Knowledge · Connection Technology
Identify hydraulic threads: how to name every thread
An unknown thread on a hydraulic component costs time the moment you guess it. Three measurements name any tube fitting for certain: outer diameter, pitch, taper. The flank angle finally separates the families that look alike at the caliper, from BSP through metric and NPT to JIC and ORFS. With the four values in the thread identifier or the quick-match table you match male or female for certain and order the right fitting straight away.
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The measuring protocol
For a certain identification you need three tools: a caliper for the diameter, a pitch gauge for the pitch and an angle gauge for the flank angle. A thread ring or plug gauge covers the critical cases. Work the four steps in order and you narrow any thread down to one family and one size before you even open the charts.
The outer diameter is the primary key because you take it straight off the male thread. Pitch and taper then decide between the families that look similar in diameter. The flank angle finally separates the Whitworth threads (55 degrees) from the metric and American ones (60 degrees). Note all four values before you go to the charts: diameter, pitch, taper and flank angle. Only the combination is unambiguous; a single value quickly misleads among the similar families.
- Measure the outer diameter: take the largest diameter of the thread crests on the male thread with a caliper, or the inner diameter on a female thread. Note the value in mm.
- Check parallel or tapered: measure the outer diameter once at the frontmost and once at the rearmost full thread. Both values equal means parallel (G, M, UN); decreasing toward the tip means tapered (R, NPT, roughly 1 mm per 16 mm of length). On tapered threads set the caliper at a defined thread, for example the third.
- Determine the pitch: apply the pitch gauge until no gap remains. Read metric threads in mm, inch threads in threads per inch (TPI). The pitch is the axial distance between two adjacent threads.
- Check flank angle and sealing seat: 55 degrees (Whitworth: BSP G and R) versus 60 degrees (metric M, NPT, UN) with the angle gauge. Judge the seat: 24-degree internal cone (cutting ring, DKO), 37-degree flare (JIC), flat O-ring face (ORFS), 60-degree cone or flat gasket (BSP stud).
P = 25.4 mm / TPI
- P
- pitch, the axial distance between two adjacent threads, in mm
- TPI
- thread count in threads per inch
- 25.4 mm
- one inch in millimetres
Example: an inch thread with 18 threads per inch
- Measured: 18 threads per inch on an inch thread.
- Substitute: P = 25.4 mm / 18.
- Result: P = 1.41 mm pitch.
- Cross-check: 1.41 mm is the value for NPT 1/4 and NPT 3/8 (both 18 threads per inch) in the NPT size chart of the article on thread types in hydraulics.
Conversion between pitch and thread count is universally valid; the target value of 1.41 mm follows ANSI/ASME B1.20.1, as in the NPT chart. As of 2026-07.
Thread identifier: measure, enter, identify
Enter the measured outer diameter, the pitch or thread count and the taper, and the tool narrows down the thread family and size.
Female thread? Measure the male counterpart, or send two photos to our engineers.
Measure across the thread crests of the male thread with a caliper. Pitch and seal form narrow the result further.
Quick match: from diameter, pitch and form to the family
With the four values noted, you assign the thread to one of the five families that dominate hydraulics. The table names, per family, the feature that separates it from the others, the range of preferred sizes and the sealing seat by which you recognise the port form. The full size charts with major diameter, pitch and thread count per size, plus the cross-reference by dash and tube OD, are in the article thread types in hydraulics.
Two rules often save the trip to the charts entirely: a 55-degree flank angle is always BSP, and a tapered thread is always R or NPT. What remains is a parallel 60-degree thread, and there the pitch decides: metric in plain millimetre values, JIC and ORFS in threads per inch.
| Family | Form | Flank angle | Pitch | Major diameter | Sealing seat |
|---|---|---|---|---|---|
| BSP: G and R | G parallel, R tapered 1:16 | 55 degrees | 28, 19, 14 or 11 TPI (0.907 to 2.309 mm) | 9.7 to 59.6 mm (1/8 to 2 inch) | G: 60-degree cone, flat or soft gasket, O-ring at the stud base. R: metallic in the thread |
| Metric M | parallel | 60 degrees | 1.0, 1.5 or 2.0 mm | 10 to 52 mm (nominal) | Gasket ring, sealing edge or O-ring on the stud (DIN 3852 forms A, B, E); 24-degree cone on cutting ring and DKO |
| NPT and NPTF | tapered 1:16 | 60 degrees | 27, 18, 14 or 11.5 TPI (0.94 to 2.21 mm) | 10.3 to 60.3 mm | metallic in the thread, NPT with sealant, NPTF dry |
| JIC 37 degrees | parallel (UNF, UN) | 60 degrees | 20, 18, 16, 14 or 12 TPI | 11.1 to 63.5 mm (dash -04 to -32) | 37-degree flare cone |
| ORFS | parallel (UN) | 60 degrees | 18, 16, 14 or 12 TPI | 14.3 to 50.8 mm (dash -04 to -24) | O-ring in the flat face |
Risk of confusion
Four pairs regularly lead to mistakes because the threads barely differ at the caliper or even engage by hand. When in doubt, always check the definitive feature, not just the diameter. If a mix-up already causes a leak, the article hydraulic fitting leaking helps with the diagnosis.
| Confusion pair | Why it confuses | Definitive feature |
|---|---|---|
| G 1/4, NPT 1/4 and M14x1.5 | Outer diameter almost equal (roughly 13.2 to 14.0 mm) | Check taper (NPT tapered, G and M parallel), pitch (G 1/4 = 19 TPI, NPT 1/4 = 18 TPI, M14x1.5 = 1.5 mm) and flank angle (G = 55 degrees, NPT and M = 60 degrees) |
| G versus R (same size) | Core data and TPI identical, only the form differs | Measure the outer diameter front and back: constant = G (parallel), decreasing toward the tip = R (tapered) |
| NPT versus BSPT (R) | Both tapered 1:16, at 1/2 and 3/4 even the same 14 TPI | The flank angle decides: NPT = 60 degrees, R = 55 degrees. At 1/2 and 3/4 only that plus a marginally larger NPT diameter separates them |
| JIC -06 versus ORFS -04 | Physically the same 9/16-18 UNF thread, engages by hand | The sealing seat decides: JIC has a 37-degree flare cone, ORFS a flat face with an O-ring groove |
Thread identified, matching fitting from stock
You know the family and size? Then you find the matching tube fitting, adapter or transition piece in the PTS range. If the identification stays unclear, engineering sends the right connection from a photo and measurements instead of leaving you to guess.
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