Clutch

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

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

The output shaft of the gearbox is splined, and the clutch disc fits over these splines.

Pressure Plate

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

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.

Common Problem

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.