Rebuilding an 8-3/4 - Get Your Rear In Gear
We Show The Ins And Outs Of Building And Installing An 8 3/4 Centersection
As automotive enthusiasts, we often talk about the different methods of making engine power, as much of our interest lies under the hood of our Mopars. The pros and cons of automatic versus manual transmissions are also a subject of our conversations, as are the various ways to get our cars to run quicker elapsed times, or make more power on the dyno. When it comes to the rear differential in our Mopars, however, most of us are aware that gear ratio selection is an important factor determining how a car will perform, but aside from that we have very little understanding of how the differential actually works.
Since most of our Mopars came with the Chrysler 8 3/4 rear end, we seldom have any major problems with this area of the car. In fact, even a factory 8 3/4 can handle substantial engine power without fatigue, and we've seen high-mileage rear ends handle modified big-blocks without much trouble. So, barring a catastrophic failure of the rear end, we generally just tend to forget about how many parts are in the rear differential and the various ways they can wear out, negatively affecting the performance of our vehicles. Worn ring and pinion gears will whine, and more serious problems will cause vibrations, excess drag, and even rear end lockup.
Swapping the rear end for another used one is an option, and most of us have swapped the centersection of our 8 3/4 to gain more optimal gearing as well. But as even the most recently produced factory 8 3/4 rear ends are 30 or more years old, the centersection you find to install in your car may be just as worn as the one you removed. Thankfully, most 8 3/4 differentials can be rebuilt, and many new parts are available. Since we needed a solid rear for our '71 Road Runner, we contacted Randy's Ring and Pinion to check out our options for the 8 3/4 in our car.
Like most Mopar enthusiasts, we like to perform much of the work on our cars ourselves. One area that we tend to leave to the experts, however, is the rear differential, and specifically the Sure Grip unit and ring and pinion gears of the car. It's not that the differential is beyond our technical expertise so much as this type of work requires specialty tools and must be done correctly so that the rear end has minimal drag and the appropriate amount of gear noise for the application. For these reasons, we decided to let the pros at Randy's Ring and Pinion build our 8 3/4 centersection, and we followed along to show you how it's done the right way.
When choosing a rear differential, the first determination that must be made is the intention of the car. For all-out drag cars with slicks that don't see street duty, a spool (solid) rear axle makes the most sense. The spool is lighter and locks the rear axles together, ensuring both tires spin at equal rpm at all times. At the other end of the spectrum, if you're just replacing the rear in your '74 Dart four-door with a Slant-Six, an inexpensive open style unit will work just fine. For the majority of us, however, a limited-slip type differential is the best choice for our street cars, or even cars that see a blend of street and strip duty. Chrysler called their limited-slip differentials Sure Grip and they were used extensively in muscle cars, trucks, and even station wagons.
There are several versions of the 8 3/4 centersection, and up until the mid-'60s, the differential had a bolt-together pinion yoke, making those undesirable for performance builds. The most common casting numbers for the 8 3/4 centersections end in the numbers 741, 742, and 489, with the 741 case being the least desirable due to its small diameter pinion bearing. Inside the cases, there were two types of Chrysler Sure Grip, commonly called the clutch style and the cone style, used to apply power to both axles during hard acceleration, but let the inside axle slip to maintain control when cornering. Both of these differentials worked well, but the clutch style is considered better of the factory Sure Grips because of its simpler design and its ability to be rebuilt when worn out. And speaking of wearing out, any factory 8 3/4 that has been used extensively, especially in a muscle car, is likely worn out by now. Since the rear in our Road Runner was suffering multiple ailments, including gear whine and chatter during tight turns, we decided to replace it.
