Project Corn Star: Our Fourth-Gen F-Body Gets a Rearend Upgrade

Project Corn Star: Our Fourth-Gen F-Body Gets a Rearend Upgrade

Anyone that has spent anytime at all around fourth-generation (or even third-generation) F-bodies, knows the 7.5-inch, 10-bolt rearend is one of the most anemic pieces of hardware General Motors ever saw fit to bestow upon a high-performance vehicle. Ironically, the General then went on to make the very questionable decision of pairing the flimsy 10-bolt rearend with the 4L60 automatic transmission in what is possibly the worst drivetrain combination ever put together. If you’re new to the F-body, first, welcome! Second, Google “10-bolt” or “4L60” memes and start to grasp what you’ve just gotten yourself into. 

While GM made some suspicious rearend and transmission choices when it came to the fourth-gen F-body, they did make one incredibly good decision. That decision was the LS1. Well, that and they look great. But any serious hot rodder knows that, while fourth-gen F-bodies can be incredibly quick — thanks LS1! — it takes a few key upgrades to get there.  And that’s exactly where we are with Project Corn Star.

Achilles Heel

So far, Project Corn Star has been fitted with a 370-cubic-inch, iron-block engine from the guys over at Blueprint. It was then equipped with a Huron Speed turbo kit, which spins a Precision PT7675 Gen II snail. Go juice is E85 — thus the project’s namesake. You can check out everything we’ve done on Project Corn Star here, Needless to say, its rearend has quickly become the project’s Achilles’ heel. With plans to make around 900 horsepower at the tire, our factory 10-bolt was one wrong look or stiff breeze away from being a pile of scrap on Corn Star’s first launch. 

We were lucky enough in that Corn Star came with a T56 manual transmission — which will take a beating for a while (it’s no 4L60, that’s for sure). Since the clutch has been upgraded to a unit from SPEC, capable of holding up to 1,200 lb-ft of torque, our last item on the upgrade list is our 10-bolt. For that, we turned to the guys at Strange Engineering for what they would recommend as far as a replacement.

35-spline alloy axles come standard with the S60. The Chromoly yoke on our driveshaft, however, is definitely an upgrade.

Gettin’ Our Rear In Gear

Since the 10-bolt rearend is such a known under-performer, there are a lot of options for replacements. Everything from Ford 9-inch rearends to GM 12-bolts — and even a few Ford 8.8s in between. So, how do you go about choosing the right one for your build? That comes down to what you want to do with it and how much weight or power loss through the unit is important to you. 

For our decision, we turned to the experts at Strange Engineering for a little help. They’ve practically seen and done it all, especially when it comes to fourth-gen F-bodies. They were quick to jump in and recommend the S60 and explain a little bit more about how to choose the right rearend for your build. 

There are a lot of options for these cars, but the Strange S60 is one of the best available in terms of strength and affordability. – JC Casico

“There are a lot of options for these cars, but the Strange S60 is one of the best available in terms of strength and affordability,” said JC Casico of Strange Engineering. “The strength and reliability of the S60 is superior to the 12-bolt rear. Strange uses Spicer gear sets in the S60 street/strip applications, which allow the setups to be quiet while maintaining its strength as compared to the aftermarket gears used in the 12-bolt assemblies.”

But what about a 9-inch? Those have been around forever, and are known for their strength, right? Absolutely! The 9-inch is a great unit but, typically costs a bit more.

“When compared to the Ford 9-inch, the S60 offers comparable strength,” JC explained. “However, the S60 is less expensive than a similar 9-inch assembly and only weighs 15-20 pounds more. The S60 is also the type of assembly that can be bolted in and requires minimal maintenance.”

While strength is typically the first consideration taken into account when selecting a new rearend, it should be far from the last. You’re also going to want to consider the efficiency of the unit at transferring power to the tires and, ultimately, overall weight. However, these factors can often be a give and take. Typically, a rearend can be made more efficient by using lighter-weight components in them. This frees up horsepower that would otherwise be used to bring heavy parts up to speed. On the other hand, lighter parts are typically weaker and more susceptible to breaking with increased loads.

