A replacement pneumatic fitting may have two sizes in its description: one for the tube and another for the equipment port. Treating them as one dimension leaves an important gap in the specification. A clear replacement record identifies the tube interface, thread form and sealing arrangement separately, then checks them against a drawing.
Start with the tube outside diameter
Record the tube's outside diameter and whether the dimensions are metric or imperial. Where available, retain the tube manufacturer's marking and part reference. For an unidentified sample, obtain measurements from a clean, undamaged section using suitable measuring tools; also record the tube material and any stated dimensional tolerance. Festo's QS documentation lists tube outside diameter separately from the threaded connection. The visible bore alone is not a complete ordering description.
Write the full thread designation
Next identify the equipment port and the fitting's mating end. State male or female, the thread family, nominal designation and pitch when required. Parker's identification manual treats NPT, BSPT, BSPP and metric threads separately. A useful inspection record includes measured diameter, pitch-gauge observations, whether the thread is tapered or parallel, and photographs of any marking. Have uncertain identification checked against the equipment drawing or appropriate gauges rather than approving it from appearance.
Locate the intended sealing surface
Thread engagement and pressure sealing are separate design questions. ISO 228-1's public abstract describes parallel threads whose pressure-tight joint is made outside the threads with an appropriate seal. Festo also distinguishes sealing arrangements in its QS family documentation. Examine the specified washer, O-ring, seat or other sealing feature on the selected design. Ask for the correct seal arrangement and installation instruction; do not assume that adding more thread tape can resolve an unidentified interface.
Use one record for purchasing and inspection
Combine the findings in a compact schedule: tube outside diameter and units; tube reference; fitting shape; full thread designation; sealing method; and the equipment drawing revision. Label photographs so the supplier knows which image shows the tube end and which shows the port. Record uncertain details explicitly instead of filling them with an approximate equivalent.
Before approving an alternative, ask for a dimensioned drawing that resolves those uncertainties. The responsible engineer can then compare the proposal with the machine's actual connection. This article provides an identification workflow, not permission to force mismatched threads together or a guarantee that any listed connector fits a particular machine.
Official references: Festo QS connection documentation; Parker thread identification manual; ISO 228-1 public abstract.