What is a Thread?
A thread is a "bump" wrapped around a connector body, or inside a locknut like a spiral.  When the lump is big and the coils spaced apart, it forms a coarse thread like NPT or BSP, or if it is small and neat then it can be made into a UNEF or metric thread. Different thread types mean a different pattern of "bumps" or "spiral" has been used on that connector or locknut.

The Pitch of a thread is the distance from "bump" to "bump".

As threads on connector bodies can vary, it is important to specify the correct size of thread, particularly with threaded entries. This will ensure that connectors are suitable for either the threaded entries or locknuts they are used with, and vice versa!

Adaptors and Reducers

There are two types of threads which can be used for connectors.  These are female types on the inside, like a nut, and male types around the outside like a screw.

If the female is larger than the male one, then the resulting component is called an adaptor.  This allows a large connector to be fitted into a small hole.  Alternatively if the female thread is small and can be made inside the male, then the component is known as a reducer so it can make a big hole smaller.

Adaptors and reducers can be made from metal or plastic, depending on where they are used.  They are usually steel or aluminium to go with pliable / steel flexible connectors and plastic such as nylon to match Plastic conduits and connectors.

Flameproof Adaptors are generally made from brass so they will not produce a spark when hit accidentally. To specify either an adaptor or reducer the size and thread type of both holes, as well as whether male or female, needs to be known.