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I guess so, but would like to hear other opinions: this car transmission (Fiat 126p) comes with the gearbox and differential in the same unit. When driving it, and more precisely when turning a corner it lost transmission completely, with a horrendous clanking noise in the differential area. With the car jacked and 1st speed engaged, if I turned a wheel I could replicate the clanking nasty noise while the other wheel wouldn't move.

With the unit off the car and disassembled, I found one of the differential bearings attached to the ring gear, which supports the differential unit on it the gearbox body, was virtually disintegrated: all the rollers were piled up in a corner while the cage was just two rings. If I try to "squeeze" both axles attached to the differential unit, I can't replicate the clanking noise, so I think the differential inner parts are ok. At a quick sight, I don't see gear damages.

So my question is: can a virtually nonexistent differential bearing affect the pinion/ring gear coupling SO dramatically, it could make the pinion jump and clank over the ring gear, making the differential "dead"?

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In a word: Absolutely.

The bearing you are pointing out is the carrier bearing for the differential, which keeps lateral movement (forward/aft) of the ring gear to a minimum (if not non-existent). If the carrier bearing is destroyed, chances are your ring/pinion gears are both destroyed as well.

(NOTE: The above is changed after a clarifying comment from @blacksmith37 - The below is my original answer when I understood the bearing which was disintegrated to be the pinion bearing, not the carrier bearing.)

The pinion bearings places the pinion so movement of it is limited to rotation. It keeps it stable in the three directions: x (left/right), y (forward/aft), and z (up/down). Without the bearing, the pinion is able to move in all three of these directions. I'd suggest if your pinion bearing is gone, the ring and pinion both are now toast.

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  • Bearings in diffs (and gearboxes) are often pre-loaded so that the movement is limited to a few thou, if you think of the power of the engine being transmitted through one or two pairs of teeth then it becomes obvious as to why...
    – Solar Mike
    Commented Nov 12, 2018 at 6:15
  • Not to be redundant but, a small axial movement by the pinion can cause improper engagement with the ring and "skipping" over gear teeth. That would be caused by failure of the thrust bearing ( primarily) in the pinion.
    – blacksmith37
    Commented Nov 12, 2018 at 16:34
  • @blacksmith37 - Actually, you make sense and I misunderstood the question a bit ... I'll adjust my answer a bit.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Nov 12, 2018 at 16:39
  • Let's call it good luck: I disassembled the whole unit, no further damage, just the bearing got disintegrated, and some rubbing inside the body at that side. Seems that bearing was dead long time ago, its track looks partially blurred, no "mirror finish".. Perhaps because this car has a little 2 pistons, 650cc engine not powerful enough to break things in there. The bearing failure reason: bad installation, it was about 4mm too deep into the bearing housing, i.e., 4mm too far from the differential unit, and the adjuster nut was not locked.
    – Aram Alvarez
    Commented Nov 13, 2018 at 3:34

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