The flywheel provides a friction surface for the clutch disc, a torque buffering mass, a mounting surface for the pressure plate, a mounting for the starter driven gear, and on some engines the flywheel is a factor in engine balance.
The condition of the friction surface of the flywheel is important for proper clutch function. The surface should be smooth and free of burned spots and surface cracks. Used flywheels can be resurfaced. This should be done by grinding rather than lathe turning as less material is removed. The amount of material removed from the face can affect which clutch release bearing should be used. A flywheel should always be checked for runout on the engine it will
This is the other half of the driving friction surface. It mounts on the flywheel. It consists of four main parts and is more correctly called a clutch cover assembly. These parts are the pressure plate itself, the springs (or spring, if a diaphragm type), the clutch cover, and the release arms. There are two basic designs of clutches usually referred to by the spring type.
These are the Rockford™ (diaphragm spring type) and the Borg and Beck™ (coil spring type). The coil spring type is also called a three-finger type, referring to the three release arms this style requires to compress the coil springs.
The "softest" clutch is the diaphragm type. It also requires the least amount of travel to release. The diaphragm type clutch works good in lightweight, low geared vehicles. It is not the best clutch for high RPM use as the diaphragm spring will stay "flat" or released from the centrifugal force generated by the RPM. A variation of the diaphragm type was used for a while by GM, that to some extent helped this problem. This was called the Hi-Cone diaphragm type and was designed so the spring - instead of being flat when released - still had a slight bevel. These Hi-Cone units were not bad but still won't hold like the Borg and Beck coil spring type. Aftermarket units like the Centerforce®, use centrifugal weights to counteract this high-rpm flattening and subsequent loosening. It should be noted that this is not typically a concern of the Jeep enthusiast as high RPM horsepower is not as much an interest as low-RPM torque. It should be pointed out that the spring itself is the "release arms" of a diaphragm type clutch. Note that when interchanging from one type to the other, you will require a different throwout bearing. The three-finger style requires a longer throwout vs. the diaphram type, which uses a shorter throwout bearing. More on this later...
The fourth part of the pressure plate assembly is the cover. The pressure plate, spring (or springs) and release arms are attached to the cover in such a manner that, when the release bearing pushes on the three arms or the diaphragm spring, it causes a leveraged action to take place. This counteracts the spring pressure and lifts the pressure plate off the clutch disc, releasing the clutch.
As stated above, the diaphragm type clutch takes slightly less travel to release and requires about .030 total air gap when released. The coil spring type requires about .040 to .050 total air gap when released. Air gap is the clearance between the clutch disc, flywheel, and pressure plate with the clutch released. A total air gap of .050 will measure .025 between each side of the disc.
This is the "driven" part of the clutch. It has a friction material riveted to each side of a wavy spring (called a marcel). This is attached to a splined hub that the transmission input gear protrudes into.
There are basically two common types of friction material used for clutch lining. These are organic and metallic. The organic is best for all around use. The metallic is preferred by some for severe duty applications but requires high spring pressures and is hard on the flywheel and pressure plate friction surfaces. Avoid solid hub clutches and clutches without marcel as they will always chatter when used in vehicles with a rear differential mounted on springs (as opposed to a transaxle design).Pilot Bushing
In most cases, this is a porous bronze, pre-lubed bushing rather than an actual bearing, as it is often called. A few applications still use an actual bearing and others use a needle roller type bearing, but by far, the most common type is bronze. You cannot use a roller bearing on a transmission shaft originally designed for a bronze bushing due to different type of heat treatment on the shafts
The pilot bushing is seldom thought of as a part of the clutch system but it is one of the most vital parts of the system. It pilots the end of the transmission input gear in the crankshaft. If it is worn or not running "true", it can cause serious clutch problems or transmission failure. Pilot bushing bore runout should always be checked with a dial indicator and should be within .002 total. The bronze bushing type should be a press fit in the crankshaft bore. It must be installed carefully. It should have between .002 and .003 clearance on the transmission shaft when installed. The pilot bushing is only functional when the clutch is disengaged but it is a factor in input gear alignment at ALL times.
Most people have no idea what an important part the pilot bushing plays in the life of the transmission and clutch. The job of the pilot bushing is to support the end of the transmission input (main drive) gear in the crankshaft and it only acts as a bushing when the clutch is depressed. This pilot bushing should be a light drive fit into the crank bore. Care should be taken when installing any pilot bushing as they are soft and easily damaged by crude installation techniques. A damaged pilot bushing can bind on the input gear giving symptoms of clutch drag. Transmission damage and early failure can be caused by a pilot bushing or crankshaft bore that "runs out" in relation to the transmission locating bore in the bellhousing. It is advisable to check the bore of the crank with a dial indicator before installing the pilot bushing (see below). If the bore runs out more than .003 total, the crank should be set up in a lathe and the bore trued up OR a special pilot bushing should be made that runs out the same amount as the crank bore. The run out in the bore of a pilot bushing is put 180 degrees off from the crank bore run out and the pilot bushing installed. If properly done, this can put the bore of the pilot bushing well within the .003 required. We have used this method to save engine disassembly many times. A disadvantage of this method shows up at pilot bushing replacement time as a special pilot bushing will have to be reproduced.
It is always a good idea to use an input gear (of the proper diameter) or clutch aligning tool when installing the clutch on any engine. With the clutch disc aligned on the pilot bushing it becomes a simple matter when installing the transmission to engage the splines and bolt up the transmission . If this simple procedure is not done, the transmission shaft won't line up and the temptation will be great to "pull it up with the bolts" which damages the front transmission bearing, pilot bushing, and more than likely will break an ear off the transmission or adapter. The transmission should slip in freely to mate up with the face of the bellhousing.
