The coiled insert is the most different from all the other types of insert. Instead of a solid tube, it consists of a diamond cross-section spring. Coiled inserts are threaded in and their shape means the insert can be contracted for installation and expand when in place, securing it further.
The threads on a coiled insert are created by the diamond shape cross-section, which forms both the internal and external threads. This spring design allows the insert to have a thinner wall than the other insert types, which means coiled inserts are lighter and can be installed closer together, if necessary.
Coiled inserts can come with a tang. This is a small section at the end of the wire, which is bent inwards to form a bar across the bottom of the insert. The tang is used to guide the installation tool when fitting the insert and is generally removed afterwards, to get it out of the way.
Coiled inserts are used in metal. There are some specialised coiled inserts for use with spark plugs.