A clutch kit is a group of manual transmission parts that are either purchased as a unit or assembled piecemeal. When bought together, a clutch kit will usually consist of a pressure plate, clutch disc, and a release bearing. In certain applications a pilot bearing will be included, and many will also come with an alignment tool of some kind. Some manual transmissions are designed so that the release bearing is an integral part of the slave cylinder, in which case this will often be included as well. Most clutch kits are designed to the same specifications as the original parts, though it is sometimes possible to find a performance or racing kit.
The purpose of a clutch kit is to provide all the necessary parts to repair the clutch on a manual transmission vehicle. Clutch discs are wear items that will eventually need to be replaced, regardless of how carefully a vehicle is driven. While other parts, like the release bearing and pressure plate, may not wear out at the same speed, they may be replaced anyway since removing a transmission is often labor intensive.
In addition to the parts included in many clutch kits, certain other parts may be replaced at the same time. The flywheel, which the clutch disc is typically mounted to by means of a pressure plate, is usually removed and machined as part of a clutch job. In certain circumstances, it may be more economical to simply replace the flywheel, especially if it is cracked or missing teeth from the ring gear. The clutch slave and master cylinders are often replaced as well, as problems with these hydraulic parts may cause a clutch disc to wear out prematurely.
Many clutch kits include an alignment tool. These tools can be used in more than one clutch repair job, though they must have the same number of splines as the clutch disc. In applications with a pilot bearing, the alignment tool must also be able to fit into the bearing in the same way the pilot shaft of the transmission does. Since there are so many different spline and pilot bearing configurations, many clutch kit manufacturers simply include the correct tool for ease of installation.
Stock kits are usually well suited to every day driving, though it is possible to find performance clutch kits for some vehicles. The parts contained within racing and performance kits will typically bolt up just like stock parts, but may provide improved holding power and other benefits. Some performance kits may include additional parts, such as a lightweight aluminum flywheel.