How do I determine a pipe thread?

Determining a pipe thread often causes confusion – especially because the imperial measurements do not correspond to the actual millimetre dimensions.

Many think that a 1" thread should have an outer diameter of 25.4 mm. However, this is incorrect.

Measuring external threads – the easiest way

Basically, it is easiest to measure theexternal thread. zu messen.

Without acaliper, accurate determination is hardly possible, as the deviations in millimetres are crucial.

Why is 1" ≠ 25.4 mm incorrect?

The mistake originated in the early 20th century:

• At that time, the imperial measurements referred to theinternal diameterof pipes.
• A 1" pipe had a clear width of about 25.4 mm.
• With the wall thicknesses of the time, this resulted in anouter diameter of about 33 mm..
• All fittings and tools were adjusted to this outer diameter.

With the improvement of steel quality, the pipe walls became thinner. This changed the internal diameter, but the thread sizes remained unchanged.

Today, pipe sizes are defined in metric – but the imperial measurements have remained astechnical designations. geblieben.

Standards and regulations

TheDIN EN ISO 228-1specifies the dimensions for cylindrical pipe threads.

• Whitworth pipe thread (G thread): Cylindrical pipe thread for non-sealing connections (corresponds to BSPP).
• BSP R (tapered): Tapered external thread, measurement is taken at the end, as it tapers towards the front.

Important:

• G thread = cylindrical (straight), 55° flank angle
• R thread = tapered, 55° flank angle
• Both types of threads arenot compatible.

Practical tips for determination

1. Measurement with caliper: Determine the outer diameter of the thread (for external threads) or the core diameter (for internal threads).
2. Cross-check with table: Only in this way can the correct inch value be determined.
3. Note the difference between G and R: Straight or tapered? – This is crucial.
4. When in doubt: send a sample piece or photowith measurements to the supplier.

Conclusion

The inch specification for threads hasnothing to do with the conversion inch → mmat all.

Anyone measuring a 1" thread will find adiameter of about 33 mmoutside diameter.

For a reliable determination,measurement and comparison with a thread tableare essential.
Here is the table with inch measurements