GM Corvette 1963-1982 Repair Guide

Understanding the Clutch

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See Figures 1 and 2

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Fig. Fig. 1: Exploded view of a common clutch assembly



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Fig. Fig. 2: Cross-sectional view of a dual disc clutch

The purpose of the clutch is to disconnect and connect engine power at the transmission. A vehicle at rest requires a lot of engine torque to get all that weight moving. An internal combustion engine does not develop a high starting torque (unlike steam engines) so it must be allowed to operate without any load until it builds up enough torque to move the vehicle. To a point, torque increases with engine rpm. The clutch allows the engine to build up torque by physically disconnecting the engine from the transmission, relieving the engine of any load or resistance.

The transfer of engine power to the transmission (the load) must be smooth and gradual; if it werent, drive line components would wear out or break quickly. This gradual power transfer is made possible by gradually releasing the clutch pedal. The clutch disc and pressure plate are the connecting link between the engine and transmission. When the clutch pedal is released, the disc and plate contact each other (the clutch is engaged) physically joining the engine and transmission. When the pedal is pushed in, the disc and plate separate (the clutch is disengaged) disconnecting the engine from the transmission.

Most clutch assemblies consists of the flywheel, the clutch disc, the clutch pressure plate, the throw out bearing and fork, the actuating linkage and the pedal. The flywheel and clutch pressure plate (driving members) are connected to the engine crankshaft and rotate with it. The clutch disc is located between the flywheel and pressure plate, and is splined to the transmission shaft. A driving member is one that is attached to the engine and transfers engine power to a driven member (clutch disc) on the transmission shaft. A driving member (pressure plate) rotates (drives) a driven member (clutch disc) on contact and, in so doing, turns the transmission shaft.

There is a circular diaphragm spring within the pressure plate cover (transmission side). In a relaxed state (when the clutch pedal is fully released) this spring is convex; that is, it is dished outward toward the transmission. Pushing in the clutch pedal actuates the attached linkage. Connected to the other end of this is the throw out fork, which hold the throw out bearing. When the clutch pedal is depressed, the clutch linkage pushes the fork and bearing forward to contact the diaphragm spring of the pressure plate. The outer edges of the spring are secured to the pressure plate and are pivoted on rings so that when the center of the spring is compressed by the throw out bearing, the outer edges bow outward and, by so doing, pull the pressure plate in the same direction away from the clutch disc. This action separates the disc from the plate, disengaging the clutch and allowing the transmission to be shifted into another gear. A coil type clutch return spring attached to the clutch pedal arm permits full release of the pedal. Releasing the pedal pulls the throw out bearing away from the diaphragm spring resulting in a reversal of spring position. As bearing pressure is gradually released from the spring center, the outer edges of the spring bow outward, pushing the pressure plate into closer contact with the clutch disc. As the disc and plate move closer together, friction between the two increases and slippage is reduced until, when full spring pressure is applied (by fully releasing the pedal) the speed of the disc and plate are the same. This stops all slipping, creating a direct connection between the plate and disc which results in the transfer of power from the engine to the transmission. The clutch disc is now rotating with the pressure plate at engine speed and, because it is splined to the transmission shaft, the shaft now turns at the same engine speed.

The clutch is operating properly if:

  1. It will stall the engine when released with the vehicle held stationary.
  2.  
  3. The shift lever can be moved freely between 1st and reverse gears when the vehicle is stationary and the clutch disengaged.
  4.  

Two types of mechanical clutch assemblies have been available on Stingray. The standard clutch is a bent-finger, centrifugal diaphragm unit first introduced in 1963. This design permits heavy plate loads, yet allows low pedal effort without resorting to center booster springs. The second type of clutch is a dual-plate, bent-finger diaphragm model that was available in 1969 and 1970. It was optional on the 427/454 high performance engines and requires a 14 in. diameter ring gear. The throwout bearing used with 1963 and 1981 models is 1 7 / 32 " long; the bearing used with 1964-80 models is 1 1 / 4 " long. DO NOT replace either of these bearings with the 1 7 / 8 " unit which is popular in other Chevrolet models.

Before attempting to repair the clutch, transmission or related linkages for any reason other than an obvious failure, the problem and probable cause should be identified. A large percentage of clutch and manual transmission problems are manifested by shifting difficulties such as high shift effort, gear clash and grinding or transmission blockout. When any of these problems occur, a careful analysis of these difficulties should be accomplished and the basic checks and adjustments performed before removing the clutch or transmission for repairs. Run the engine at a normal idle with transmission in neutral and clutch engaged. Disengage the clutch, wait about 10 seconds and shift the transmission to reverse. No grinding noise should be heard. A grinding noise indicates incorrect clutch travel, lost motion, clutch misalignment, or internal problems such as failed dampers, facings, cushion springs, diaphragm spring fingers, pressure plate drive straps, pivot rings, etc.


CAUTION
When servicing clutch parts, do not create dust by grinding or sanding clutch disc or by cleaning parts with a dry brush or with compressed air. (A water dampened cloth-NOT SOAKED-should be used) The clutch disc contains asbestos fibers which can become airborne if dust is created during servicing. Breathing dust containing asbestos fibers may cause serious bodily harm.

