Manual gearboxes are limited mostly to sports and performance cars, and include 4-, 5- and 6-speed transmissions. The top gear in 5- and 6-speed transmissions is usually an overdrive ratio to improve highway fuel economy. Manual gearboxes are relatively trouble-free and long-lived provided they are not abused too severely. Gears and synchronizers (which allow you to shift from one gear to another smoothly) can be damaged by abusive driving.
The primary wear components in a manual drivetrain are the clutch and release bearing. The clutch is bolted to the flywheel on the back of the engine. A “pressure plate” exerts pressure against the clutch disc to hold it firmly against the flywheel when the clutch is engaged. This allows engine torque to pass directly to the transmission and drivetrain. When the clutch pedal is depressed to disengage the clutch, the release bearing is pushed against the spring fingers on the pressure plate, causing the clutch to release.
Most clutches have a diaphragm spring but some older vehicles have coil spring clutches with 9 to 12 coil springs. Diaphragm clutches are used on most newer vehicles because they require less pedal pressure to release than coil spring clutches, are less complicated, last longer and actually increase the clamp load on the clutch disc as it wears (up to a point).
The clutch disc acts as friction linings on both sides that grab the flywheel and pressure plate. The disc is mounted on the transmission input shaft with a splined hub, which often has five to eight springs to help cushion clutch engagement. Engaging the clutch creates friction which generates heat. Consequently, the clutch linings can get very hot if the clutch is allowed to slip too much. The flywheel and pressure plate both act as heat sinks to help carry heat away from the clutch and cool it. But if the clutch gets too hot from excessive slippage or overloading, the linings may burn damaging the clutch. Oil leaks from the engine or transmission can also cause the clutch to slip and fail.
The release bearing that releases the clutch has ball bearings to reduce friction. If the release bearing is worn, it can make noise when the clutch pedal is depressed. It can also damage the clutch spring or release fingers if it binds up.
On most clutches, the release bearing is held in a yoke or fork that pivots on a ball stud when the clutch linkage moves. On some vehicles, a telescoping hydraulic release bearing is used inside the bellhousing to operate the clutch. Wear or damage to any of these components may affect the operation of the clutch.
Older vehicles mostly use a mechanical linkage or a cable connected to the clutch pedal to operate the clutch. Most newer vehicles have a hydraulic clutch linkage. A master cylinder attached to the clutch pedal generates hydraulic pressure that moves a slave cylinder attached to the release fork on the transmission bellhousing, or operates a hydraulic release bearing directly. Fluid leaks in a hydraulic system can prevent the clutch from disengaging. Internal corrosion can also damage the piston seals in the master or slave cylinder, preventing the unit from holding pressure.
Replacing a clutch is a major job because of its buried location between the engine and transmission or transaxle. The labor required to do this job can be four to six hours or more on most vehicles. Because of this, most experts recommend replacing all of the major clutch system components at the same time. Installing a complete clutch kit that includes the clutch pressure plate, disc, release bearing and pilot bearing or bushing (if used) will restore the clutch system to like-new condition, and reduce the risk of a comeback. Clutch kits also eliminate the risk of mismatched parts, which can sometimes happen when different clutch components are sourced from different suppliers. Upgrading to a stiffer, stronger performance clutch may also be recommended for hard use applications.
If a customer is replacing a clutch, he should also inspect the flywheel to see if it is clean, smooth and flat. After years of use, most flywheels are often scored and grooved. If runout exceeds specifications and/or the surface is worn, the flywheel must be resurfaced. Minor surface heat cracking is normal, but large cracks are dangerous and require the flywheel to be replaced. Some clutch suppliers will not honor a clutch warranty if the flywheel was not resurfaced when the clutch was installed.
In most rear-wheel drive vehicles, a pilot bearing or bushing is located in the center of the flywheel to support the end of the transmission input shaft. This should also be replaced when changing a clutch.
Some trucks (typically diesel-powered pickups) have a “dual mass” flywheel, which is like two flywheels in one. A dual mass flywheel helps dampen engine vibrations and cushions clutch engagement for smoother operation. If a dual mass flywheel is cracked, damaged or the internal springs have failed, it must be replaced. Resurfacing dual mass flywheels is not recommended. Dual mass flywheels are very expensive, so there are conventional one-piece solid flywheels available as a less costly repair option. But these one-piece flywheels may require a different clutch, and may increase engagement harshness and vibration.
Installing a new clutch usually requires a “pilot tool” to center and align the disc with the transmission input shaft. Other items that may also be needed include new motor or transmission mounts, engine or transmission seals (if oil leakage caused the old clutch to fail), and gear oil or ATF (if low).