Clutch University - Chapter 5

RELEASE MECHANISMS

All clutch systems require a release mechanism to operate the clutch.  There are several types of systems that have been used over the years by the various manufacturers.

Mechanical Linkage

Mechanical linkage is most prevalent in vehicles produced pre-1980 and in race vehicles.  It usually consists of a bellcrank that pivots between the frame and the engine block.  One side of the bell crank attaches to the clutch pedal, and the opposite side attaches to the adjustment rod for clutch release.  The adjustment rod is inserted into the clutch fork and lengthened or shortened to achieve the proper release and gap between the clutch fingers and the bearing.  Typically the ratio between the release mechanism and the pedal effort is between three and four to one.  Mechanical linkages provide the most flexibility in clutch adjustment.

Hydraulic Master/Slave Cylinder Release System


Master cylinder assembly


Slave cylinder assembly

Hydraulic master/slave release systems are common in vehicles produced after 1980.  This system uses a master cylinder mounted on the firewall and a slave cylinder, usually mounted on the bellhousing.  A hydraulic line connects the two. The slave cylinder will usually have a rod that fits into the clutch fork and either pulls or pushes the fork when the pedal is depressed.  Some hydraulic systems incorporate an adjustable slave cylinder, where you can manually set the clutch release.  Some later GM systems use non-adjustable slave cylinders and are more difficult to deal with when installing non-stock clutches.

Often when changing clutches in vehicles with non-adjustable hydraulics, if the flywheel is resurfaced more than .020 inch, a flywheel shim must be used between the crank flange and the flywheel to shim the flywheel to its original position.  Since the slave has no adjustment, the only way to make these systems function properly is to have the pressure plate mount so that the fingers are in the exact same position as stock in order for the clutch to release properly.

Cable Release System

Cable release systems use a cable connected to the clutch pedal and the clutch fork to actuate the release bearing.  When the clutch pedal is depressed, the cable is pulled and in turn pulls the clutch fork to disengage the clutch.  Most cable systems are self adjusting, using a ratchet mechanism mounted under the dashboard to keep the release bearing in position for the clutch to release.

Aftermarket adjustable cables are available for many aftermarket clutch systems, the most popular being the late model Ford Mustang.

Internal Hydraulic Release Bearings


Typical internal hydraulic bearing


RAM hydraulic bearing for oval track

Internal hydraulic release bearings are the newest release systems used in late model vehicles.  Typically, a slave cylinder that houses the release bearing mounts to the rear of the bellhousing or on the front of the transmission.  A hydraulic line connects the slave to the master cylinder mounted on the firewall.  When the clutch pedal is depressed, the slave cylinder pushes the release bearing out and disengages the clutch.  Most of these systems are non adjustable and require the clutch to install so that the fingers are in the stock location, or the clutch may not release.  You may also hear these bearings referred to as concentric release bearings.

Internal bearings are not new to the aftermarket.  Several manufacturers including RAM have produced these units for use in oval track racing, where space limitations preclude using a fork release system.  They are also very popular for kit car installations.

Types of Release Bearings

Two types of bearings are used for these release mechanisms.  Standard release bearings have the bearing and race pressed on the bearing collar. The bearing is static on a standard bearing.  SELF ALIGNING release bearings are assembled in the same manner, but have floating design that allows the bearing to move about on the collar so it can find its exact center once it comes in contact with the pressure plate fingers.  Self aligning bearings are used in most all late model applications. 

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