GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Understanding the Clutch

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See Figure 1

The purpose of the clutch is to smoothly connect and disconnect engine power from the transmission. A car 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 car. 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 weren't, 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 (clutch engagement), 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.



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Fig. Fig. 1: Operation of the diaphragm spring clutch

The clutch assembly consists of the flywheel, clutch disc, clutch pressure plate, throwout bearing and fork, the actuating linkage and the clutch 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 it, it is dished outward toward the transmission. Pushing in the clutch pedal actuates an attached linkage rod. Connected to the other end of this rod is the throwout bearing fork. 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 throwout 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 allows this coil spring to pull the throwout 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 diaphragm 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. Understanding clutch operation can be rather difficult at first; if you're still confused after reading this, consider the following analogy. The action of the diaphragm spring can be compared to that of an oil can bottom. The bottom of an oil can is shaped very much like the clutch diaphragm spring and pushing in on the can bottom and then releasing it produces a similar effect. As mentioned earlier, the clutch pedal return spring permits full release of the pedal and reduces linkage slack due to wear. As the linkage wears, clutch free-pedal travel will increase, while free-travel will decrease as the clutch wears. Free-travel is actually throwout bearing lash.

The diaphragm spring type clutches used are available in two different designs: flat diaphragm springs or bent spring. The bent fingers are bent back to create a centrifugal boost ensuring quick re-engagement at higher engine speeds. This design enables pressure plate load to increase as the clutch disc wears and makes low pedal effort possible even with a heavy-duty clutch. The throwout bearing used with the bent finger design is 1 1 / 4 in. long and is shorter than the bearing used with the flat finger design. These bearings are not interchangeable. If the longer bearing is used with the bent finger clutch, free-pedal travel will not exist. This results in clutch slippage and rapid wear.

The transmission varies the gear ratio between the engine and rear wheels. It can be shifted to change engine speed as driving conditions and loads change. The transmission allows disengaging and reversing power from the engine to the wheels.

CLUTCH CROSS-SHAFT LUBRICATION



Once every 36,000 miles or sooner if necessary, remove the plug, install a fitting and lubricate with water resistant EP (Extreme Pressure) chassis lubricant.

LINKAGE INSPECTION



A clutch may have all the symptoms of going bad when the real trouble lies in the linkage. To avoid the unnecessary replacement of a clutch, make the following linkage checks when a problem is suspected:

  1. Start the engine and depress the clutch pedal until it is about 1 / 2 in. from the floor mat, then move the shift lever between 1st and reverse (1st and 2nd on a 4-speed) several times. If this can be done smoothly without any grinding, the clutch is releasing fully. If the shifting is not smooth, the clutch is not releasing fully and adjustment is necessary.
  2.  
  3. Check the condition of the clutch pedal bushings for signs of sticking or excessive wear.
  4.  
  5. Check the throwout bearing fork for proper installation on the ball stud. The fork could possibly be pulled off the ball if not properly lubricated.
  6.  
  7. Check the cross-shaft levers for distortion or damage.
  8.  
  9. Check the car for loose or damaged motor mounts. Bad motor mounts can cause the engine to shift under acceleration and bind the clutch linkage at the cross-shaft. There must be some clearance between the cross-shaft and the motor mount.
  10.  
  11. Check the throwout bearing clearance between the clutch spring fingers and the front bearing retainer on the transmission. If there is no clearance, the fork may be improperly installed on the ball stud or the clutch disc may be worn out.
  12.  

FREE-PLAY ADJUSTMENT



See Figures 2 and 3

This adjustment must be made under the vehicle on the clutch operating linkage. Free-play is measured at the clutch pedal. For access on most vehicles, the car must be raise and supported safely using jackstands.

1964-67 Vehicles

This adjustment is made from under the car. Free-play is measured at the clutch pedal.

  1. Disconnect the spring between the cross-shaft lever and the clutch fork.
  2.  
  3. Loosen the pushrod locknut approximately three turns.
  4.  
  5. If no free travel is present, adjust the pushrod until it is free of the clutch fork.
  6.  
  7. While holding the clutch fork to the rear, adjust the rod until it just touches the fork seat.
  8.  
  9. Turn the locknut until 3 / 16 in. clearance exists between the nut and the sleeve.
  10.  
  11. Turn the rod until the nut just contacts the sleeve, then hold the rod with a wrench and tighten the nut.
  12.  
  13. Free pedal clearance should be 3 / 4 -1 1 / 8 in. (1964-65 vehicles) or 1-1 1 / 2 in. (1966-67 vehicles).
  14.  



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Fig. Fig. 2: Clutch pedal free-play adjustment-1964-67 vehicles



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Fig. Fig. 3: Clutch pedal free-play adjustment-1968-72 vehicles

1968-70 Vehicles
  1. Disconnect the return spring at the clutch operating fork.
  2.  
  3. Use the linkage to push the clutch pedal up against its rubber bumper stop.
  4.  
  5. Loosen the operating rod locknut and lengthen the adjustment rod until it pushes the fork back enough that the release bearing can just be left to contact the pressure plate fingers.
  6.  
  7. Shorten the rod three turns and tighten the locknut.
  8.  
  9. Replace the spring and check the free-play at the pedal pad. It should be about 1 in. or more.
  10.  

1971 and Later Vehicles

You can use this procedure on any earlier models that have a gauge hole in the clutch pivot shaft arm.

  1. Disconnect the return spring at the clutch operating fork.
  2.  
  3. Use the linkage to push the clutch pedal up against its rubber bumper stop. On the 1973 Chevelle, more clearance can be obtained by loosening the rubber bumper bracket and moving the bracket.
  4.  
  5. Push the end of the clutch operating fork to the rear until the release bearing can just be felt to contact the pressure plate fingers.
  6.  
  7. Detach the front end of the operating rod from the clutch pivot shaft arm and place it in the gauge hole on the arm.
  8.  
  9. Loosen the locknut and lengthen the rod just enough to take all the play out of the linkage. Tighten the locknut.
  10.  
  11. Replace the operating rod in its original location.
  12.  
  13. Replace the return spring and check the free-play at the pedal pad. It should be about 1 in. or more.
  14.  

 
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