7 3/4" and 8 1/4" rear weaknesses? (and other ?'s)

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derrangedgadgeteer

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Well, I've read over and over that for a performance application, the 7 1/4" rearends that come underneath our a-bodies leave much to be desired. I've also read that the 8 1/4" rear is better, but still far from ideal. My first question is this:

When you push a 7 1/4" or 8 1/4" rear, how does it break? Do the teeth get stripped off the pinion or ring? Do the axles snap? does the case split? What's the weak point? Also, How much does keeping fresh oil in them and setting the backlash make? I know the chevy guys always say the only reason for a rear to blow up is if it's set wrong. (or you're trying to put a dumptruck into the 9's)

My other questions regard alternatives to these two...

Between craigslist and local wrecking yards, I've found that 8 3/4" rear ends are few and far between, and those that I can find are all heavy duty truck units. Ford 9" and Dana 60 rear ends are the same except replace "scarce" with "EXPENSIVE" However, My local wrecking yards have tons upon tons of old jeeps and cherokees. I stuck a tape measure under a couple, and they seem like they'll fit if I relocate the spring perches.

I've looked on the 4-wheeler forums, and they say change 'em out for serious off-roading, but that they're fine for most driven rigs. But I can't find any info about people putting the dana 44's under Cars. I'd figure it'd be a bit less strong than a 60 or a 9" but how does it stack up with the other chrysler rears? Is there any reason that a freshened dana 44 with 3.55's can't wrangle an a body well into the 11's?
 
I don't hear too much about the 8 1/4 going bad, more the 7 1/4 and the new Dakota 9 1/4 the 7 1/4 usually snaps the spider gears or the pin holding them, or the pinion strips. the newer 9 1/4 Dakota rear ends are notorious for the pinion bearings going bad and stripping the pinion gear and ring gear. best rear end for a mild build to severe is the 8 1/4 Dakota rear the 8 3/4 or the Ferd 9"
 
You can't look on a 4x forum and expect the right answer for the performance world. . . . They say the D-35 sucks, and i have been putting 500+ hp thru one for years.
Everything depends on use, they use them in a abusive environment and im putting power to it equally and not more to one wheel then the other or bouncing it (loading it and unloading it which kills gears and axles).

Same as you can't just say a rear is junk because you blew it up hopping it all over...

What is it you expect to use the car for, and what is the expected power you think you're going to put to it...

If you want decent performance lose the 7.25

The 9.25 like MANY other diffs has a issue (some of them) the pinion nut loosens up and the steering wheel operator IGNORES the noise and blows up the 9.25 and then they scream it's junk.....
The same happens to D-60's, D-70's and even 80's, even seen it happen to 8.75, 12 bolts a couple 14 bolts, 1 aam 11" . . . . . However some 9.25's with some miles on them do run into the loosening nut more often and well whenever you IGNORE a noise your car/truck is making you deserve what comes next. . . .

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Yup, 7-1/4 will break spider gears or crosshaft and bust the spider shafts out of the carrier with not much persuasion. 8-1/4 are actually fairly tough and I would think it would take a bit of effort and some serious torque to break one. 8-1/4 should last in a street car . It's not ideal or the toughest thing you can put in but acceptable.
 
I beat the heck out of 8-1/4's for many years. 3:55's & 4:10's with a 4 speed. It does have richmond gears, auburn unit & all new bearings, etc. Never broke one but have spider cracked a few adjusters over the years. The real limiting factor is slicks, imo. I would not recommend one for anything other than a street tire small block car.
 
7-1/4" rear weakness comes from the small size of the gears, the 7-1/4" ring gear means an proportionately small pinion gear, and tiny little side gears and planetary gears, these are the weak spot, they are so small it doesn't take much abuse to break them, they are fine for normal driving, but if you put any hard loads on them, (like hard launches) they just break, a 7-1/4" with sure grip is stronger only because the sure grip splits the load between both side gears, so each side only takes half as much abuse, with the other side taking the other half, resulting in a stronger longer lasting rear,

8-1/4" rears do not suffer from these problems, there only weakness is due to how the axles are retained, 8-1/4" rears use C-clips to hold the axles in (like a ford 8.8 rear in a fox body mustang) the bad rep that the 8-1/4 got is not due to it being a BAD rear (its actually a very good rear), but rather from MOPAR deciding to use it to replace the superior and venerable 8-3/4"
 
The 7.25" rear is junk , even behind a 170 Slant Six / 904 combo , with an old lady driving .
Both cars I've driven with that stupid waste of metal were :

1. ) 1972 Swinger 318/904 . I bought it when it was 17 years old ; and , its prior (original) owner kept-up with all services (showed me receipts and everything) . I had the car for less than a year and that 7.25" growled , howled and protested .
I swapped it for an 8.25" junk yard rear (3.21 gear w/ Sure Grip) from a '74 Duster . I did nothing except change its fluid and install it . Over 10 years and engine upgrades , that 8.25" held up perfectly (again , with servicing it every other year).

2.) 1967 Dart GT 225/904 . This was a $500 special my friend bought in 1996 . The car wasn't perfect , but it was presentable and its miles were low and original (35,000 if I recall) . It was used as a "lead-in" car for a classic car lot .
The 7 mile home drive after purchasing it revealed a grumbling , growling , howling 7.25" .
Next day , a trip to the pick-a-part found us an 8.25" out of a '75 or '76 Dart sedan . Ratio was 2.94 , open .
We also ganked its driveshaft (no rust , dents , nor anything obviously wrong with it).
Took 'er home , filled 'er up with 75w-90 synthetic (he's not as vigilant about servicing his vehicles as I am) , spanked its on its butt and drove away .

I'd say that the 8.25" is a good diff which got a "bad" name when it replaced the coveted 8 3/4 in '73 'A' Bodies (though the 8 3/4 survived through '74 in 'B' , 'C' and 'E' cars and '76 in trucks ).

Its advantages are :
- Lighter weight
- Less "reciprocating" weight = *potential* better fuel economy
- Parts are on-the-shelf at auto parts stores
- 5x4.5" bolt circle wheel pattern
- Dozens of 'em in the 'yards
- Can handle decent amounts of torque and h.p.
( e.g. , a warmed 225 , 318 or mild 360 )

The 9.25" is an oft-overlooked alternative . They're prevalent in '74 & later 'B' , 'C' , 'M' and 'R' bodies , and numerous pickups and vans
( albeit , with a larger 5x5" bolt circle or larger in trucks and vans; custom or redrilled axles needed).

The advantages of the 9.25" are similar to the 8.25" .

 
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