In-depth With Hydraulics


In the following paragraphs we will discuss each major component of the system, its function, and general location in the vehicle.

Clutch Hydraulic System

Clutch pedal

While not actually a part of the hydraulic system, the pedal connects to and actuates the clutch master cylinder. The pedal assembly is located within the dash. If you were to crawl underneath the steering column with your back on the floor and appear up, you will see something such as this:

Figure 1: Take a look at Clutch Pedal Assembly Located Under Dash

You can see the clutch pedal spring in this picture along with the clutch adjustment rod. This component undergoes a boot in the firewall (which may also be seen) and connects for the master cylinder. Just a point of interested on most cars, this rod can be turned left or right to alter the point of clutch engagement above the floor, closer or farther.

Clutch Master Cylinder

The master cylinder is bolted to firewall in the back of the engine bay. It is simple to locate it as it is connected by a short hose on the clutch fluid reservoir. While I do not have a picture of an actual master cylinder, it is as the name suggests a simple cylinder with a steel line coming from the end. The inner piston is connected to the clutch adjustment rod. Thus as the clutch pedal is pressed, the fluid in the master cylinder needs out of the cylinder into the steel clutch fluid line that runs towards the slave cylinder.

Clutch Slave Cylinder

The slave cylinder is bolted to the actual transmission itself. One end of the cylinder is ready to accept the clutch fluid line. On the other side a steel rod connects the piston in the cylinder for the clutch fork on the bell housing of your tranny. A sketch from the cylinder is show below.

Figure 2: Drawing of Clutch Slave Cylinder and Related Compenents

When clutch fluid is forced to exit the master cylinder from depression of the clutch pedal, it flows with the steel line and in the slave cylinder. The force of the moving fluid actuates the piston in the slave cylinder and forces the slave rod outwards against the clutch fork.

This covers the most important elements of the clutch hydraulic system, however certain vehicles will have extra fluid reservoirs and/or restrictors in-between the two cylinders to help smooth out clutch disengagement and engagement. They are not essential components and they are not discussed. Now, let’s take a quick recap of how the hydraulic system functions

· Clutch pedal is pressed

· Master cylinder is actuated

· Fluid is forced out of your master cylinder and in to the slave cylinder

· Slave cylinder is actuated, forcing the slave rod out against the clutch fork

Clutch Mechanical System

Clutch Fork

The clutch fork connects the slave cylinder on the clutch release bearing, commonly called the get rid of bearing. We can easily see this by looking at the same figure above, this time together with the fork and related components labeled.

Figure 3: Look at Clutch Fork and Related Components

When the master cylinder is actuated and the slave rod extends, pressure is applied to just one end from the clutch fork. This causes the fork to rotate around the pivot ball and slide the discharge bearing over the transmission input shaft in the clutch unit itself.

Release Bearing (Throw Out Bearing)

As mentioned before, the release bearing is coupled to the end of the fork within the transmission bell housing and rests in the tranny input shaft and the clutch unit. As soon as the fork swings, the bearing applies pressure to the clutch and gives the force necessary for disengagement.

Clutch Unit

I made a decision to break this section up into the elements of the clutch to better explain the purpose of the different elements of the clutch itself. Whenever a clutch kit is purchased, it contains two pieces, a pressure plate and a clutch disc. These pieces are then bolted to the flywheel.

Pressure Plate

The pressure plate is simply a steel diaphragm composed of a friction surface that mates to the clutch disc and a number of extremely stiff springs. A standard pressure plate can been seen in the next picture:

Figure 4: Pressure Plate Without Cover Showing Friction Surface and Springs

Within this picture, there is absolutely no pressure plate cover. This is the often colorful piece that bolts the plate on the flywheel. Next, taking a look at that now familiar inner bell housing figure, we’ll have a look at the location of the clutch components

Figure 5: Location of Clutch Components Throughout the Transmission

Here, when the release bearing is forced against the pressure plate it causes the metal springs of the plate to bend. This bending action warps the friction surface of the causes and plate it to chill out is hold on the clutch disc. Simply put this “unlocks” the clutch disc through the friction top of the flywheel allowing the flywheel to free spin. At this time, the clutch is disengaged and everything looks like this:

Figure 6: View of Clutch Components When Clutch is Disengaged

Figure 7: Look at Clutch Unit Close Up

When the clutch pedal is raised, everything works in reverse, the clutch reengages and now there exists a picture that looks like this:

Figure 8: Look at Clutch Components When Clutch is Engaged

Figure9: Take a look at Clutch Unit Close Up

In the first picture, the angle of your pressure plate springs is exaggerated for that purposes of comparison between the engaged and disengaged states. In reality, the springs are completely flat when engaged, like the second picture, and conical when disengaged.


The flywheel connects the clutch and driveline to the engine. On one side it is bolted directly to the crank shaft, on the other the clutch assembly. Whenever a clutch is installed, pressure plate is bolted on the flywheel with all the clutch disc wedged in-in between the two. As there is volumes more information on flywheels, this is the essential information relating them to clutch operation. An assembled clutch looks like this:

Figure 10: Exposed Clutch Installed on an Engine

Clutch Disc

The clutch disc is merely a disc with each side covered in a high friction material to mate with the friction surfaces in the pressure plate and flywheel. It is also the only real part of the entire clutch system that connects to the transmission. The center of the disc is splined to match the splines on the input shaft of the tranny. As soon as the clutch is engaged, the disc is wedged against the flywheel and transfers power from the crankshaft to the transmission shaft. When disengaged, the pressure plate relaxes and the disc is no longer pressed against the flywheel. Simply put, at this time there isn’t enough clamping force to result in the disc to spin with the flywheel, so no power is transferred.

When a clutch goes bad, it can be typically as the frication material wears out on the disc, another point of interest. When this happens, even thought the pressure plate is engaged and applying a clamping force, there isn’t enough friction in between the disc and also the flywheel to transfer power and the clutch “slips”. This feels like the clutch is disengaged when it is actually engaged. At this point, normally the disc and pressure plate are replaced. There are many other modes of clutch failure, including explosion, breakage, warpage, etc. but those won’t be discussed here.

At this moment, an overview of the mechanical element of clutch disengagement might help within the understanding of all this information.

To begin with:

The slave rod pushes against the clutch fork

The fork transfers this motion and forces the discharge bearing from the pressure plate

The springs of the pressure plate warp and distort the friction surface

This removes pressure from the clutch disc allowing slippage to occur between the disc and the friction surface of the flywheel

At this point, no power is now being transferred to the transmission and gear changes can be made

When the clutch is reengaged, as in after a shift, the force from the distorted pressure plate springs actually supplies enough force to reverse the entire disengagement process and raise the clutch pedal off the floor. Thus, pressure you feel about the clutch pedal when you depress it is the springs of the pressure plate fighting back.

Hopefully this article may help in the idea of the entire clutch system and its particular function. This is by no means a summary of all things clutch related as there are entire books published on just the design of the mechanical advantage of the hydraulic system alone; however this needs to be a good basic overview for anyone who desires to learn just what continues when the clutch pedal is pushed.