The most senior CIK-FIA karting class with a gearbox is KZ.
KZ is the primary gearbox class used in FIA Karting’s major championships and is often described as the “Formula One of karting.” It is for drivers aged 15 and older and combines a 125 cc water-cooled two-stroke engine with a manual clutch and a six-speed sequential gearbox. Unlike direct-drive classes, the driver actively shifts gears while racing.
The modern KZ class has a long lineage. It began as Formula C in 1974, became Super-ICC in 2002, and was replaced by KZ1 in 2007. In 2013, KZ1 was renamed KZ and returned to the World Championship. KZ2 remains the closely related secondary gearbox class.
The names can be confusing because KZ1 was historically the top KZ category, while KZ2 was the more standardized secondary class. In current terminology, however, KZ is the principal senior gearbox category. OK is the senior direct-drive class, not a gearbox class, while X30 is a separate one-make engine category.