A classic muslcecar never seems to be done; we are forever tinkering on it, fixing it, modifying it, or searching for parts at a wrecking yard. But one of the problems with parts from a wrecking yard is that we already have used parts on our car, so finding more used parts sometimes doesn’t fix the problem because we don’t know the condition of the replacement parts.

For those who have a Mopar, finding a decent rearend to replace a worn-out 8-3/4-inch is a crap shoot, at best. If you’re lucky, you’ll come across a 489-case Sure Grip rearend, because who wants to buy a one-legger? You’ll also need to be sure that the housing isn’t bent from whatever accident brought the car to the wrecking yard in the first place.

If you do find a decent rearend, the next step is getting the unit home to begin the teardown so you can assess what is needed. This can include rebuilding the Sure Grip, installing new axle bearings and seals, and possibly replacing the axles. Bottom line: you just don’t know how hard the car was driven or how worn out the parts are until you tear it all down.

All of this can cost you quite a bit of time and money, and when you’re done with your junkyard rearend, you realize you just spent a lot on the rebuild. A fresh coat of paint for it is even more work and it all adds up rather quickly. Wouldn’t it be nice if you could spend just a little bit more and get a complete, brand new 8-3/4-inch rearend for your car? Moser Engineering has a solution, and you get exactly what you want instead of taking what you can get.

Avoid The Wrecking Yards, Get New Parts

Moser Engineering is well known for providing enthusiasts and racers with new, high-performance parts for differentials and rearends, and now they’re building brand new replacement 8-3/4-inch housings for Mopars. These stamped housings are made with all brand new parts, welded in-house at their facility in Indiana.

The housings are made with new, heavy-duty cores with seamless steel axle tubes, new brackets, and a Ford 9-inch based center with the Mopar 489 case flange. They build it on their jig based on a factory Mopar differential, and the housing and axle package also includes your choice of 30, 33, or 35 spline alloy axles, axle bearings, and heavy-duty bearing retainer plates.

In addition to the Mopar housing package, Moser also offers brand new 489 cases that can be set up with a number of different third members. That means that you can now order a complete brand new rearend for your Mopar that will bolt directly into your car.

This is a component that you can install in your own driveway or garage with just basic hand tools. Moser sets up the third member so that all you have to do is bolt it into the housing and install the axles.

We sometimes spend a couple of days scouring the wrecking yards just trying to find a donor rearend, and in that same amount of time – just two business days – Moser can have a brand new differential housing and third member ready to ship out to you. They’ve made it simple by providing an online order form that allows you to make all of your selections on one screen and submit your order when you’re done.

With all the attention that is paid to the rest of the drivetrain, it makes sense to pay more attention to the rearend as well. This solution from Moser Engineering will provide the security of knowing that everything is brand new, which means no parts to fail from unforeseen situations after a junkyard rebuild.

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Moser starts with their stamped steel center section based on the Ford 9-inch, DOM seamless tubes, and their own Mopar style flanges. The end result is a fresh, brand new Mopar 8-3/4 rearend.

Ordering The Differential Housing Online

We decided to freshen up our 1965 Plymouth Belvedere, Project Track Attack, and went online with Moser Engineering to order a brand new differential housing and axle package. All available options are listed and their staff is ready to answer any questions or provide expert advice when it’s needed. The only thing we had to do was choose which options we wanted for our new rearend, all based on how we use the car. It should be noted that these are performance components, and are not exact reproductions for the restoration crowd.

They’re a direct bolt-in unit, but provide more choices than what the factory offered up back in the 1960s and 1970s. Any modifications you’ve done to your car can be incorporated into the housing, whether it’s to accommodate a spring relocation kit or even a narrowed housing with shorter axles. Disc brake kits are available as an option, or if you’ve already installed disc brakes and want to replace the reared, simply inform Moser which kit you have as they are familiar with just about every kit out there.

We ordered both differential housing and axle packages, and the third member package, and went through the online order process step by step until we were ready to submit our order. Moser Engineering also provides the gear oil that you’ll need with either package, that decision depends on what third member is going into the housing.

The differentials are available for A, B, & E body Mopars, and that includes the early and late styles, as well as the 2009 and newer Drag Pak Challengers. Some of the early cars had tapered bearings, but since these are direct bolt-in housings with new axles, that problem child is eliminated with this package.

