Fastener & Thread Identification

Measure the fastener before choosing a replacement or repair

Fastener identification depends on more than the wrench that fits the head. Nominal thread diameter, pitch or threads per inch, thread system, length, head style and sometimes strength grade all matter. A near match can start by hand and still damage both components after a few turns.

Use the right measurement for the feature

Measure an external thread across the crests with a caliper and keep the jaws square to the fastener axis. Use a thread-pitch gauge when possible. For metric threads, pitch is the distance between adjacent thread crests. Unified inch threads are commonly described by threads per inch. Head or wrench size is a separate dimension.

Do not force a close match

Some metric and inch diameters are numerically close. Similar diameter does not make thread forms interchangeable. If a fastener does not run in freely by hand, stop and confirm the specification rather than using a wrench to prove the match.

Identification becomes critical before machining

Exact diameter and pitch matter before tapping, drilling for an insert or ordering a replacement for a loaded joint. If the original thread is too damaged to measure, use an undamaged mating fastener, drawing, service information or gauge.

Use this hub with the measurement tools

The guides below explain diameter, pitch, metric vs SAE/Unified systems, UNC vs UNF, coarse vs fine threads, blind holes and practical ways to identify an unknown fastener.