The black powdercoating is optional but in our opinion, worth it.

This can be a real catch-22. For example, the 10-bolt is light and pretty efficient at getting power to the ground. However, that’s because its ring gear is only 7.5-inches in diameter, compared to the 9-inch or S60, which are 9- and 9.75-inches, respectively. This obviously adds some weight, but increases strength exponentially. The same goes for the axles — 10-bolts were equipped with either 26- or 28-spline axles. compare that to the 35-spline axles that come with the S60, and you begin to understand where the additional weight comes in.

While the S60 actually uses a bigger ring gear than the 9-inch, it’s more efficient at getting power in and out of the unit due to its unique design. With all of the improvements in strength and efficiency, you can see why Strange was quick to recommend its S60 for our application.

Why Does The 10-Bolt Suck So Bad?

With stats like that, you can quickly start to see why the 10-bolt is pretty incapable. But, you might be thinking, “I can upgrade my 10-bolt to take more power.” The reality is, there isn’t a lot that can be done with the 7.5-inch rearend to make it really handle all that much more power, JC explained. 

“The 7.5-inch, 10-bolt rear is purely limited by the size of the ring-and-pinion,” he explained. “Regardless of upgrading the posi, axles, caps, cover, etc., the ring-and-pinion will always be the weak link. The diameter of the S60 ring gear is 9.75 inches, which adds a tremendous amount of strength to the rear over the 7.5-inch ring gear. Also, the S60 comes standard with 35-spline axles, compared to the small 26- or 28-spline axles in the 10-bolt.”

Case flex is also a very common problem with the 10-bolt. Any flex in the case allows the gears to come out of lash when large amounts of torque are applied. Obviously, when the gears lose lash, it creates serious problems. Unfortunately, there isn’t a lot that can be done about the small ring gear or housing flex. The simplest solution? Pull it out and throw it in the trash can next to that 4L60.

The S60 Difference

After we spoke with Strange and they recommended the S60, our first question was, what makes it so much better than the tried-and-true Dana 60 to which the rearend is based? The answer: A LOT!

“The Strange S60 casting is a vast improvement over the factory Dana 60, but will still accept the same components,” Cascio explained. “The casting and oversized main caps are constructed from a proprietary nodular iron, which is substantially stronger than cast iron. This allows the casting to be made not only thinner and lighter, but also stronger. The heavy-duty main caps are fitted with adjuster nuts that eliminate the side carrier shims. Backlash and preload are adjusted by a turn of a wrench, greatly reducing the time and effort to change a gear or differential. All castings feature a drain plug located at the bottom. Mopar applications have a machined provision to accept factory or aftermarket pinion snubbers. GM F-body applications have the torque arm mounting location cast into the unit, while A-body and G-body feature the upper control mounts.”

Unlike many rearends, Strange’s S60 comes standard with 35-spline alloy axles that can take a real beating. It also comes with Strange’s S-Trac helical carrier, which, much like the rest of the assembly, can take some serious horsepower. Other included items are your choice of wheel stud kit (M12 or 1/2-inch studs — we went with 1/2-inch), your selection of gear ratios, S-series pinion yokes that are considerably stronger than standard cast pieces, and an extended sway-bar link kit for our application. As for upgrades, you can get 5/8-inch wheel studs, a Chromoly yoke, and a black powdercoat finish if you don’t want to paint it yourself. 

The Strange S60 rear comes standard with 35-spline axles. The axles carry a five-year warranty against torsional breakage. – JC Casico

Oh, did we mention it all comes with a warranty, as well? “The Strange S60 rear comes standard with 35-spline axles. The axles carry a five-year warranty against torsional breakage,” Cascio said. “The S60 is also available with the Strange S-Trac differential, which carries a lifetime warranty to the original buyer. The rest of the components and complete assemblies have a one-year warranty.”

