I was wondering (1) is the 742 rearend better/stonger than the 489 rear diff (2) would the 69 roadrunner have one vs the other specific rear end saying that do all roadrunners have the same rear end other than a dana ???
Thanx ....Mike

the power lock sure grip type in the 742 case is far stronger than the cone type sure grip in the 489 case sure grip. There was a recent post on when the changeover was & iirc it was mid 68 model year but you know how Ma was with carryovers/poor record keeping & yours could have swapped out. Cant miss the big cast numbers on the side

I thought the 489 first came out in 1970?

IMO the weak point in all of them is the threaded end of the pinon shaft.

A different take on the issue:

1. When the aftermarket decided to make an aluminum center section they picked the 742. I bet at least some of that reason was because they thought it was stronger. And I'm assuming that the aftermarket designers knew the crush sleeve of the 489 case could be replaced with a spacer to take out that weak link.

2. For every story about twisting off a pinion shaft, there are 10 or more stories about breaking gears. What happens to break a differential is that the two gears are trying to push away from each other, and the farther they push apart, the more leverage the tooth has on the root. In other words, when the gears push apart the gear teeth are apt to break off. Now as each of the three types has the same diameter ring gear and pinion gear, the only difference is the stiffness in bending of the pinion itself. Again, the 489 case in this example has a spacer, not a crush sleeve.

So IMHO there is not a lot of difference between the two, and I'll also say if you are worrying about breaking one when the "stronger" one may not break, you are wasting your time. Get a Dana 60 and be done with worrying.

R.

I agree.
Thats why my 440 68 Dart has a Dana 60.

Yep, a whole lotta splitting of the hairs tween 741, 742, and 489s.

As long as the diff is not a cone, and if it's a 489 it's got a solid spacer, I'm good with any of them for a street car, street tires.

I believe the 742 and 489 are similar in strength... with a solid crush sleeve the 489 may be just a bit better.

Agree the cone type sure grip is not the best for performance use. With parts swapping and replacement either rear could have either type by now. I'm running a 489 with aftermarket clutch type sure grip behind a 496/5 speed with no problems in 10k+ miles. I romp on it but no hard drag launches or slicks. Drag would want a decently built D60 in that car, especially with MT.

This debate is very interesting to me and I like reading it every time it comes up. Each time, the end result is maybe I need to forget about getting another center for my 4-speed Bee and go straight to the Dana. It will see more street use, but it will get regular track flogging as well.