Because our Road Runner will see mostly street driving, with some aggressive driving thrown in during weekend trips to the dragstrip, we decided that a Sure Grip unit would be the perfect choice for our B-Body. Additionally, since our A-833 four-speed transmission ends in a one-to-one ratio (no overdrive), we needed a set of gears that would allow highway cruising, counting on the relatively low first gear of the four-speed to get our weighty B-Body out of the hole. Knowing we're planning a future engine change to a larger and more powerful engine than the 383 currently in the car, and having experience driving cars with a multitude of engine and gear combinations, we decided that a 3.55 gear ratio would be about right. This gear ratio is a bit of a compromise on both ends, sacrificing a little bottom end and a little top speed, but is a great all-around gear that will allow us to drive the speed limit on the interstate while keeping our engine at a reasonable rpm.
With our decision made, we called Randy's Ring and Pinion to see what our options were. Building rear differentials for everything from industrial applications to all-out high-horsepower racing vehicles, the team at Randy's has good experience and quickly suggested, although they did have aluminum centersections for the 8 3/4, that we should build a differential using a factory 742 case for our application. Inside, Randy's installed a new Auburn Sure Grip unit, new heavy-duty 3.55 ratio ring and pinion set, and all new bearings and seals. Additionally, we chose to install a billet pinion yoke, taking away the weak link of the somewhat brittle factory yoke.
Once ordered, Randy's Ring and Pinion shipped the completed centersection directly to us, and local shop Inline Performance Specialists was nice enough to let us use one of their lifts to install it. The difference? We'll just say dramatic. The new differential is far quieter than our worn out 3.91 geared 489 case factory rear, and we even took away an annoying vibration that we later found was caused by a worn pinion bearing in the unit we removed. Even better than our newfound smooth and quiet operation, the 3.55 gears allow us to cruise at highway speeds without excessively revving the engine. Not bad for a phone call and a couple of hours on the lift.
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|Originally posted by cageman|
The one sure grip that turns part way each direction looks like a locker, and after doing some reading on them, that is what they do, so maybe that is normal, IDK.
I was looking at the 3.91 I broke, and it appears to have scrapes on the carrier that look like the pinion was gouging it, but it all appears tight and it has the proper back lash. Im not sure if it was from before or now. I got the sure grip in a box of parts ten years ago, don't remember if they were there then or not.
I'm running Mobil 1 synthetic gear lube, and I had friction crap in the last one as it chirped going around corners, so I added it, I only put 120 miles on the 323, but it appeared to work fine till it let loose. It tries to move if I put it in gear and let out the clutch, but nothing in reverse.
To clarify for speed readers, I am breaking the sure grips, not gears or axles, same thing with the Dana 60s, the sure grips are what I have broken twice. I have never taken one apart, so I have no clue what has gone bad, i just sold them as being in need of a rebuild, and they sell right away.
Finding sure grips these days is tough, so I plan on holding on to these three. I was talking to a local, and he says the hardest part of rebuilding them is getting all the stuff in line, so you have to use an axle in each side, which is awkward.
I have never read anywhere about any unreliability problems with the clutch-type (Power-Lok by Dana) Sure Grips, while the only issue I've heard of with the Borg-Warner/Auburn units is that they wear and you need to do some careful machining to restore them. Not that it's ever impossibly to break anything.
So for this reason, I'm really keen to see what you find inside. I've got a big investment in both a couple of Power-Loks and a Borg-Warner and I'd like to know what problems I should look for.
Also, the 'speed reading' award I think is perhaps yours... I don't think anyone is suggesting you've broken axles or sun gears or anything, they are just saying that they only ever hear of this problem, not of the Dana bits breaking.
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All Center Sections are assembled with the following Moser Engineering designed and manufactured components: Items that are part of the Center Section that are assembled but sourced from an outside supplier;
- 489 Nodular Iron Case
- Bearings & Seals
- Fasteners (all grade 8)
- Limited Slip Differentials
- Ring & Pinions (Gears) - Moser Engineering does not manufacture any gears! These are sourced but not limited to the following suppliers; US Gear, Hoosier Gear, Motive/Richmond Gear & Yukon Gear
She did not answer, crying even harder. I carefully unbuckled the handcuffs on her arms and secured her left hand on the long bar of the head of the bed, tucked her down. And covered her with a blanket.
8 3 suregrip mopar 4
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