Clutch: Parts and function 

Here you get an overview of how a clutch system works and which parts are included.

The different parts are divided into categories as shown below. These headings together give an idea of what parts can be used and whether an upgrade is necessary or not.

The various parts of the clutch system are as follows:

1. Master clutch cylinder
2. Clutch hose
3. Slave cylinder
4. Reservoir / container
5. Release bearing
6. Pressure plate / Clutch cover
7. Clutch disc
8. Flywheel
9. Flywheel bolts (and pressure plate bolts)
10. Clutch fluid

Master clutch cylinder

A clutch master cylinder sits together with the clutch pedal. This is a separate system of three parts together with a slave cylinder and a hose that connects them together. When the clutch pedal presses the master cylinder, the fluid pushes out the slave cylinder that is mounted adainst the clutch. In this way, you can "disconnect" the gearbox and the engine when shifting.

A master cylinder can be mounted in the engine compartment or in the cabin. In the cabin, it can also point backwards depending on the design of the pedal box.

The cylinder itself can look different, but the function is the same for both OEM and racing / aftermarket.

Clutch hose

The only function a clutch hose has is to conduct fluid between the master cylinder to the clutch cylinder.

Something to think about is the dimensioning of this. The flow of fluid between master and slave cylinder is higher than in a brake system. Therefore, a thicker ID hose may be required if the clutch pedal does not spring back quickly enough. This is because a hose with an inner diameter that is too small acts as a restriction.

What differentiates an original and an aftermarket clutch hose is the type of connection and hose. In the aftermarket, standardized fittings such as AN fittings. The hose can also be steel braided/reinforced.

Slave cylinder

A slave cylinder is connected to a master clutch cylinder in a separate system. Through a hose from the master cylinder, clutch fluid is pressed to the slave cylinder, which in turn, with the help of a release bearing, pushes the pressure plate so that it can be "disconnected"

There are two types of slave cylinder.

• Mounted in the bell housing directly against the clutch / pressure plate. This is called a hydraulic release bearing.
• Mounted outside of the bell housing together with linkage / fork that push on the pressure plate. This is called an external slave cylinder.

Reservoir / container

Just like a brake system, a slave cylinder needs to be fed with fluid / oil. Brake fluid is used here, just like for the brake system. You can use the same fluid from the brake system's brake fluid reservoir to the clutch system. From a common container, fluid is led down into the clutch and brake master cylinder.

When fluid is divided in this way, it is important that the cylinder of the clutch is located under the brake fluid reservoir so that fluid can flow down freely. If this does not work, the alternative is to use a separate container for the clutch master cylinder.

Original and aftermarket fluid reservoirs have the same function. -Fluid must be led down to the master cylinder.

Release bearing

A release bearing is the part that push the pressure plate, more specifically on the spring of the pressure plate. This bearing allow a permanently mounted slave cylinder to press against a rotating clutch. This bearing sit against the clutch and spins with it all the time when you press the clutch pedal -> Master cylinder -> hose -> Slave cylinder -> release bearing, finally the pressure plate spring is pushed in and the clutch "disengages".

The release bearing must be adjusted so that it always rests against and rotates with the clutch. If not, it will wear out in no time. Also to bear in mind is that the diameter of the bearing should match the diameter of the pressure plate spring. For example, a 184mm clutch has a smaller inner diameter bearing than the corresponding 240mm clutch.

Note!
Push type pressure plates:
The release bearing must have contact with the pressure plate clutch diaphragm spring at all times, but when fully compressed, you must have at least 2mm play between them. When the clutch disc becomes worn, the pressure plate clutch diaphragm spring will rise and therefore this clearance of at least 2mm is needed. If this clearance is not present, the pressure plate will lose clamping force as the clutch disc wears.

Pressure plate / Clutch cover

The role of a pressure plate is to clamp the clutch disc together with the flywheel. They are mounted as follows:
Pressure plate -> Clutch disc -> Flywheel.
The flywheel and pressure plate wear surfaces are made of metal. Clutch discs are available with different material choices for different amount of friction and different character.

The following determine how much power a clutch can handle:

• Clutch total contact surface (cm2)
• The clamping force of the pressure plate
• Clutch disc friction coefficient

When mounting the pressure plate, the manufacturer's guidelines must be followed. That is, how much the pressure plate's spring is tensioned when the pressure plate is tightened. Too much or too little tension drastically reduces the clutch clamping force and power resistance. Here you have smalll margin of error.

Both original and aftermarket pressure plates are used in racing / performance.

Clutch disc

The clutch disc is mounted between the pressure plate and the flywheel. This largely determines what character the clutch will have. There are a few different types of material to use for clutch discs. All with different friction coefficients depending on the area of use.

Heat resistance is also something that differentiates the different materials depending on whether you will be using it for the street, track racing or drag racing, different materials are used.

Original clutches usually use a so-called organic friction material that provides a soft and drivable clutch for the street. This does not withstand as much heat and does not have as high a coefficient of friction as the corresponding sinter material. Therefore, different sintered materials are used for performance use.

The different types available:

• Organic
  Friction number: Low to medium
  Heat resistance: Low to medium
• Sinter pad
  Friction number: High
  Heat resistance: High
• Sinter
  Friction number: High
  Heat resistance: High

Flywheel

The flywheel is the part that is mounted on the engine and, together with the clutch disc and pressure plate, connects the engine and gearbox. The flywheels often have a starter ring and trigger ring built in, but it can differ completely between different models, so this needs to be checked on specific items.

When the flywheel is assembled with the rest of the clutch, it is important that the correct clamping pressure is achieved in the same way as described in the the section about the pressure plate.

OEM flywheels are often a dual mass type, which should reduce vibrations. This type is not used as performance flywheels. Then you have a so-called single mass flywheel.

Flywheel bolts (and pressure plate bolts)

OEM flywheel bolts and bolts for the pressure plate can be used also in performance.
However, you do not want to reuse these when they are stretched. Instead of buying original bolts, you can use reinforced bolts instead. These are often cheaper than originals.

Bolts for the flywheel and pressure plate must always be torqued according to the manufacturer's guidelines. Even if the flywheel and pressure plate are of the performance / aftermarket type, the original length of bolts can usually be used.

The three most common types of bolts that are used:

• Original
• 12,9
• ARP2000

Clutch fluid

Clutch fluid is actually brake fluid. Clutch and brake fluid often share the same container, and therefore the brake fluid that is in the brake system is usually used for the clutch as well.