This Resource Centre provides general guidance only. Always follow the equipment manufacturer’s instructions, the product technical data sheet and your site’s safe system of work. Isolate stored energy before maintenance. For safety-critical, regulated or uncertain applications, ask a competent engineer or the product manufacturer to confirm the specification.
You will need a clean fitting, a vernier calliper, a thread-pitch gauge and a reliable identification chart.
1. Make the component safe
Isolate the machine, release stored pressure and confirm the line is safe to disconnect. Clean the thread without removing metal. Replace badly damaged fittings rather than relying on uncertain measurements.
2. Decide whether the thread is male or female
A male thread is external; a female thread is internal. Note the fitting style and where the connection appears to seal.
3. Check for parallel or tapered form
Measure the thread diameter near the first full thread and again farther along.
- A parallel thread stays substantially the same diameter.
- A tapered thread changes diameter along its length.
Wear, plating and measurement position can affect the reading, so use this as one part of the identification.
4. Measure diameter
For a male thread, measure across the outside crests. For a female thread, measure the internal opening carefully. The measured diameter may not equal the nominal trade size—this is especially common with pipe threads.
5. Measure pitch
Match a pitch gauge to the threads with no visible gap.
- Metric pitch is normally stated as the distance between crests in millimetres, such as M12 × 1.5.
- Inch-based threads are commonly stated as threads per inch, such as 18 TPI.
If a gauge is unavailable, measure across as many complete threads as possible and divide, but treat the result as an estimate.
6. Identify the sealing method
The thread may provide mechanical retention while the seal is made elsewhere. Look for:
- a bonded washer;
- an O-ring at the face or port;
- a 37° flare;
- a 24° cone;
- a compression olive;
- a tapered thread;
- a sealant on the thread.
7. Compare all features
Use diameter, pitch, taper, thread angle and sealing face together. Never identify a pressure connection from diameter alone.
8. Confirm before use
Check the fitting against the equipment documentation or a manufacturer gauge. Confirm material, pressure rating, temperature range and fluid compatibility as well as thread type.
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