Clutch

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Alongside the gearbox, the Clutch is one of the most misunderstood mechanical components of the car. But it doesn't need to be.

If you think your clutch is on its way out, learn more about replacing the clutch.

Contents


Overview

The clutch mechanism is made up of three key parts; the Clutch Disc, the Pressure Plate and the Release Bearing. These parts bolt together into a unit that sits between the flywheel of the engine and the gearbox, and determine when the power is sent from one to the other, and how much.

The Pressure Plate's job is to pull the Clutch Disc onto the flywheel with a large amount of spring force. The Clutch Disc bites onto the surface of the flywheel, just like a brake pad rubs against the brake disc, using surface friction.

When you press the clutch pedal, you actually compress a hydraulic syringe called the Master Cylinder. This pumps fluid to push a rod out of the Slave Cylinder, which will pull the Clutch Disc away from the flywheel, thus disengaging the drive. When you release your foot, the springs of the Pressure Plate pull Clutch Disc back onto the flywheel, subsequently bringing power back into the gearbox and through to the wheels.

It is this combination of surface friction, spring force and hydraulic pressure that makes the clutch so complicated. Understanding each part will help you get the most out of your clutch, and therefore your engine.

Components

Clutch Disc

The clutch disc is the surface that presses up against the engine flywheel. It's outer rim is covered in friction pads, made of an asbestos-like material that grips the face of the flywheel. It's when these friction pads wear down that the clutch is effectively dead, and needs replacing.

The centre hole of the clutch disc is splined; its teeth fit over the output shaft of the gearbox to make it spin. But the centre hole isn't directly bolted to the outer rim; it's held in place via a series of tight but powerful springs, which give the outer rim a degree of rotational flex. This means that if the clutch disc is slammed against the flywheel (when dumping the clutch), it doesn't snap the gearbox shaft; some of the shock is absorbed by these springs.

Because of the size of each engine's flywheel, the clutch disc is specific to the engine family, so M40 clutch discs cannot be fitted to an M20 engine.

Pressure Plate

The Pressure Plate is a huge metal circle with fingers of metal pointing towards a hole in the middle. This all works as one massive, powerful spring, and is what holds the clutch disc against the flywheel with such force.

The Pressure Plate bolts directly to the flywheel via six bolts around its edge, and completely covers the clutch disc. It also houses the release bearing, which fits between the plate's massive springs and the clutch disc.

Although all M20 clutches are the same, the Pressure Plate fitted to the 325i is slightly more powerful than the 320i. Despite that, clutches can be swapped between these cars with no ill effects.

Release Bearing

The Release Bearing is how we engage and disengage the clutch. When no pedals are pressed, the Pressure Plate presses the Bearing against the clutch disc, engaging drive. When the pedal is pressed, the Release Bearing is pushed away from the clutch disc, allowing the clutch to turn with the gearbox shaft, but not to spin with the flywheel.

When the pedal is then released, the Pressure Plate pulls the Release Bearing back against the clutch disc, engaging drive and transferring the power from the engine to the wheels.

The Release Bearing is operated by a release lever within the Pressure Plate, which is controlled by the Slave Cylinder.

Unlike the Disc and Plate, the release bearing is specific to the gearbox, and therefore can't be swapped between the 320i and 325i.

Master Cylinder

The master cylinder is a big fluid syringe, and bolts underneath your clutch pedal. It receives fluid from the braking system, and uses that fluid to drive the slave cylinder. When the master cylinder is pressed in, the slave cylinder is pressed out, which operates the clutch mechanism.

Slave Cylinder

The Slave Cylinder is another fluid syringe, just like the Master Cylinder. When the Master is pressed in by the clutch pedal, the Slave is pushed out, which presses the release lever inside the Pressure Plate.

The Slave cylinder is bolted directly to the gearbox with two 13mm bolts, the top one of which is notoriously fiddly to access. It is also fitted with a bleed nipple to remove air from the hydraulic system.

Common Problems

Pedal Feel

If your pedal is becoming either too hard or too soft, or the biting point is rising and falling, then it is very likely that either your Master or Slave cylinder is failing. Check under the car and under the pedal for signs of fluid leaks. If you believe either cylinder has failed, then replace both at the same time. Failure to replace both will lead to increased pressure on the older cylinder, causing it to fail imminently anyway. Learn how to replace the clutch cylinders.

Fluid Leaks

An oily patch appears, either under the car or in the driver's footwell. 99% of the time, this means that one of your clutch cylinders has failed; your Master cylinder leaks into the footwell, and the slave leaks under the car.

When this happens, you need to change both cylinders as a pair. Changing one is false economy; the stronger condition of a new cylinder will cause the older of the pair to fail in a matter of weeks, so do them both together. Learn how to replace the clutch cylinders.

Bleeding

Just like the Brakes, air in the hydraulic system will cause issues such as soft pedal feel and engagement issues. To bleed the clutch:

  1. Crack off the bleed screw with a proper brake spanner to reduce the risk of rounding off the bolts.
  2. Unscrew one quarter turn
  3. Have assistant press AND HOLD the clutch pedal
  4. Retighten bleed screw
  5. Have assistant release clutch pedal
  6. Repeat steps 2-5 until fluid seeps from the bleed screw. At this point, tighten the screw fully.