Hydraulic Thread Identification Chart
Four things tell you what thread you are holding: the thread pitch, the outside diameter, the shape of the sealing surface, and whether the thread is straight or tapered. Work through them in that order and you will not go wrong.
Step 1 — Is the thread straight or tapered?
Lay a straight edge along the crests of the thread. If the crests run parallel, it is a straight thread and it seals on a cone, an O-ring or a flat face. If the crests taper, it is a pipe thread — NPT, NPTF or BSPT — and it seals on the thread flanks themselves.
Step 2 — Count the pitch
Use a thread pitch gauge. Inch threads are counted in threads per inch; metric threads are measured as the distance between crests in millimetres. This one measurement separates most look-alikes: NPT 1/2″ is 14 threads per inch, BSPT 1/2″ is also 14, but a 1/4″ NPT is 18 and a 1/4″ BSP is 19. That single thread of difference is why the two will start and then bind.
Step 3 — Measure the outside diameter
Measure the male thread across the crests with calipers. On a female port, measure the inside diameter across the thread roots. Pipe sizes are nominal, not actual — a 1/2″ NPT male measures about 0.840″, not 0.500″. Use the tables below to convert what you measured into a size.
Step 4 — Look at the seat
- 37° cone — JIC / SAE J514. The most common straight-thread connection on North American equipment.
- Flat face with an O-ring in a groove — ORFS / SAE J1453. Best choice for high vibration.
- Straight thread with an O-ring at the base — SAE O-Ring Boss. Threads match JIC, but the seal is at the shoulder, not the tip.
- 24° cone, metric thread — DIN 2353, light (L) or heavy (S) series.
- 30° internal flare — Komatsu style. Often mistaken for JIC.
- 60° cone with a BSP thread — JIS / BSP 60°, common on Japanese equipment.
- No seat at all — tapered pipe thread, sealing on the threads.
JIC 37° flare (SAE J514)
| Dash size | Male thread (UN/UNF) | Tube OD |
|---|---|---|
| -04 | 7/16-20 | 1/4″ |
| -05 | 1/2-20 | 5/16″ |
| -06 | 9/16-18 | 3/8″ |
| -08 | 3/4-16 | 1/2″ |
| -10 | 7/8-14 | 5/8″ |
| -12 | 1-1/16-12 | 3/4″ |
| -14 | 1-3/16-12 | 7/8″ |
| -16 | 1-5/16-12 | 1″ |
| -20 | 1-5/8-12 | 1-1/4″ |
| -24 | 1-7/8-12 | 1-1/2″ |
| -32 | 2-1/2-12 | 2″ |
SAE O-Ring Boss (straight thread)
The thread sizes are identical to JIC, which is exactly why these two get confused. The difference is the seal: O-Ring Boss carries an O-ring at the base of the thread and seats against a flat shoulder in the port. If you thread a JIC male into an O-Ring Boss port it will tighten and still leak.
| Dash size | Male thread (UN/UNF) |
|---|---|
| -04 | 7/16-20 |
| -05 | 1/2-20 |
| -06 | 9/16-18 |
| -08 | 3/4-16 |
| -10 | 7/8-14 |
| -12 | 1-1/16-12 |
| -14 | 1-3/16-12 |
| -16 | 1-5/16-12 |
| -20 | 1-5/8-12 |
| -24 | 1-7/8-12 |
O-Ring Face Seal / ORFS (SAE J1453)
| Dash size | Male thread (UN/UNF) | Tube OD |
|---|---|---|
| -04 | 9/16-18 | 1/4″ |
| -06 | 11/16-16 | 3/8″ |
| -08 | 13/16-16 | 1/2″ |
| -10 | 1-14 | 5/8″ |
| -12 | 1-3/16-12 | 3/4″ |
| -16 | 1-7/16-12 | 1″ |
| -20 | 1-11/16-12 | 1-1/4″ |
NPT and NPTF pipe thread
NPT and NPTF share the same dimensions and will assemble together. NPTF is designed to seal without tape or sealant by deforming the thread crests and roots; NPT normally needs a sealant. Thread outside diameters below are the pipe outside diameter, which is what you will actually measure.
| Nominal pipe size | Threads per inch | Thread OD (approx.) |
|---|---|---|
| 1/8″ | 27 | 0.405″ |
| 1/4″ | 18 | 0.540″ |
| 3/8″ | 18 | 0.675″ |
| 1/2″ | 14 | 0.840″ |
| 3/4″ | 14 | 1.050″ |
| 1″ | 11-1/2 | 1.315″ |
| 1-1/4″ | 11-1/2 | 1.660″ |
| 1-1/2″ | 11-1/2 | 1.900″ |
| 2″ | 11-1/2 | 2.375″ |
BSPP and BSPT (British Standard Pipe)
BSPP is parallel and seals on a bonded washer, an O-ring or a 60° cone. BSPT is tapered and seals on the thread. Both use a 55° thread form against NPT’s 60°, so even where the thread count matches the flanks do not.
| Nominal size | Threads per inch | Thread OD (approx.) |
|---|---|---|
| 1/8″ | 28 | 0.383″ |
| 1/4″ | 19 | 0.518″ |
| 3/8″ | 19 | 0.656″ |
| 1/2″ | 14 | 0.825″ |
| 3/4″ | 14 | 1.041″ |
| 1″ | 11 | 1.309″ |
| 1-1/4″ | 11 | 1.650″ |
| 1-1/2″ | 11 | 1.882″ |
| 2″ | 11 | 2.347″ |
Metric DIN 24° cone (DIN 2353)
Metric DIN comes in two series. Light (L, also written DKO-L) is for lower pressure; heavy (S, DKO-S) uses a larger thread on the same tube size. Compare a 10 mm tube: light series is M16 x 1.5, heavy series is M18 x 1.5. Check the thread, not the tube, before you order.
Light series (DKO-L)
| Tube OD | Metric thread |
|---|---|
| 6 mm | M12 x 1.5 |
| 8 mm | M14 x 1.5 |
| 10 mm | M16 x 1.5 |
| 12 mm | M18 x 1.5 |
| 15 mm | M22 x 1.5 |
| 18 mm | M26 x 1.5 |
| 22 mm | M30 x 2 |
| 28 mm | M36 x 2 |
| 35 mm | M45 x 2 |
| 42 mm | M52 x 2 |
Heavy series (DKO-S)
| Tube OD | Metric thread |
|---|---|
| 6 mm | M14 x 1.5 |
| 8 mm | M16 x 1.5 |
| 10 mm | M18 x 1.5 |
| 12 mm | M20 x 1.5 |
| 14 mm | M22 x 1.5 |
| 16 mm | M24 x 1.5 |
| 20 mm | M30 x 2 |
| 25 mm | M36 x 2 |
| 30 mm | M42 x 2 |
| 38 mm | M52 x 2 |
The four pairs that cause the most leaks
- JIC 37° and SAE O-Ring Boss — same threads, different seal. Look for the O-ring groove at the base.
- NPT and BSPT — 60° versus 55° thread form. They will start, then gall.
- DIN light and DIN heavy — same tube size, different thread.
- JIC 37° and Komatsu 30° — the cone angle is the only visible difference and it is easy to miss by eye.
Values in these tables are nominal industry standard dimensions for identification. Always confirm against the actual part before ordering, and check the manufacturer’s rating for pressure and temperature.
Get the right fitting
We stock JIC 37°, ORFS, NPTF, BSP, SAE O-Ring Boss, metric DIN and Code 61/62 flange hose ends in one and two piece styles. Shop hose ends and hose fittings, build a custom hose, or see the other reference charts — including the AS568 O-ring chart and the hose size chart.