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Discussion Starter #1 (Edited)
To the C3 Corvette Readers:

After realizing that there is a deficiency in the specification for the C3 differential carrier bearing preload (CBP), I am offering the following as a first step in deriving a numerical specification for the CBP. The currently accepted way of achieving CBP is to add a 0.004" shim to each side of the carrier after shimming to be snug. The 1978 C3 factory unit repair manual does not describe the use of a differential case stretch tool, but it does state to add 0.008" shim thickness (I added 0.004" per side) to produce the correct amount of CBP. That method of shimming to be snug is very subjective and results in a wide variation of CBP results. One person's snug could be another's tight or loose. We ought to be able to do better than that. The first step is for us to understand and accept the basic physics and mathematics of determining CBP by using an inch pound (in-lb) torque measurement gage. The procedure that could be used would be to measure it directly, which is neither simple nor practical to do because of the lack of fixturing, tooling or gaging to rotate the mounted carrier alone. There is, however, an easy way to determine the CBP by measuring the pinion bearing preload (PBP) and the total preload comprised of the PBP combined with the CBP as long as the ring and pinion ratio is known. The explanation is as follows: The ring and pinion ratio has nothing to do with measuring pinion bearing preload, but it has everything to do with measuring and calculating the carrier bearing preload. The reason why is that the ratio is the only factor that can compensate for the torque multiplication of the ring and pinion gears. In my 3.73 ratio case, when the pinion gear is rotated 3.73 times, the ring gear rotates 1.00 time. That fact represents the torque multiplication for that particular gear set. Knowledge of that ratio allows the calculation of carrier bearing preload after measuring pinion bearing preload and the combined pinion and carrier bearing preload. The correct formula to calculate only the carrier bearing preload is:

Carrier Bearing Preload = (total preload - pinion bearing preload) x the differential ratio. For example (24 in-lbs - 21 in-lbs) x 3.73 = 11.2 inch-pounds.

I am waiting for a response from the GM Heritage Center for their opinion of using the measured CBP instead of the judgmental snug procedure. I also have fabricated a differential case stretching tool that will be used to determine the CBP as a function of case stretch, and also to determine the CBP when using the currently used method of hammering the shim packs in. I will publish the results when available.

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