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Differential set up, long post Posted by CMyles [Email] ![]() ![]() In Reply to: What is the issue with final drive pre-load, Bob_SAAB_101 ![]() |
Bob,
The spring loaded buttons in the ends of the inner drivers just provide a method by which they may bear against the differential cross pin and be laterally stabilized. Those springs have no effect on backlash or pre-load. The differential is suspended between two opposed, tapered bearings, one right and one left. If shims are moved from one side to the other the bearings shift over and so does the differential. That changes the relationship of ring gear to pinion gear bringing the teeth closer together or farther apart. There is an ideal amount of distance between ring and pinion gear teeth and the free motion is referred to as "Backlash". The main purpose of backlash is to make the gears run quietly, too much or too little produces noise. Generally, the quieter they run the longer they will run. Backlash can be measured with a dial indicator gauge after lifting the car on a rack and removing the differential cover. The "factory determined" ideal backlash for any gear set is marked on the ring gear. A better approach for used gears is to do a transfer pattern test which is where you paint a sticky substance (like white artist's oil color) on the ring gear teeth, roll the assembly and then examine the pattern. You can find a picture of an ideal pattern in many common service manuals.
Proper bearing pre-load is created by starting with the bearings mounted so they are neither loaded nor loose and then removing a small amount of shim so that the bearings are slightly "crushed". If you are confident in the left shim pack but uncertain about the right, just check the backlash and check for looseness in the bearings (that would be left to right freeplay of the differential). You could quantify the bearing pre-load by removing the right inner driver and then replacing it with the spring, button and shims removed, then carefully tighten the six bolts until the bearing slack just disappears. (You'll have to gauge the slack with a small prybar since the spring is still in the left side assembly.) Now measure the gap between the bearing housing flange and the main case and create a shim pack that is .008" thinner (measure the existing shims, they may be correct now). You can do all of this with the car on a lift but it's a job. Good luck
posted by 198.233....
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