatic and low-pressure hydraulic systems still use. Seals on thread interference — the threads themselves deform to make the seal. NPT is the standard; NPTF (“Dryseal”) has tighter root/crest tolerances for higher reliability.
- How to spot it: tapered thread (you can see and measure the taper), straight hex, no flare or face seal
- Common sizes: 1/8″-27, 1/4″-18, 3/8″-18, 1/2″-14, 3/4″-14, 1″-11.5
- Pressure rating: 1,500 to 3,000 psi depending on size and material
- Do not: reuse NPT fittings without re-tape, mix with BSPT (similar but 55° vs. 60° thread angle and different pitch — they will not seal and may strip)
BSPP (ISO 228) — British Standard Parallel Pipe
The European standard. Straight thread that seals via a bonded washer (or an O-ring in some variants). Often confused with NPT — they have similar nominal sizes but different pitch and thread angle, and BSPP is parallel where NPT is tapered.
- How to spot it: straight threads (no taper), 55° thread angle, bonded seal washer at the base of the boss
- Common sizes: G 1/8, G 1/4, G 3/8, G 1/2, G 3/4, G 1 (denoted with “G” prefix per ISO 228)
- Where you find it: European hydraulics, imported machinery, increasingly common in U.S. plants servicing international equipment
The Other Players
BSPT (ISO 7) — British Standard Tapered
The tapered cousin of BSPP. Looks like NPT but with 55° threads and different pitch. Will not mate to NPT — do not try. Identify by pitch (~ 28 TPI on smaller sizes versus NPT’s 27).
SAE O-Ring Boss (ORB / SAE J1926)
Used at the port end of cylinders, valves, and manifolds. Straight thread plus an O-ring captured under a hex shoulder. Common dash sizes from -02 to -32. The thread pitches match JIC, which is convenient but also the source of many wrong assemblies.
Flat Face O-Ring (SAE J518) Code 61 / Code 62
The split-flange standard for large-bore high-pressure hydraulics. Four bolts hold a flat-face fitting against an O-ring captured in a groove. Code 61 is rated to ~3,000 psi; Code 62 is the same flange pattern but heavier wall for 6,000+ psi service. Used on pump and motor port flanges from -16 dash and up.
DIN 2353 Bite-Type (Compression)
Common on imported European hydraulics. A ferrule “bites” into the tubing OD when the nut is torqued. Straight metric threads. Easy to identify by the metric pitch and the ferrule visible at assembly.
Identification Workflow
When you have an unknown fitting in hand:
- Measure thread OD with calipers. Record to 0.001″.
- Count TPI with a thread pitch gauge. Or measure across N threads and divide.
- Check for taper. Roll the thread along a straightedge. Tapered = NPT or BSPT. Straight = JIC, ORFS, ORB, BSPP.
- Inspect the seat. 37° cone = JIC. Flat with O-ring groove = ORFS. Shoulder with O-ring = ORB. Bonded washer = BSPP. None of the above = thread-seal NPT/BSPT.
- Cross-reference dash size. Each family has its own dash-to-thread chart.
The Thread Identifier automates steps 4 and 5 — give it OD, TPI, and seat type and it returns the family and dash.
Look-Alikes That Will Bite You
- JIC -06 (9/16″-18) vs. ORFS -04 (9/16″-18): identical thread size. Different seat. They will start by hand and you will not know it is wrong until you torque it and the cone or face is damaged.
- NPT 1/2″-14 vs. BSPT 1/2″-14: same nominal pitch but different thread angle. They will engage by 2 turns and bind, often shaving the softer thread.
- SAE ORB -06 (9/16″-18) vs. JIC -06 (9/16″-18): same straight thread. Different seal mechanism. An ORB fitting screwed into a JIC port leaves the O-ring exposed; the JIC fitting in an ORB port has nothing to seal against.
- UN/UNF straight thread used for plumbing: common amateur mistake. Straight threads do not seal — you need a face seal, flare cone, or O-ring.
Torque Matters as Much as Identification
Even the right fitting will leak if assembled wrong. Under-torque leaks. Over-torque cracks. For JIC and ORFS, the manufacturer publishes flat-tightening procedures: hand-tight plus a specified number of flats per dash size. For ORB and BSPP, follow the seated-torque table for the bonded washer or O-ring.
Use the Fitting Torque Calculator to look up the right value by family and dash. It also covers the F.F.F.T. (flats-from-finger-tight) method when you do not have a torque wrench on the boom of an excavator.
Closing Notes
Hydraulic fittings are one of those topics where a one-page reference saves more hours per year than any single calculator. Bookmark this article. Keep a pitch gauge, calipers, and a printed thread identification chart in the truck. When in doubt, pull the fitting and measure — guessing is how leaks become incidents.
For the rest of the fluid power suite, browse our Fluid & Force category. New thread identification, pressure-drop, and hose-selection tools are being added regularly.