Reading pitch on UN and UNF threads

Technical explanation

For inch threads, pitch information is usually given as threads per inch; it cannot be used directly as the same number as a metric pitch. A geometric thread match is not a match of the sealing geometry. Connections with the same or similar thread sizes may use different seal and seat designs. The thread check is therefore completed by examining the functional surface of the connection after measuring diameter and pitch. The relevant standard and manufacturer data are required for thread tolerances.

Source: S019, S023

Topic-specific checkpoints

Source: S019, S023

How is it assessed in practice?

Keep the 'mm pitch' and 'TPI' fields separate in the measurement form. For example, automatically reject a data entry that transfers 18 TPI as an 18 mm pitch. When storing inch fractions, separate the designation from the calculated value; the original standard's designation can be retained in the order. Do not copy a connection's assembly instructions from another part merely because it has a UNF thread. Do not ask a supplier for an equivalent without the definition of the sealing surface and the mating port in addition to the thread information.

Original editorial recommendation; not a clause of a standard.

Frequently asked questions

Does the thread pitch get larger as the TPI value increases?

No. More threads per inch means a smaller axial pitch; the units must not be confused.

Source: S019, S023