With all of those standard features and a warranty like that, the S60 was a no-brainer for Project Corn Star.

Kick In The Rearend

Since we knew Project Corn Star would take a beating, we optioned our rearend (PN PRSF05) with a few upgrades. We opted for the black powdercoating (a $159 option; PN H1199P-BLK) and 1/2-inch wheel studs. We also went with the Chromoly yoke (PN OPRS18), and Strange sent us two quarts of Lucas gear oil. When you purchase any rearend, you also get your choice of gear ratio. We went with 3.73 gears at Strange’s recommendation. Considering our tire height and power level, the 3.73 gears would have us crossing the line at just about the optimal RPM with a little room to grow.

The custom-length driveshaft is a stout piece and is designed to take power levels well north of 1,000 horsepower.

However, one option we did not opt for was to send in our brake backing plates so that Strange could completely assemble the rearend for us. While this is a very nice option to have, we figured many of you, like us, wouldn’t have the luxury of having the old rearend apart while waiting for the backing plates to ship to Strange, be installed, and for the rearend to ship back to us. While Strange is very good at turning these things around quickly, we thought this would be an excellent opportunity to cover everything that it takes to install an S60 from start to finish — even if you install the axles yourself. 

Now, you may be wondering why Strange needs your brake backing plates, or why you have to reuse your old ones. Well, the answer is two-fold. First, this is so you can fully retain your stock braking system. The second is because the new S60 uses the backing plates to hold the axles in the rearend. Since the S60 doesn’t use C-clips on the ends of the axles (which increases strength and brings the rearend in line with NHRA compliance), the axle bearings that are pressed on the axle are what actually butt up against the backing plates and hold the axles in their homes.

As we stated before, you can choose to pull your stockers and send them to Strange, who will then press the axles together and install them for you, or you can do the axle assembly yourself at home. It’s a little harder, but it requires your car to be down for less time. We opted for the latter. 

Getting It Together

Arguably one of the easiest parts of our install was removing the rearend. We won’t go through it step-by-step, but it’s only a matter of disconnecting the brake lines at the rear distribution block, unbolting the lower control arms and torque arm, disconnecting the Panhard bar, unbolting the shocks at the lower mounts, disconnecting the sway bar, disconnecting the E-brake, and then removing the driveshaft. After that, the old 10-bolt can swing down out of the way, and you can now sell it for scrap metal, because no one in their right mind is going to buy it from you.

Here, we disconnected some critical pieces. First, the sway bar end links. Our new installation kit came with a new set so we weren't worried about breaking the old ones on the way out. We then disconnected the lower control arms and brake lines.

Next was disconnecting the Panhard bar, shocks, parking-brake cables, and torque arm. After that, our old rearend was ready to come out.

With the old unit out, we stripped it down and moved any necessary parts over to our new S60.

After our old unit was out of the way, we set about assembling the new axles. The first order of business was removing the factory backing plates. Four 10mm bolts sit behind the rotor that holds them on. After the calipers and discs are removed, you will need to open the differential cover on the 10-bolt and remove the C-clips holding the stock axles in place. The axles have to be removed to take out the old backing plates. This is accomplished by removing the 8mm bolt in the torsion carrier and then removing the block that holds the axles outward. Once this is accomplished, the axles can push in, exposing the C-clips and allowing you to remove them with a magnet. Once the axles are removed, the backing plates simply unbolt and pull off the old rearend.

After removing the cover on our 10-bolt, we were able to remove the axle-retaining block in our Torsen carrier that then allows the stock axles to be pushed in to remove the C-clips.

With the axles out of our 10-bolt, we removed the backing plates and prepared our Harbor Freight press to put together the new 35-spline axles.

With our backing plates now removed, it was time to assemble the new axles. For this, you will need an 11-15 ton press, according to Strange. We purchased a cheap 12-ton press from Harbor Freight and it got the job done, but your results may vary. With the new axle in hand, there is a specific order the axles need to be pressed together.