REMOVAL & INSTALLATION



See Figures 3 through 7

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Fig. Fig. 3: Location of the clutch release bearing lubrication points



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Fig. Fig. 4: Be sure that the flywheel surface is clean, before installing the clutch



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Fig. Fig. 5: Install a clutch alignment arbor, to align the clutch assembly during installation



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Fig. Fig. 6: Clutch plate installed with the arbor in place



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Fig. Fig. 7: Pressure plate-to-flywheel bolt holes should align

  1. Remove the transmission from the car as outlined in this section.
  2.  
  3. Disconnect the clutch-fork pushrod and spring.
  4.  
  5. Remove the bell housing.
  6.  
  7. Install a clutch pilot tool (wooden versions available at most automotive parts stores) to hold the clutch plate during removal.
  8.  
  9. The flywheel and clutch cover are marked with x for correct assembly, if these are not visible, scribe new marks.
  10.  
  11. Gradually loosen the clutch-to-flywheel bolts one turn at a time until all spring pressure is released.
  12.  
  13. Remove the bolts and remove the clutch assembly.
  14.  

To install:
  1. Crank the engine over by hand until the x-mark on the flywheel is on the bottom.
  2.  
  3. Position the clutch disc and pressure plate in the same relative location as removed and support with the clutch pilot tool.
  4.  

On single-disc models, the clutch disc is installed with the damper springs and slinger toward the transmission. On dual disc models, the discs are installed with the springs away from the flywheel, as shown.

  1. Rotate the clutch assembly until the x-marks on the flywheel and clutch assembly align. Align the cover bolt holes with those in the flywheel.
  2.  
  3. Install bolts in every hole and tighten down evenly and gradually. Install the remaining bolts.
  4.  
  5. Remove the clutch pilot tool.
  6.  
  7. Lubricate the ball socket on the clutch fork and reinstall on the ball stud.
  8.  
  9. Pack the recess on the inside of the throwout bearing collar and the throwout groove with graphite grease.
  10.  
  11. Install the bell housing.
  12.  
  13. Install the throwout bearing on the fork. Lubricate the bearing groove.
  14.  
  15. Install the transmission as previously outlined.
  16.  
  17. Connect the fork pushrod and spring.
  18.  
  19. Adjust the free pedal play and check the clutch release position. Lubricate all linkage pivot points.
  20.  

The clutch pilot bearing is an oil impregnated type bearing pressed into the crankshaft. This bearing requires attention when the clutch is removed from the vehicle, at which time it should be cleaned and inspected for excessive wear or damage and should be replaced if necessary.

PEDAL REPOSITIONING



1 / 2 in. The reduced travel will make the release faster and permit faster gear shifting.

See Figure 8

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Fig. Fig. 8: View of the clutch linkage

Earlier Corvette clutch linkage includes a provision for a second clutch setting which reduces total pedal travel approximately 1

  1. Remove the clutch return spring at the cross-shaft and the pedal pushrod at the pedal.
  2.  
  3. Loosen the pedal bracket lower bolt, remove the upper bolt, and rotate the bracket so that it will align with the extra upper bolt hole. Install the upper bolt.
  4.  
  5. Disconnect the pedal pushrod at the cross-shaft and turn it 1 / 2 turn. Reconnect the pushrod at the cross-shaft.
  6.  
  7. Tighten the bracket bolts and connect the pedal pushrod.
  8.  

CLUTCH ADJUSTMENT



1963-74 Vehicles

  1. Disconnect the spring between the clutch pushrod and cross shaft lever.
  2.  
  3. While holding the clutch pedal against the stop, loosen the two locknuts enough to allow the adjusting rod to move against the clutch fork until the throwout bearing lightly touches the pressure plate springs.
  4.  
  5. Turn the upper nut against the swivel and then back it off 4 1 / 2 turns. Tighten the bottom locknut to lock the swivel against the top nut.
  6.  
  7. Reinstall the return spring. Pedal free travel, the distance the pedal can be moved before the throwout bearing contacts the pressure plate spring, should be:

    1963-64 Corvette- 3 / 4 - 1 in.
     
    1965-71 Corvette-1 1 / 4 - 2 in.
     
    1969-70 Corvette with HD clutch-2 - 2 1 / 2 in.
     
    1972 Corvette-1 1 / 4 - 1 3 / 4 in.
     
    1973 Corvette-1 1 / 4 - 1 1 / 2 in.
     
    1974 Corvette-1 - 1 1 / 2 in.
     

  8.  

1975-81 Vehicles

No manual transmission was available in 1982.

  1. Disconnect the return spring between the floor and the cross shaft.
  2.  
  3. Push the clutch lever and shaft assembly until the clutch pedal is tightly against the rubber stop under the dash.
  4.  
  5. Loosen the two locknuts on the shaft.
  6.  
  7. Push the shaft until the throwout bearing just touches the pressure plate spring.
  8.  
  9. Tighten the top locknut toward the swivel until the distance between it and the swivel is 0.4 in.
  10.  
  11. Tighten the bottom locknut against the swivel.
  12.  
  13. Check pedal free travel. It should be 1 - 1 1 / 2 in.
  14.  

 
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