The 489 obsession

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autopar3000

autopar3000

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Conventional wisdom would tell us the 489 is the strongest 8 ¾ third member, then the 742, and the 741 is the weakest. Who here has ever broken a 741 on the street using street tires?

I'm asking because I have been passively looking for a third member with a 3.73 or 3.91 ever since I had the 46rh OD installed, and I have noticed people tend to value 489 third members like they're made of gold. I have been beating the ever living hell out of my car's the 741 diff (3.23 Sure Grip) for going on 25 years and I have never even had it apart. And none of my local Mopar buddies have ever broken a 741 on the street. They've got stories about breaking every diff you could imagine at the racetrack, but with sticky tires and a prepped track you can break almost anything.

So, is the 741 actually weaker than the others or do we just think it's weaker because it should be weaker?
 
One drawback is that it's starting to get tough to find aftermarket gearsets for the 741 in some ratios. The others currently have better aftermarket support.
 
Cass has 3.55 and 3.73 ratios on his website. Someone wrote up an article comparing the 741 to something else, 12 bolt or 8.8 maybe. Pinion is same diameter. No issues w a 741 unless it’s wide tires and a drag start.
 
I ran a 741 in my 62 max wedge with sticky tires for a coupla years before I found out it was the "weak" one. . . lol
 
Buy every cheap 741 you can find. If you actually look at the pinion gears (the supposed weak point of a 741) the small part of all three shafts are the same size (weakest link?) and the 741 doesn't have the potential stress riser of the abrupt increase in size.
As mentioned above, the major drawback of a 741 is a lack of ratios available in new gear sets.
 
The 741 has a larger pinion stem than a 9" Ford. However, the Fjord has three pinion bearings, the third one solely to support the head of the pinion, which gives it a lot of strength (the pinion head is loaded in shear by the ring gear). If you're going to break a 741, it will probably be right at the pinion head, the area of greatest stress.
The other two cases have a much-larger pinion stem at the rear, the 489 being 1-7/8" and the 742 being 1-3/4" if I'm remembering correctly. That means you've got a lot more support at the most critical spot on the pinion. However, both are 1-3/8" at the front bearing, same as the 741. When the bigger pinions break, they usually shear just behind the yoke or the front bearing. Also, the 489 can suck the pinion head into the diff if the crush-sleeve arrangement is retained. I've owned more than one diff scarred by such behavior, but I received them that way. I've used solid spacers for years.
Personally I've never broken any of 'em, but I've seen all three break, generally as I've described and almost always under extreme duress--slicks, trans brakes/clutch dumps, etc. I've also seen them endure enormous abuse--some at my own hand. Still, if you don't plan to beat the ever-livin' shyte out of your car, you'll probably never have an issue with any of 'em.
If you do expect to engage in such shenanigans, you might want to meet my friend Dana.
 
Took the 742 W/323s SG out of my 68 R/R for a 741 W/2.7s open Moon shot gears. The duster has a 742 W/391s SG. A 489 and the R/R gears are tucked away. Nothing wrong with a 741 or decent used gears.
 
autopar3000 said:
Conventional wisdom would tell us the 489 is the strongest 8 ¾ third member, then the 742, and the 741 is the weakest. Who here has ever broken a 741 on the street using street tires?

I'm asking because I have been passively looking for a third member with a 3.73 or 3.91 ever since I had the 46rh OD installed, and I have noticed people tend to value 489 third members like they're made of gold. I have been beating the ever living hell out of my car's the 741 diff (3.23 Sure Grip) for going on 25 years and I have never even had it apart. And none of my local Mopar buddies have ever broken a 741 on the street. They've got stories about breaking every diff you could imagine at the racetrack, but with sticky tires and a prepped track you can break almost anything.

So, is the 741 actually weaker than the others or do we just think it's weaker because it should be weaker?
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It's all the pinion diameter.
 
I know Scott went through a small pinion 8 3/4. He replaced it with a Strange S60. That was years ago. And a lot of abuse.

527500CA-A05F-4E5C-B5AF-B59566FF13E4.jpeg
 
Actually that may be your conventional wisdom, but technically the 742 is the strongest. .......and I say that very lightly as it's really splittin hairs between the three. Now, if you use a crush sleeve eliminator with the 489, then it becomes the strongest.....but by "how much" are they all three separated? Not much, I can tell you. If I had any of the three, I'd use it and not look back.
 
HemiSSDart said:
741's are in all the trucks and vans up to 1972 for sure...
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Really? I've seen them in every unmodded A body I've ever looked at. Never paid attention to the trucks though. Evidently, Chrysler didn't think much about the strength differences, either. lol
 
inertia said:
The broken 8 3/4s I've seen, either stripped teeth off the ring-gear, or blown the caps off .
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The guy that rebuilt my A833 and also does diffs said the same thing basically.....pinion wasn't the typical failure point. I grew up believing 489 was the best but I'll run my 741 without fear on the street :) I broke a 742 years ago doing a clutch drop. The old worn spider gears gave out.
 
For me the 742 is the most trusted. It was the last 8-3/4 3rd member you could buy a 4.86 Pro gear for as I remember. No longer (too bad). The MP aluminum case was also based on the 742. I last ran the MP aluminum case with 4.86 pro gear with no failures. I smashed a few 489 gears in my life. Probably not the fault of the 489 though. I can't really quote experience with the 741. FWIW.
 
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