GM Firebird 1967-1981 Repair Guide

Driveshaft and U-Joints

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GENERAL INFORMATION



See Figure 1

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Fig. Fig. 1: Simplified example of a universal joint (U-joint)

The driveshaft (propeller shaft) is a long steel tube that transmits engine power from the transmission to the rear axle assembly. It is connected to, and revolves with, the transmission output shaft (remember, the transmission shaft is connected to and revolves with the engine crankshaft) whenever the transmission is put into gear. With the transmission in neutral, the driveshaft does not move. Located at each end of the driveshaft is a flexible joint that rotates with the shaft. These flexible joints, known as U-joints (universal joints) perform an important function. The rear axle assembly moves with the car. It moves up and down with every bump or dip in the road. The driveshaft by itself is a rigid tube incapable of bending. When combined with the flexing capabilities of the U-joints, however, it can do so. A slip joint is coupled to the front of the driveshaft by a universal joint. This U-joint allows the yoke (slip joint) to move up or down with the car. The yoke is a cylinder containing splines that slides over and meshes with the splines on the transmission output shaft. When the rear axle moves up and down, the yoke slides back and forth a small amount on the transmission shaft. Therefore, it combines with the U-joints in allowing the driveshaft to move with the movements of the car. The rear universal joint is secured to a companion flange which is attached to, and revolves with, the rear axle drive pinion.

A U-joint consists of a cross piece (trunnion) and, on each of the four ends, a dust seal and a series of needle bearings that fit into a bearing cup. Each U-joint connects one yoke with another and the bearings allow the joints to revolve within each yoke.

1967-68 Firebird driveshafts use a conventional snapring to hold each bearing cup in the yoke. The snapring fits into a groove located in each yoke end just on top of each bearing cup. The 1969-81 driveshaft has its U-joints attached differently. Nylon material is injected through a small hole in the yoke and flows along a circular groove between the U-joint and the yoke, creating a synthetic snapring. Disassembly of the later model U-joint requires the joint to be pressed from the yoke. This results in damage to the bearing cups and destruction of the nylon rings. Replacement kits include new bearing cups and snaprings to replace the original nylon rings. These replacement rings must go inboard of the yoke in contrast to outboard mounting of the early models. Previous service to the later U-joints can be recognized by the presence of snaprings inboard of the yoke.

Bad U-joints, requiring replacement, will produce a clunking sound when the car is put into gear. This is due to worn needle bearings or a scored trunnion end possibly caused by improper lubrication during assembly. Firebird U-joints require no periodic maintenance and therefore have no lubrication fittings. A clunking sound can also be produced by two other components. The Firebird three-speed automatic transmission has a rear seal that prevents the transmission from lubricating the front slip joint. All other transmissions allow a slight lubrication of this slip joint because they contain a different type of rear seal. If a driveline clunk should develop in a Firebird with a three-speed automatic, clean the slip joint and pack it with one tablespoon of chassis lube. A similar clunk can be the result of improper rear end gear lash but, due to its complexity, should be checked only after checking the more probable causes.

Some driveshafts (generally heavy-duty applications) use a damper as part of the slip joint. This vibration damper cannot be serviced separately from the slip joint. If either component goes bad, the two must be replaced as a unit.

REMOVAL & INSTALLATION



See Figures 2, 3 and 4

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Fig. Fig. 2: Exploded view of the strap-type driveshaft retainer used to secure the driveshaft to the differential flange



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Fig. Fig. 3: Exploded view of the two types of driveshaft-to-differential flange mounting found in Firebird vehicles



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Fig. Fig. 4: Remove the clamps and mounting bolts to separate the driveshaft from the differential flange

  1. Raise the vehicle and safely support it on jackstands. Paint a reference line from the rear end of the driveshaft to the companion flange so that they can be reassembled in the same position.
  2.  
  3. Disconnect the rear universal joint by removing the U-bolts or retaining straps.
  4.  
  5. To prevent loss of the needle bearings, tape the bearing caps to the trunnion.
  6.  
  7. Remove the driveshaft from the transmission by sliding it rearward.
  8.  

Do not be alarmed by oil leakage at the transmission output shaft. This oil is there to lubricate the splines of the front yoke.

  1. Check the yoke seal in the transmission case extension and replace it if necessary.
  2.  
  3. Position the driveshaft and insert the front yoke into the transmission so that the splines mesh with those of the transmission shaft.
  4.  
  5. Using reference marks made during removal, align the driveshaft with the companion flange and secure it with U-bolts or retaining straps.
  6.  

U-JOINT OVERHAUL



See Figures 5, 6 and 7

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Fig. Fig. 5: Exploded view of a typical internal snapring Universal Joint (U-joint) assembly



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Fig. Fig. 6: Once the new U-joint is positioned in the yoke, install the replacement snaprings



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Fig. Fig. 7: If the retaining clips will not seat correctly, tap the yoke ear as shown until they do

  1. Remove the driveshaft as explained above and remove the snaprings from the ends of the bearing cup.
  2.  
  3. After removing the snaprings, place the driveshaft on the floor and place a large diameter socket under one of the bearing cups. Tap on the berring opposite this one with a hammer and a drift. This will push the trunnion through the yoke enough to force the bearing cup out of the yoke and into the socket. Repeat this procedure for the other bearing cups. If a hammer fails to loosen the cups, a press may be necessary.
  4.  

A 1969-81 driveshaft secures its U-joints in a different manner than the conventional snaprings of the 1967-68 designs.

On the later design, nylon is injected through a small hole in the yoke and flows along a circular groove between the U-joint and the yoke, thus creating a synthetic snapring. Disassembly requires the joint to be pressed from the yoke. If a press is not available, it may be carefully hammered out using the same procedure (Step 2) as the early design, although it may require more force to break the nylon ring. Either method, press or hammer, will damage the bearing cups, and requires metal snaprings to replace the original nylon rings.

  1. Thoroughly clean the entire U-joint assembly with solvent. Inspect for excessive wear in the yoke bores and on the four ends of the trunnion. The needle bearings should not be scored, broken, or loose in their cups. Bearing cups may suffer slight distortion during removal and should be replaced.
  2.  
  3. Pack the bearings with chassis lube (lithium base) and completely fill each trunnion end with the same lubricant.
  4.  
  5. Place new dust seals on the trunnions with the cavity of the seal toward the end of the trunnion. Care must be taken to avoid distortion of the seal. A suitable size socket and a vise can be used to press on the seal.
  6.  
  7. Insert one bearing cup about a quarter of the way into the yoke and place the trunnion into the yoke and bearing cup. Install another bearing cup, press in both cups, and install the snaprings. Snaprings on the 1967-68 shafts must go on the outside of the yoke while the 1969-81 shaft requires that the rings go on the inside of the yoke. The gap in the later model ring must face toward the yoke. Once installed, the trunnion must move freely in the yoke.
  8.  

The 1969-81 shaft uses two different sizes of bearing cup at the differential end. The larger cups (the ones with the groove) fit into the driveshaft yoke.

 
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