Five hydraulic thread families are common, three of them look almost identical to the naked eye, and mismatching any two will leak — sometimes immediately, sometimes after a hundred pressure cycles. This guide is the field procedure: thread pitch, sealing geometry, and the visual cues that tell them apart.
If you have the fitting in hand, the fastest path is the Hydraulic Thread Identifier — enter measured OD or ID and pitch, and it returns the matching standard. For torque after identification, see the Hydraulic Fitting Torque Reference.
The Five Common Families
| Standard | Origin | Seal Method | Thread Form |
|---|---|---|---|
| JIC 37° (SAE J514) | USA | 37° flare cone | UN/UNF (60°) |
| ORFS (SAE J1453) | USA | O-ring face seal | UN/UNF (60°) |
| SAE Straight (J1926) | USA | O-ring boss | UN/UNF (60°) |
| NPT / NPTF | USA | Tapered thread interference | 60°, tapered 1:16 |
| BSPP (G) | UK / ISO 228 | Bonded washer or 30° cone | Whitworth 55°, parallel |
| BSPT (R) | UK / ISO 7 | Tapered thread interference | Whitworth 55°, tapered 1:16 |
| Metric DIN 2353 | EU | 24° cone or O-ring (DKO) | Metric (60°), parallel |
Step 1: Parallel or Tapered?
Lay a steel rule along the threads. If the thread crests sit perfectly flat against the rule, it’s a parallel (straight) thread. If the rule rocks or shows a visible gap at the small end, it’s tapered. Tapered means NPT, NPTF, or BSPT. Parallel means everything else.
Parallel threads always need a separate seal — an O-ring on the face (ORFS), a 37° cone (JIC), a 24° cone (DIN), an O-ring on a chamfered seat (SAE straight, BSPP), or a bonded washer. If you’re looking at a parallel thread fitting and there’s no O-ring or sealing cone, you have a problem before you even start.
Step 2: Measure Thread Pitch
Use a thread pitch gauge — a $5 tool that pays for itself the first time. Inch threads are TPI (threads per inch); metric is mm pitch.
| Family | Pitch Series | Common Pitches |
|---|---|---|
| JIC 37° | UNF | 14, 16, 18, 20 TPI |
| ORFS | UN | 14, 16, 18 TPI |
| NPT | NPT | 11.5, 14, 18, 27 TPI |
| BSPP / BSPT | Whitworth | 11, 14, 19, 28 TPI |
| Metric DIN | ISO 261 | 1.0, 1.5, 2.0 mm |
BSPP/BSPT and NPT share several nominal sizes but with different pitches — 1/2″ NPT is 14 TPI; 1/2″ BSPP is 14 TPI also (coincidence). Tell them apart by thread angle: NPT is 60°, BSPP/BSPT is 55° Whitworth. The 5° difference is visible if you place the thread profile against a printed chart.
Step 3: Measure the OD (Male) or ID (Female)
Measure across the thread crests for males, across the thread roots for females. Caliper accuracy of 0.01 in (0.25 mm) is plenty.
| Nominal | JIC OD | ORFS OD | NPT OD | BSPP OD |
|---|---|---|---|---|
| 1/4″ | 0.4375 (7/16-20) | 0.5625 (9/16-18) | 0.540 | 0.518 (G 1/4) |
| 3/8″ | 0.5625 (9/16-18) | 0.6875 (11/16-16) | 0.675 | 0.656 (G 3/8) |
| 1/2″ | 0.750 (3/4-16) | 0.8125 (13/16-16) | 0.840 | 0.825 (G 1/2) |
| 3/4″ | 1.0625 (1-1/16-12) | 1.1875 (1-3/16-12) | 1.050 | 1.041 (G 3/4) |
Step 4: Look at the Sealing Surface
This is the unambiguous tiebreaker:
- JIC 37°: male has a polished 37° flare cone at the tip; female has a matching seat. No O-ring.
- ORFS: male has a flat face with a captive O-ring in a groove. Female has a flat face. The O-ring is the seal.
- SAE Straight (boss): male has an O-ring sitting against a chamfered seat at the base of the threads. Used for port connections, not tube-to-tube.
- NPT / NPTF / BSPT: nothing on the face. Sealing happens entirely on the tapered threads. NPTF is the dry-seal version; NPT typically needs PTFE tape or sealant.
- BSPP: often paired with a bonded sealing washer at the base; some variants (BS 5200) use a 30° cone. Look for the washer.
- Metric DIN 2353: 24° cone seat. Distinct from JIC’s 37° — measurable with a thread/angle gauge.
Common Mismatch Mistakes
- JIC into ORFS port: threads might engage but there’s no sealing surface match — leaks immediately.
- NPT into BSPT: thread angle differs by 5°. They will start, ratchet a few turns, then split the female casting.
- BSPP into BSPT: one is parallel, one is tapered. Will not seal under pressure.
- Metric DIN into JIC: 24° vs 37°. Looks plausible at a glance; always leaks at the cone.
Related Tools and References
- Hydraulic Thread Identifier — interactive lookup by thread dimension
- Hydraulic Fitting Torque — assembly torques by thread family and size
- Hydraulic Hose Selector — match hose dash size to fitting
- Pressure Drop Calculator — verify the line you just spec’d
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