Vehicles with manual transmissions account for only a small segment of the overall car population (less than 6 percent), though in recent years there has been an uptick in manual sales. The fuel economy advantage that manual transmissions have traditionally had over automatics has eroded in recent years thanks to new fuel-efficient Continuously Variable Transmissions (CVT) and 6-, 7- and 8-speed automatics. Many younger drivers today don’t even know how to operate a manual transmission.
In spite of these trends, there are still more than 15 million manual-equipped vehicles on the road that will eventually need a replacement clutch (some more than once). So manual clutch kits will be with us for many years to come.
When a customer needs a new clutch, always recommend a complete clutch kit. A kit includes a clutch disc, pressure plate and release bearing that is guaranteed to fit the application. Mixing parts from different suppliers can sometimes result in mismatched components that won’t fit or cause release or engagement problems.
Piecemeal clutch repairs seldom last, and the labor involved to replace individual clutch components is no more than to replace everything at the same time. Installing a complete clutch kit, therefore, can avoid additional repair expenses down the road while extending the life of the clutch system.
The function of the clutch is fairly simple: it momentarily disengages the engine from the drivetrain when shifting gears and when stopping. Depressing the clutch pedal pushes the release bearing against the pressure plate. This relieves pressure on the clutch disc, allowing it to slip and disengage the engine from the transmission.
After many miles and years of use, the clutch may lose its grip and slip, or fail to engage or release. Oil contamination is a common cause of clutch slippage, and is usually due to a leaky rear main crankshaft seal or leaky transmission input shaft seal. A badly worn clutch disc or a weak pressure plate can also allow the clutch to slip. In either case, the vehicle needs a new clutch. Any oil leaks should also be repaired before the new clutch is installed.
If a clutch fails to release, the problem is often a bad master clutch cylinder, slave cylinder or hydraulic concentric release bearing. Replacing the master and slave cylinder at the same time will reduce the risk of future problems.
On applications that have a concentric slave cylinder, the release bearing may be part of the slave cylinder, or a separate component. A new release bearing should be included if it is not part of the slave cylinder. Slave cylinders may have a plastic strap that holds the cylinder in a fully compressed position. This strap must not be removed during installation. It is designed to break free the first time the clutch is engaged.
On older vehicles that use a cable release mechanism, the clutch may not release if the cable is broken or misadjusted. A problem with the pivot fork that operates the release bearing can cause similar problems.
Clutch noise is usually caused by a bad release bearing, but it can also be caused by a bad pilot bushing in vehicles that have a pilot bushing in the back end of the crankshaft. The bushing supports the tip of the transmission input shaft. This may or may not be included in a clutch kit depending on the application.
The condition of the flywheel also is important. The surface of the flywheel must be clean, smooth and flat. If rough or worn, it should be resurfaced or replaced. Some stepped flywheels and dual mass flywheels cannot be resurfaced and must be replaced if worn or defective. If cracked or damaged, replacement is the only option.