Short answer
To identify a thread, the parallel or tapered form, thread pitch and diameter are examined together. The sealing method of the connection must also be defined separately. The fact that parts screw together for a few turns does not prove a correct match.
Source: S019
Separate measurement from identification
First compare the diameter at two different points on the thread: on an accessible male thread, hold the caliper jaws perpendicular to the thread crests and apply them at the area where the full thread begins and at an area farther from the end. An approximately constant diameter suggests a parallel form, a regular change suggests a tapered form; do not take a measurement from a damaged first thread. Then determine the pitch with a suitable thread gauge. Metric pitch is expressed in millimeters, common inch threads in threads per inch. Parker's four steps use taper, pitch, size and designation together.
Source: S019
Connection identification card
| Record | Example type of information |
|---|---|
| Thread family | BSPP / BSPT / NPT / metric / UN-UNF or unknown |
| Measurement | Diameter, pitch and the measurement method used |
| Geometry | Parallel/tapered; male/female |
| Sealing | Cone, face, O-ring, washer or thread; not yet verified option |
| Mating part | Part code, technical drawing or photo |
Original editorial recommendation; not a clause of a standard.
If in doubt, stop the order
A photo alone does not provide a definitive thread match. The measurement result, part marking, mating part drawing and sealing surface must be verified together. Measuring the thread and determining the sealing method are separate steps: if an O-ring, washer, face or cone is visible, do not draw a conclusion from the thread family alone. If access to the female thread or the measurement is insufficient, do not guess; stop the order without a mating part or manufacturer drawing.
Original editorial recommendation; not a clause of a standard.
Sequence for verifying thread identity
- Clean the part in a depressurized and safe condition and inspect it for damage.
- Determine by measurement whether the thread is parallel or tapered.
- Record the pitch with a suitable thread gauge and the relevant diameter with a caliper.
- Use the correct sizing reference for pipe threads and non-pipe threads.
- Compare the thread family and size-pitch designation with the reference table.
- Separately verify the sealing method via O-ring, washer, cone or thread.
- If in doubt, do not match parts by trial or by forcing them.
Source: S019
Frequently asked questions
Are two threads of the same diameter compatible with each other?
Diameter alone is not sufficient; pitch and thread form are also checked.
Source: S019
Will you identify a fitting definitively from a phone photo?
No. A photo is only supporting input; taper, pitch, diameter and sealing surface are verified by measurement.
Original editorial recommendation; not a clause of a standard.
Source coverage
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S019
4 Easy Steps to Identify Hydraulic Threads Parker Hannifin · accessed 2026-10-19 PDF page 1-2; Steps 1-4PDF page 1; introduction and Steps 1-3