Clutch:

For the average driver, stock BMW clutches tend to last in excess of 100,000 miles. Even most performance models such as the M3 and M5 will provide flawless service for 100k miles. This is not always true of the Z3 and Z4. Z models average about 70,000 miles. Over-the-top performance drivers and inexperienced drivers should not be surprised to get considerably lower mileage.

BMW clutches present a few engineering variations that owners should take into account when clutch problems arise.

Hydraulics:

BMW clutches are operated by a master and slave hydraulic system (two separate cylinders, one on the firewall, one mounted on the transmission). Layout and design is conventional enough. Components look and operate as do many other vehicles. The unusual feature is that the clutch hydraulic system shares fluid and reservoir with the brake system. Although not exclusive to BMW, this is not a common configuration.

The brake fluid reservoir is larger than most of those used on clutch systems. When a small leak occurs in one of the clutch cylinders, additional brake fluid feeds into the system to replace what is lost. The large volume of available fluid tends to obscure symptoms that are usually expected from a leaking clutch cylinder. The system continues to work as long as fluid is available. The clutch pedal probably seems a little soft and shifting the transmission may feel a bit sticky. There will be some brake fluid loss.

Be sure to check the entire hydraulic system (clutch and brake) when diagnosing clutch or brake fluid loss.

Flywheel:

BMW uses both solid, single-piece flywheels and dual-mass flywheels. Newer models are tending toward dual-mass.

Dual-mass flywheels act as momentary impact/thrust absorption implements (absorbing the initial shock of clutch engagement) while transferring torque efficiently to the transmission. By placing the absorption mechanism in the flywheel rather than in the clutch disc (as has been done for decades), a great amount of shock can be absorbed. This helps prevent clutch chatter and engagement 'jump' while protecting transmission and differential parts from damage under aggressive driving.

But, dual-mass flywheels do not last forever, and they are expensive. It is unusual for a dual-mass flywheel to retail for less that $400 and it is not unusual to see certain part numbers retailing as high as $1595!

During most clutch replacement operations on vehicles that have dual-mass flywheels, the flywheel should be replaced. Their life expectancy is not much greater than the clutch itself.

When doing a clutch job, replace the entire clutch kit (plate, disc, release bearing, and pilot bearing), carefully inspect the flywheel and replace if hotspots are present (solid flywheels, if dual mass, we recommend replacing the flywheel with each clutch). Check motor mounts and transmission mount for breakage and/or sagging (BMWs are especially susceptible to sagging motor mounts). Inspect universal joints and drive shaft coupler.

SMG - Sequential Manual Gearbox:

This technology, available on M3s and standard on M5s and M6s, is just a bit more than amazing. Driving it feels like cheating! Performance exceeds human capabilities. On its most aggressive setting the SMG changes gears in 0.08 seconds! That is less than half the time required by Ferrari's version of the same technology (0.015 seconds) and far superior to Toyota's version.

When a vehicle's own control abilities exceed human potential by such a wide margin, driving starts to feel more like being driven. The vehicle is in control, not the driver. This phenomenon offers an entirely new kind of performance thrill. Start the car, press the accelerator, hang on!

Despite such cutting edge performance, SMGs do require maintenance and repair. Clutches last less than 100,000 miles. Although parts are costly, the actual replacement operation is pretty straight-forward. Clutch Wizard shops can usually accomplish the entire operation in less than one full work day.

According to United States Patent #6962533:
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Torsional vibration damper, especially a dual-mass flywheel


Abstract: A torsional vibration damper, especially a dual-mass flywheel, has a primary side and a secondary side, which is free to rotate around an axis of rotation relative to the primary side. Starting from a rotation in a direction of rotation around a forward base angle, a forward base spring arrangement exerts a forward base restoring force on the secondary side. Starting from rotation around a forward auxiliary angle, which is greater than the forward base angle, a forward auxiliary spring arrangement exerts an additional forward auxiliary restoring force. Upon rotation around a forward limit angle, both spring arrangements come to a stop together.