I'm changing the rear end on my 69 B body, 440 car. I have 2 cases and was thinking a 3:23 gear. What would you guys suggest the 742 or 489 case? What are the pros and cons of each? Thanks.
742!!!! It came with the clutch style poss...........Even tho it will fit in 489 housing. the 489 poss was a "cone style poss".............they work great as well BUT when it go's bad you chuck it in the trash and by a new on The clutch style, you just by New clutches and it good as new..........742 has a large straight pinion (compared to the 741 housing)the 489 housing has a large tapered pinion..........The 742 is considered the stronger of the two.......But they are real close to equal in my book!!!!!
The 489 is the stronger of the two. It has a 1-7/8" pinion and the 742 has a 1-3/4" pinion. The differential cases (posi units) are interchangeable between all 8-3/4 diffs (741 724 489) as long as you use the matching bearings. In one of my cars I have a clutch type posi from a 741 in a 489 with 3.91 gears. A 742 might have a cone type posi if it was built after 1970. The cone types are fine for street use, but not for serious drag racing. You can rebuild the cone type by machining 0.030" off of the bottom of the cones. As when they wear they tend to bottom out and lose there grip.Don
the 742 is early modle then came 489 which is stronger