If you have a kart engine made without taking into account the approval regulations imposed by FIA Karting (to learn more about this topic we recommend, "Dossier | FIA approvals: what they are, what they are for, how long they last... the ultimate guide!"
), our advice is to consult the manual provided by the manufacturer of your engine, since each brand may opt for different sizes. If, on the other hand, you have an engine produced according to the dictates of the CIK-FIA regulations, then it is the FIA Karting regulation that imposes precise "sizes" on manufacturers, including spark plugs. Specifically, all spark plugs must have a thread with a diameter of M14 with a pitch of 1.25 mm. As regards, however, the length of the threaded part inside the cylinder head is fixed at 18.5 mm. Thus, the length of the thread of the male spark plug is 19 mm (0.75 inches), so that combustion takes place inside the chamber. The female thread of the spark plug in the cylinder head can also be replaced by a threaded insert. Regarding the protrusion of the ceramic insulator, the FIA regulations state that the insulation of the electrode (not including the electrode itself) must not protrude beyond the metal body of the spark plug. The insulation (usually white), must not, therefore, come out of the metal ring that is beyond the end of the thread.
In summary, kart spark plugs must comply with the FIA technical regulations, which impose the following measurements:
Thread diameter: M14
Thread pitch: 1.25 mm
Female thread cylinder head length: 18.5 mm
Spark plug thread length: 19 mm
Notes: no protrusion of the ceramic insulator from the metal body (as per Appendix 5 of the FIA Karting technical regulations)
By checking the technical data sheet to ensure the sizes correspond to those of the regulation, you will not have problems either in assembly or during the technical verification of your kart during race weekends.