Identifying diameter and pitch on metric connections

Technical explanation

In thread identification, diameter, pitch and parallel/tapered form are measured separately. Two connections with a metric thread may belong to different cone seat, O-ring or port designs. The measured outside diameter can be affected by tolerance, plating and wear; a product decision should not be made from a single measurement. The thread pitch and the pipe or hose diameter are not the same information. Series designations such as L and S used in the metric fitting family cannot be reduced to thread size alone either.

Source: S019

Topic-specific checkpoints

Source: S019

How is it assessed in practice?

For a sample, record the diameter measured with a caliper and the thread gauge result as raw observations; then compare them with the standard product definition. Add side and front photos of the sealing area. Use the full designation with units instead of a number such as '18'. In the field, a thread starting only when forced is not evidence of a correct match. Do not create an assembly through corrections such as re-threading, shortening or thickening the seal before the correct part has been verified.

Original editorial recommendation; not a clause of a standard.

Frequently asked questions

If thread diameter and pitch are equal, are two metric fittings compatible?

These two dimensions may be necessary but are not sufficient. The seating surface, series and service limits must also match.

Source: S019