These are all "K-type" thermocouples and they come in sizes 10mm, 12mm, 14mm and 18mm. The sizes refer to the thread diameter of the spark plug the probes are used on. Most automotive engines use 14mm plugs, the Jabiru engine and Rotax 912/914 use 12mm plugs. The big Lycomings and Continentals use 18mm plugs. Some motorbike engines use small 10mm plugs. The probes are supplied with 3 ft of Teflon insulated cable, heat resistant to 300 degrees C.