742 to 489 and 741

[Don Dulmage <big-d@xxxxxxxxxxxx>]

All have the same size shaft at the spline. The 742 is 1 3/4 at the rear of the pinion by the gear end and the 489 is 1 7/8 the X or 741 is 1 3/8. HOWEVER!!!! (big word aint it. ) they are all the same size where the spline attaches (nut or threaded end of the pinion) and since no piece is stronger than its weakest part the idea that  a 741 or 742 is too weak for serious street use does not wash. The bigger bearing inside increases load carrying abilty by the gear  but in practice the smaller bearing , the front pinion bearing is the one that will fail if you do have any trouble. and it is realitively similar in all three diffs. 
Limited slips (cone and clutch can often be swapped without trouble. Sometimes the carrier bearings are different but with the matching races will fit nicely. I have done this many many times over the years. Axle adjustment or shimming can be tricky sometimes however a green bearing kit makes this detail moot. 
Don
PS there are two splines often seen in all diffs , at least all 742 and 489s) fine spline and course splines. Often the front seal is the same number as well BTW regardless of the case number 
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