The 8 1/4 thread

Not sure if this has been mentioned yet in this thread, but the way I understand it, the difference in carriers has to do with the thickness and/or location of the carrier mounting flange where the ring gear bolts up.

If you start with a 4.10 gear and work backwards, to 3.91, 3.55, etc., you will notice the pinion gear diameter growing to accommodate the increasing number of teeth, while the ring gear thickness decreases.

The reason a 4.10 gear has such a thick ring gear is so it can reach the small diameter pinion gear and mesh properly. Have a look at 2.2 and 2.45 ring gears and you will notice there is not a ton of material other than the gear teeth themselves.

You cannot use numerically higher than 2.45 gears in a 2.45 carrier, because the mounting surface of that carrier is moved too far away from the pinion centerline for the gears to mesh properly.

Conversely, you cannot use 2.45 or 2.2 gears with the same carrier your 2.76s and up came on, because the mesh will either be too tight or downright impossible to mesh at all. In this case, the carrier mounting flange is moved closer to the pinion centerline.

I hope I didn't muddle anything up. As I said, that is just the way I understand it. I welcome any corrections. Maybe this reply would do better with pictures.