How to identify a hydraulic thread
Two measurements and one visual check are usually enough. Twelve-year technicians still do this the same way, because it is faster than looking up a part number that may not exist any more.
Step 1 — Measure the major diameter
Across the crests of the male thread, with a caliper. Not the wrench flats, not the hex. Write it in both inches and millimetres; that alone eliminates most wrong answers.
Step 2 — Count the threads per inch
Lay a steel rule along the thread and count the crests in exactly one inch. Below about 1/8" pitch that is hard by eye, so a thread pitch gauge is worth the few dollars. For metric threads, measure the distance from crest to crest in millimetres instead — you will normally land on 1.5 mm, 2.0 mm or 2.5 mm.
Step 3 — Read the seat
| What you see at the sealing end | Family | Standard |
|---|---|---|
| Conical seat, straight thread, ~37° included angle | JIC 37° flare | SAE J514 |
| Conical seat, straight thread, ~45° included angle | SAE 45° flare | SAE J512 (mostly automotive, LPG, air) |
| Flat face with a recessed O-ring groove | ORFS | SAE J1453 |
| Straight thread with a captive O-ring and a washer-like shoulder | SAE O-ring boss | SAE J1926 |
| Conical seat, straight thread, 24° included angle, metric pitch | Metric 24° cone (DKO) | DIN 2353 / ISO 8434-1 |
| Parallel thread, 55° flank angle, sealed by a bonded washer | BSPP (G) | ISO 228-1 |
| Tapered thread, 55° flank, seals on the threads | BSPT (R) | ISO 7-1 |
| Tapered thread, 60° flank, seals on the threads | NPT / NPTF | ANSI/ASME B1.20.1 / B1.20.3 |
Worked examples
You measured 0.5625" (9/16") and counted 18 threads per inch
9/16-18 UNF. That is JIC -06 or ORB -06 — both use 9/16-18. Now look at the seat: a 37° cone means JIC; a flat face with an O-ring groove means ORFS, and an O-ring in a groove behind the hex with a shoulder means O-ring boss. Two families share the thread; only the seat separates them.
You measured 0.825" (about 21 mm) and counted 14 threads per inch
That is G1/2 BSPP. The 14 TPI figure is the giveaway — 1/2" UNF would be 20 TPI. If it is also tapered, it is BSPT R1/2.
You measured 13 mm and the pitch gauge lands on 1.5 mm
M14×1.5. Look for a 24° cone: that is a DIN 2353 / ISO 8434-1 light-series connection and it pairs with a specific series — L and S series have different tube and nut dimensions and do not interchange.
The three mistakes that cost the most
- Reading the hex. The hex across flats is sized for a wrench, not for the thread. Always measure the thread crests.
- Assuming "BSP 1/2" means half an inch. It means nominal bore. The thread is 20.96 mm. Same trap applies to NPT and to tube sizes.
- Assuming matching threads means interchangeable parts. JIC -04 and ORB -04 are both 7/16-20. Screw an ORB part into a JIC seat and it will not seal. See JIC vs SAE.
Tools that make this reliable
- Digital caliper, 0.01 mm resolution
- Thread pitch gauge set covering UNF, BSP and metric
- A set of thread plug gauges for the sizes you handle every day — the only way to be certain on worn or damaged threads
- A phone camera and a small ruler in the same frame