Clutch Release Bearing
As its name implies, this is the bearing that releases the clutch. It is often referred to as a "throw-out" bearing. They come on a number of different style carriers. The carriers, in some cases, vary considerably with the particular engine. In the GM line, for example, the bearings are all the same but there are several different carriers that vary about 1/2" between the shortest and longest. Which to use usually depends on the style of pressure plate being used, but substituting one length for another can often be used to the installer's great advantage. AMC, Ford & Mopar and others are far less generous with the variety of lengths available. This will be covered in more detail later in this article.
Because the release bearing only works when the clutch is being released it usually lasts quite a long time. However, improper linkage adjustment or riding the clutch with your foot when driving can wear the bearing prematurely. Normally there should be a minimum of 1/16" clearance between the face of the bearing and the three release fingers or diaphragm spring of the pressure plate when the clutch is engaged. This fact is important and will be discussed further when we get to the part about setting up the clutch linkage
Clutch Release Fork
This is the arm or lever that the linkage operates that moves the release bearing. There are several different styles of release arm. The most common in automotive use is the fork type that pivots on a rocker. This type requires a rearward force to move the release bearing forward. Note now that the following is key to your understanding of the clutch system: The ratio of the arm is the difference in length between the pivot point and the release bearing centerline divided by the length from the pivot point to where the linkage attaches. The ratio of the fork is important and will be used in the linkage setup section later in this article.
GM, Ford, and AMC all use a pivot type release arm as their most common type. Some late GM, Pinto, Jeep and a few others use a non-rocker arm. This style pivots on the passenger side of center and is direct acting. That is, it takes a forward movement of the linkage to move the release bearing forward. This is not as suitable as the rocker system as it usually complicates the linkage requirements.
Regarding GM clutch forks, there are two basic types of manufacture used for the pivot type forks. These are stamped steel and forged steel. The stamped steel type uses a flat steel retainer spring that is riveted to the fork. These forks must be used with mushroom-head type pivots. The forged steel forks must use the ball-head type pivot. (This is different than the ball-on-pedestal AMC type.) These forged forks are retained on the pivot by a spring-wire retainer that fits in a groove machined in the ball pocket in the fork.
Release Arm Pivot
As its name implies, this is the support that the release arm pivots on. There are basically two types. One pivots on a ball-ended stud that screws into the bellhousing. The other type is an actual bearing ball that sits in a pedestal type socket that is part of the bellhousing. GM, Ford, and early AMC use the screw-in type. Late AMC favors the ball type.
There is an adjustable length pivot (shown) with an adjustment range of 1-3/8 to 1-1/2 inches available for GM engines that can sometimes be used to compensate for variations in flywheel, clutch disc, and release bearing thickness. More about this in the troubleshooting section.
Both ball and mushroom-head GM pivots are available in 1-3/8 and 1-1/2" length (overall length when not [this is important] installed in the bellhousing). It is very important to use the correct style of pivot in relation to the type of arm being used.
Transmission Front Bearing Retainer
This great device has three critical functions. This first is as its name implies. The second is to provide a register on which the bellhousing must center. This is feature is sometimes overlooked with expensive consequences. Thirdly, its tubular snout is the surface on which the throwout bearing rides on its way in to depress the springs of the pressure plate. Conversions often require special and modified retainers to acheive compatibility.
This provides a mounting place for the transmission, as well as a means of aligning the transmission to the engine. In some applications it also has a structural mounting function.
The alignment function is extremely important. Unfortunately, this is the most often overlooked and least understood part about the bellhousing.
Most people who have worked on these parts realize there are aligning pins in the engine block that register with holes in the bellhousing. What they do not realize is, there can be a variation in the location of these holes and this variation can affect clutch and transmission life. How to check bellhousing alignment will be covered in its own section further on in this article.
This consists of everything between your foot and the clutch release arm. The linkags is the method of transferring the force of your left foot into the bellhousing and pressure plate release. The linkage can be mechanical, cable type or hydraulic. Note here that problems tend to show up because there are usually several choices of release arms and bearings for any particular family of engines. Choosing the wrong parts can get the linkage out of relationship and cause problems that can only be solved by removing the parts and starting over with other parts. The linkage cannot be made to compensate incorrect choice of release bearing or fork.
Cable Style Linkage
Cable linkages may seem appealing because it is easy to understand and simple to hook up. However, once past this, the installer may discover that it has high friction, stretches, sticks, rusts, freezes, frays and breaks. A cable type clutch should probably be the last choice of the three types of linkages.
Cable linkages work fine in smaller applications such as motorbikes and light cars, but they have an unsustainable record in light and heavy truck applications.
Some CJ & Commando Jeeps from 1972-1974 used a cable release, with subpar results as evidenced by the duration of their implementation.
Mechanical Style Linkage
Next is the mechanical linkage which is, with a few exceptions, the type
found on the majority of Jeeps® built prior to 1987.
There are several basic styles of Jeep mechanical linkage but all are used in nearly their original configuration when doing an engine conversion. They usually consists of a pushrod at the pedal, a bellcrank and an additional pushrod actuating the fork. Earlier systems use pullrods, bellcranks and cables in lieu of pushrods, effectively reversing the way the systems works.
The mechanical linkage is largely a successful method of clutch release. One drawback obvious to many off-roaders is the tendency of some of these to bind during frame and powertrain flex and differentiation.
Hydraulic Style Linkage
Hydraulic clutch linkage systems have moved into dominance in the past two decades, and generally with good reason.
The most common rendition of this linkage consists of the pedal pushrod against a master piston / cylinder, a high-pressure tube or line and a slave piston / cylinder whose pushrod pushes the clutch release arm.
A less common style of hydraulic release is the internal hydraulic release bearing. This design combines the piston and bearing into one unit, eliminating the pivot, fork (or release arm) and separate throwout bearing.