Metric vs Unified vs Pipe and Taper Threads

Not all screw threads are interchangeable. The standard a thread is cut to sets its form, pitch and fit, so the gauge checking it has to match the same standard.

Metric threads

Metric threads use a 60° thread angle and are gauged to DIN ISO 1502 (and the equivalent ANSI/ASME B1.16M). They're specified by diameter and pitch, for example M10 × 1.5.

Unified threads

Unified threads (UNC, UNF, UN) also use a 60° angle but are specified in inches and threads per inch, to ANSI/ASME B1.2.

Parallel pipe threads

Parallel "G" pipe threads (ISO 228-2) seal outside the thread itself, typically with a separate seal or gasket rather than the thread form.

Taper pipe threads

Taper threads such as NPT (ANSI/ASME B1.20.1), the dry-seal NPTF (ANSI/ASME B1.20.5) and ISO 7-R / BSPT (ISO 7-2, BS 21) seal by the taper itself, so the thread form, depth and taper angle all have to be correct for the joint to hold pressure.

Why it matters

Picking the wrong gauge family means it won't correctly accept or reject the part, even if the numbers look close. Always match the gauge to the standard called out on the drawing.