We spoke to Moser Engineering’s Marketing Director, Jeff Anderson, about the design behind this 8-3/4 differential. It’s based on a Ford 9-inch center section for strength, but has the mounting flange for a Mopar 8-3/4-inch. Anderson said, “Our new housings are much stronger and use a design that incorporates the strengths of a Ford 9-inch, but looks a lot like the old factory housings.”

These are based on the same housings that Moser built for the modern Drag Pak Challengers when they were first introduced in 2009. Moser built the rearends for those cars for Chrysler and incorporated some of their own parts for strength. “We use the stock location for the brackets and use a Moser designed Mopar housing end that is much more stout than the factory part.”

Although these are replacements for classic and modern musclecars, they are built for performance. For drag racing and heavy-duty applications, they can be custom made to customer specs, and Moser completes the extensive modifications, including additional bracing for hardcore racing. The DOM (drawn over mandrel) steel tubes are stronger than the original welded-seam tubes from the factory.

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Top: The new rearend housing includes studs for the axle and third member flanges.
Bottom: We had disc brakes already, and instead of the typical tabs for the brake lines, Moser installed tabs to mount the hard brake lines to the flexible lines. They're a perfect fit. The fill plug is a nice touch and is high enough to clear our rear sway bar.

“We do use American made thicker wall steel and 3-inch DOM tubing, taking from the 9-inch design for strength.” The design still retains the factory Mopar feel under the car, but is able to handle much more power than the factory rearend. Though it retains the 3-inch O.D. dimension, the wall thickness at 3/16-inch and the DOM tubing combine to strengthen the housing for high horsepower and racing applications.

Narrowing an existing rearend requires a lot of work and welding on the factory housing. Because Moser’s housing package is made to order and includes the axles, a customer can choose that option by providing all of the measurements, plus the housing will be much stronger than a narrowed, factory housing. The options for the replacement alloy axles include adjustable or non-adjustable (Green) bearings, as well as the stud type and wheel bolt pattern.

We ordered a direct replacement of our stock housing, retaining the factory housing ends that Moser builds for Mopars. For those who autocross and prefer a different axle/bearing combination, Anderson stated that Moser can build to order and accommodate customer needs.

There are also several choices for stud sizes and types, from standard Mopar 1/2-inch to 5/8-inch screw-in studs for drag racing. There are 14 choices based on size and length, but for our application we chose the 3-inch studs. Most of the stud choices are part of the package, but there is an additional charge for the larger, 5/8-inch studs.

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New hardware and brake tubes weren't even a second thought. Installation of the axles is straight forward after the third member was installed. The completed unit looks great!

Moser also accommodates customer choice in rear brakes; several disc brake conversion kits are available for an additional charge, or for those who already have rear disc brakes, entering the brake kit information is done online as well. Our brake kit was based on a factory rearend, so the standard flange that Moser makes for Mopar was perfect. For an additional fee, Moser can weld on additional back bracing, gun drill the axles, and provide star flanges for racing applications.

Our final choice for the housing was the finish and we chose the semi-gloss black powder coating even though a customer can opt for no finish at all. Anderson stated that custom colors can be applied, including color matching, for an additional fee. Once we made all of our selections for the housing, we submitted our order. The street price for our brand new custom built Mopar housing and axle package came out to $1,480.00, but depending on the options chosen it can be as low as $1,230.00.

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Ordering online with Moser Engineering just got easy and every choice is laid out for you to see. Pricing updates at the bottom, then add to cart, place your order, and in just two business days your complete rearend is ready to ship.

Third Member Choices

If you’ve looked online lately for a 489 case for your Mopar, you’ve probably noticed that the price has risen dramatically. What used to be about $350 for a used Sure Grip and 489 case has now risen to over $1,000 for a rebuilt unit.

Finding a Sure Grip in a wrecking yard, as we mentioned, is not always easy to do. Sometimes we have to refer to eBay to find a rebuilt unit or the local Craigslist seller, and you take your chances. We saw a rebuilt unit recently that listed for over $1,200 – ouch!

To complement their new rearend housings, Moser Engineering also offers brand new, nodular iron 489 center sections with your choice of third member, again based on how you’re going to drive the car. We were also able to order the complete built-to-order 489 third member online using Moser’s order form, making our selections along the way like we did with the housing and axle package.

The nodular iron case is brand new, as are the rest of the parts that make up the third member package. A choice of powdercoating is also available for an additional fee of just $70 – well worth it. The differential choice depends on the customer needs; our Mopar sees a lot of street driving with some road racing and autocross sprinkled in, so we selected the Detroit Truetrac from Eaton Performance. That will give us the best performance for all around driving.

