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.Release Distance Required?
Our research shows that a typical GM clutch pressure plate, whether three-finger style or diaphram, 10-1/2" or 11" generally requires 0.550" of travel at the fingers to release the clutch disc.
Some common Jeep pressure plates require about 0 390" of travel to release.
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).