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The pens point out the differing bearing-seat diameters needed to match the varied inner-pinion bearing sizes. The seat pressed into the 741 case (left) is machined to accept its 3.050 bearing, while the 489 case’s seat (right) is sized to take a 3.460 bearing. The 742 case (not shown) uses yet another bearing size, which is why pinion gears for 741, 742, and 489 Chrysler diffs do not interchange. Forget about trying to machine the smaller cases to accept larger bearings; it’s unexplored territory, and these things are far too common to warrant the hassle.