For hardcore drag racing, customers might want to select a steel spool giving them the best traction in a non-limited slip rearend. Other choices include the Auburn Gear limited-slip and the Detroit Locker, both for the stock 30-spline Mopar axles. The Truetrac is available with either 30- or a 33-spline upgrade, the unit we selected.

The Truetrac is not a typical limited slip in that it’s a torsion-type third member that relies on friction to operate properly. This means that there are no clutches or springs to wear out, and if proper fluid is kept in the rearend you could have this differential for a very long time. It’s a helical-geared third member that, under normal conditions, distributes power evenly to both rear tires. Should one of those tires begin to slip, the power is transferred to the tire with the most traction.

The Truetrac requires friction against the covers to work best, so both Moser and Eaton recommend not to use a friction modifier additive. Anderson said, “This will lessen the friction and thus not work as smooth, or could cause excessive heat and damage to the carrier.” He said that some synthetics are fine as long as there are no modifiers added in, but recommends non-synthetic for initial break-in.

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Not having to setup the differential is a big plus, Moser takes care of that for you.

The unit arrives already set up in the 489 case with the proper settings and side load set at Moser. Inside, where the factory had stamped steel adjusters, Moser provides new billet adjusters for additional strength. These third members are being installed on cars in excess of 600 horsepower and Moser builds these units strong enough to handle that kind of power.

Anderson recommends using the supplied third member gasket when installing the third member. Anderson said, “It acts as a shim as well as a seal, kind of like a shim in a setup kit. Silicone will squeeze out so just put a light bead on each side. Without the gasket, you actually get a very slight bind or miss alignment of the axles to the carrier, resulting in added heat and noise, vibration, and harshness (NVH).”

For gearing, we are going to be doing a lot of cruising with this car, so we opted to go more towards economy than performance with the 3.55:1 ratio using a Motive Gear ring and pinion package. There are several choices for gearing: Motive Gear, Hoosier Gear, or Richmond Gear, and gearing ranges from 3.55:1 to 4.86:1 for drag racing applications with larger tires.

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With the new rearend assembled, we tackled the old one, removing everything that is bolted to it, including the brake lines. Don't forget to support the rearend when unbolting the axle U-bolts

We installed the third member to the rearend housing first, but if you’re planning on using the factory style adjustable bearings, it’s recommended to install the axles on both sides, fully seated, and measure the distance between the splined end of the axles.

The Truetrac includes a thrust block for applications with adjustable bearings and in order to make those adjustments the thrust block may need to be machined. When using non-adjustable bearings, the thrust block is eliminated from use in the carrier.

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Top: We installed the wheels and tires on the new rearend and simply rolled it into place, making sure the car was high enough for the tires to clear the quarter panels.
Bottom: Everything was bolted back into place, brakes were bled, and Lucas gear oil was added. Try to add the gear oil with the suspension at ride height and the car on the ground if possible to get the proper level. When the gear oil level reaches the opening, insert the plug and tighten it.

We assembled the complete axle out of the car for easy access to everything and also to install our disc brakes and brake lines prior to installing the complete rearend in the car. As a one-man job, it can be done in about a day, with standard tools. We mounted a couple of spare wheels/tires to the old rearend, raised the back of the car high enough (supporting with jack stands) and disconnected the rear leaf spring hangers.

With everything disconnected, we simply rolled the old rearend out from under the car and rolled the new one into place. We bolted everything back up the same way it came out, and in one day we were done with the entire swap.

Adding fluid to the rearend once it was installed was rather simple. The filler on the top of the Moser housing has a neck that simplifies adding fluid without making a mess. The side sight plug should be removed and with the axle installed and suspension at ride height, we used three quarts of Lucas Oil Products‘ 85W-140 gear oil.

The Lucas 85W-140 is a heavy-duty gear oil utilizing Lucas’ oil stabilizer. This gear oil is fortified with special anti-wear agents and lubricity agents to control wear and drag as well as special additives to control heat. This feature is not found in conventional gear oils. This gear oil is designed for racing and high-perofrmance applications.

We added three quarts, with very little left over. Remember: the center of the housing is based on the larger, Ford 9-inch, so the capacity is going to be a little more than a stock Mopar 8-3/4 rearend.

