The clutch on a manual transmission car is a group of parts. The flywheel is bolted onto the engine’s crankshaft, and the clutch pressure plate is bolted onto the flywheel with the clutch disc squeezed in between the two. The clutch plate, or disc, is covered on both side with a fibrous friction surface that is riveted to the middle metal disc. Generally, the outer disc has a spring system that absorbs shock that connects the outer disc to the inner hub that is splined to match the transmission’s input shaft. The clutch disc is squeezed between the flywheel and the pressure plate by a large round diaphragm spring in the pressure plate. The throw out bearing rides next to the pressure plate, and pushes on the diaphragm spring when the clutch pedal is pushed, which then allows the clutch disc and transmission input shaft to stop turning while the flywheel and pressure plate whirl with the engine.
Clutch failure can manifest itself in a number of ways. A clutch can slip, chatter or fail to disengage. Occasionally, we simply find one broken, or the flywheel shattered. When a clutch just starts to slip, the driver may first notice that the engine rpm’s rise when on a steep freeway hill, but the speedometer reading does not increase at the same rate. Once slippage first occurs, the clutch can often be babied for a short time, depending on the driving environment. But each time it slips, the clutch gets worse, and degrades to undrivable rapidly. Clutch chatter is a sharp vibration felt when the clutch is released. It can be caused by warpage of the clutch cover, oil seepage onto the friction surfaces or misalignment due to improper machining or substandard parts. Often clutch chatter can be controlled but releasing the clutch at lower power or lowering power as the clutch is released. Failure to properly disengage can be caused by a clutch cable problem on an early car, or clutch hydraulics on later models. But whether the clutch fails to disengage for external or internal problems, the issue is definitely going to damage the transmission. The worse the failure, the faster the damage.
Proper clutch usage can help longevity. And there are just a few hints. Never slip a clutch in any gears except First and Reverse. The clutch should be fully engaged before any major power is transferred. Minimize slipping without violently engaging the clutch. This is easier with engines that have a lot of torque, or twisting force. One can use the handbrake to hold a car on a steep hill, rather than dragging the clutch. And with practice and careful timing, the clutch can be engaged and the hand brake released simultaneously. Be careful when downshifting. Sometimes it is necessary to momentarily apply a small amount of throttle to bring the engine rpm’s up while engaging the clutch when downshifting. Jerking the clutch while downshifting is actually the most common way a clutch gets damaged on a diesel, The springs in the center of the clutch disc can be broken. With care, the clutch can last the life of the car. It is all about the pilot and the environment.
Dual Mass Flywheel
Almost all clutches in vehicles from this century use what is known as a dual mass flywheel. As the name implies, there are two sections. The first is the outer shell with the starter ring gear on it, and it bolts directly to the crankshaft. The second section has the friction surface for the clutch plate and is held in the middle of the shell section by a large bushing. There is an array of large springs between the two sections that absorb crankshaft and clutch operation vibrations. These flywheels cannot be resurfaced and reused. There is no way to disassemble the flywheel for servicing, and no way to hold the center part secure during machining. The metal chips from the machining process would get inside the bushing and spring assemblies. These flywheels must be replaced during a clutch job, which is one reason modern clutch jobs cost so much more than similar jobs from last century. Occasionally we find bad vibrations from a damaged dual mass flywheel. We have also seen a few actually broken into pieces.
Single mass flywheels are available for most applications, and the cost is usually 30% less than the dual mass equivalent. For high performance applications, a single mass flywheel and a heavy duty clutch assembly are recommended. Extra light flywheels are an option, but not appropriate for street use. The engine idle is not very smooth and the clutch release feel is jerky and difficult. Single mass flywheels also have a noise when the clutch is released that goes away when the clutch is depressed. Called “Lay Shaft Noise”, it is the sound of the pulsations of the crankshaft that are transmitted through the clutch assembly to the transmission input shaft. Some single mass flywheel manufacturers have improved versions they call silent or quiet, and the noise is diminished, but persists just because of design.
The Fine Points During a Clutch Job
Modern flywheel bolts are all stretch bolts. They should be hand tightened progressively, torqued with a torque wrench, then a final tightening by rotating a precise amount of degrees as per the factory instructions. The job requires special tools to lock the crank/flywheel assembly. These bolts should never be just tightened with an air gun, although some “experienced” mechanics swear they can get it right. The clutch cover to flywheel bolts must be tightened progressively and in a pattern with a final precise torque as per the manual. Failure to do so can warp the clutch cover, causing the clutch to disengage near the floor and possibly chatter when released. A clutch alignment tool is usually included with the clutch kit. If the clutch disc is not properly aligned, the transmission will be hard or impossible to install, and forcing the transmission into place will damage the new clutch. Some clutch kits actually come preassembled, but the the clutch cover bolts should be checked with a torque wrench.
There is a special lubricant for the clutch disc to transmission shaft splines that is very sticky and will not drip onto the friction surfaces. The guide tube surface for the throw out bearing and the divots for the pivots need to be cleaned and lubricated with a good synthetic grease. Regular grease will dry up and fry in the clutch heat over time, and may drip on the friction surfaces causing chatter of slippage. It just takes a little more time to do the job carefully and correctly.
Best to never go cheap on parts. A clutch job is a long process, and you really want the job to go right the first time, and hopefully last for the life of the car. We never even consider rebuilt or used clutch parts. We only use Sachs and Luk clutch kits. They are the two original equipment manufacturers for Volkswagen, Audi and Porsche. For High Performance applications, we recommend South Bend Clutches.
Clutch Replacement During Engine or Transmission Jobs
Any time the engine and transmission are separated, the clutch should be evaluated. Unless one knows for sure that the clutch is less than two years old, the pressure plate should should be unbolted slowly by a half turn increments in a star pattern, and the assembly should be inspected. First just loosen the bolts until the pressure plate quits rising. If the gap between the pressure plate and the flywheel is 3 mm or less, the clutch must replaced, as it will soon be slipping. Normal gaps for a new assembly are between 8 mm and 10 mm. The measured gap indicates clearly the amount of wear life remaining.
The disc should be inspected for cracked or burned friction material, worn hubs from lack of lubrication, and loose center springs, if equipped. The pressure plate should be checked for cracks and burns, and the diaphragm spring section inspected for bent tangs, and wear from throw out bearing wobble. The flywheel should be checked for cracks, bad burn spots and depth of remaining surface for friction. A dual mass flywheel should be check to see if the center section is loose in side to side motion and tilt. Any movement near 1 mm is a problem. The pilot bearing needs to be checked for damage, and lubricated with synthetic grease. All to often pilot bearings do not get replaced and/or lubed during a cheap clutch job. The throw out bearing fork and pivots, or if equipped, the hydraulic throw out bearing cylinder should be checked for wear and any side play. A hydraulic unit should be checked for seeps or leaks.
Basically, the clutch should get replaced if dire problems are found, or if there is any history of clutch chatter. Sometimes an apparently decent clutch should be replaced for the smell of burning, as the friction material has been tortured and outgassed needed chemicals from the material. It may crumble soon. Many hours of labor are required to do a clutch job later, so make a wise and educated choice.