Things to Consider

Depending on what you had in your car before, you might need to have your driveshaft modified or a new one built. The Moser third member includes a billet pinion yoke based on the 1350 Spicer universal joint. We had the 7260/7290 combination joint on our old rearend, and it was time to upgrade to the bigger, stronger U-joint.

We also had the smaller U-joint at the transmission slip yoke, so we opted to have a completely new driveshaft made for our Plymouth with the stronger 1350 U-joints at both ends. Moser also sells complete driveshafts, but we don’t recommend ordering one until you have the new rearend installed completely, and you can provide precise measurements to Moser.

Having a friend help with this swap is always a help, and for our Garage Series of articles we sometimes have an extra pair of hands, other times we don’t. Since the third member is already assembled, and this is basically swapping parts, it’s a task that the novice mechanic should be able to accomplish. If you’ve never done something this big before, have a friend help you out. You will need to disconnect brake lines, so be prepared to cap the main fluid line to the differential.

We decided to install a set of spare wheels and tires to move it around on our own, a standard floor jack is just asking for trouble with a bulky, heavy rearend. If you have a friend, they can help guide everything into place, but always be careful and plan things out.

Another thing that is very helpful with a job like this is having two pairs of heavy-duty jack stands and a pair of floor jacks. We used all four jack stands and both floor jacks at various intervals during the swap. One floor jack is fine for raising the car initially, but once the rearend is unbolted, having a floor jack at both sides of the car for raising or lowering is a great help.

Finally, the sight plug on the passenger side requires a 9/16-inch allen wrench, if you don’t have one, a very inexpensive alternative is to use a 9/16-inch barrel nut and box-end wrench.

Proper Break-in

With everything bolted up and double-checked, it was time to get our Mopar out on the road and break it in. Break-in is very important and should be done according to the recommendations of Moser. Anderson, said, “For gear break-in, a standard non-synthetic gear oil often works better. The gear must seat during the break-in and with a very slick synthetic, sometimes the proper break-in per the gear manufacturer’s instructions doesn’t happen.”

For driving, he said, “500 miles for break in should be sufficient. I would say during the first 50 keep the car below 60 and never drive more than 15 minutes without letting it cool down for 15. This will really help with running the gears in and controlling harmonic noise issues that can occur more often than not when pushing the break in procedure.”

That easy first 500 miles gives the gears time to break-in properly. The Truetrac is built for strength and works on the friction of the helicoil gears and the case, so taking it easy is highly recommended until the break-in procedure is completed. Other sources have recommended that doing figure-8s in an empty parking lot also allows the limited slip to break in. Either way, with all new components, the proper break-in makes sense; so hold off on the burnouts until you’ve given the rearend time to seat the gears properly.

Driving Impressions/Final Thoughts

We have been driving this car pretty hard for a couple of years now since we started racing, and this complete rearend assembly from Moser Engineering is a solid unit that was set up by people who have been doing this for decades. We had installed some new parts last year but with a used ring and pinion we had a bit of clunking in the rearend and our smaller U-joints were not holding up to the kind of abuse we were putting the car through.

After the swap, we went out on our initial test drive around the block. We wanted to make sure we didn’t have any noises or problems with the install and staying close meant that we didn’t have far to go in case we forgot to tighten something. After our initial test, we drove the car for about 15 minutes as instructed and immediately noticed that it was much smoother and quieter during shifts. Power transferred immediately with all of the new parts, and even at freeway speeds we noticed that the car seemed to be a bit more responsive and smooth.

We still have a couple of hundred miles to go before it’s completely broken in, but we now know that our Belvedere is ready for just about anything and everything we want to throw at it. We’ll be making another trip to the track soon to follow up on the install and we’ll keep you up to date with Project Track Attack, so check back.

If you’re tired of scouring the wrecking yards for a Sure Grip and a 489 case rearend for your car, give the new Moser Engineering Mopar 8-3/4 rearend some thought. With the prices on eBay and Craigslist skyrocketing for rebuilt units, you can’t go wrong with a completely new unit with all brand new components. With a street price starting at just over $2,700 for the complete unit, and the ability to upgrade to better components and reinforcing, it’s not a bad choice for the security of having a rearend assembly that’s built for strength and performance.

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Top: We fabricated a bracket for the custom brake line from aluminum scraps in the garage. not exactly pretty, but functional.
Bottom: Welding pictures are always spectacular, these almost hurt your eyes.