You will notice in the axle bearing box that there is a black collar, a bearing, a seal, and a wedding ring. You start by pressing on the black collar with the chamfer facing the wheel-mating surface. This acts as a spacer for the backing plate, so the axle bearing sits in the correct location.

In the first picture, you can see the black collar that has to be pressed on first. It acts as a spacer so that the axle bearing sits in the correct location. In the second image, the axle bearing is being pressed into place after the seal and backing plate are placed in advanced. Be sure which direction your backing plates are facing so you don't make the same mistake we did here and install them backward. We had to have a machine shop press them off and install the new bearings after this mistake.

Next, we installed the seal and bearing after the backing plate had been put in place. The axle bearing is larger than the hole in the backing plate since that’s what holds the bearing and axle in position. So, the plate has to go on before the bearing. Make sure the backing plate is facing the right direction before you press the bearings on. As you can see in the images, we learned this lesson the hard way and had to have a machine shop press the bearing off and install a new one just so we could flip it around.

Once the bearing is pushed all the way flush with the black collar, we then installed the wedding ring, which is just a silver ring that ensures the axle bearing doesn’t go anywhere. After repeating the process with the second axle, we were ready to install them in our S60 housing.

The axle slides into the housing and is held in place by the brake backing plate. As you can see in the first image, there is a black spacer bracket. We RTV'd both sides of this plate to ensure it didn't leak.

The axles slide home easily and the backing plates are secured to the housing using four provided T-bolts. There is also a black four-hole spacer that goes between the axle tube and the backing plate. Be sure to put some RTV on the inside of this spacer to prevent any wheel bearing grease from leaking out. If you did not install the black axle collar right, you know immediately, because all the space for the parking brake will now be nonexistent. With the axle together, we moved on to modifying the torque arm to fit.

We modified the torque arm bracket to fit our new S60.

If you’re using your stock torque arm, you can skip this step. Project Corn Star has a BMR adjustable torque arm. Most aftermarket torque arms have two sets of holes in them to allow for adjustment. However, the way the new rearend is cast, means the additional holes interfere with the housing. To remedy this, we had to grind away the outermost holes. We accomplished this using an angle grinder. While this step can seem daunting, it was relatively easy and just requires patience. As long as you don’t grind into the second set of mounting holes, you should be just fine.

The sway bar is installed, as are the coilover-mounting brackets and new driveshaft before the rearend is lifted into place.

With the torque arm modified, we installed the brake lines using the new tabs on our S60. We also installed our sway bar with the new mounts and hardware included in the installation package. Our last step was to move over our coilover-mounting brackets for our QA1 shocks. With that, we were ready to install the new rearend. First, we slid the new, shorter, Strange driveshaft into place. It too features Chromoly yokes and 1350 U-joints, and is capable of supporting well over 1,000 horsepower. With the driveshaft in, it was only a matter of reversing our removal process, bleeding the brakes, connecting the driveshaft, and filling the new differential.

The rearend was lifted into place and the lower control arms were bolted up. Next, we attached the torque arm and reattached the brakes.

With the rearend installed, brakes bled, and the driveshaft attached, we filled our diff and were ready for break-in.

Break In

The break-in procedure consisted of heat-cycling the differential. We drove the car for 30 minutes, then let the differential cool down each time. We did this for 250 miles. Then, we put another 250 miles on the rearend — driving for an hour and then letting it cool afterward. After 500 miles, the gears are broken in and heat-treated. The last step was to drain the “old” fluid out and top it off with fresh lube.

Final Thoughts

We were amazed at how quietly the S60 functions as well as how easy the installation was. Had we not pressed our own axles together, the installation would have taken maybe six hours. For us, it took the better part of two days working at a leisurely pace. Since the installation, it’s now possibly the quietest thing on the car, and we can’t wait to get it to the track to beat on it.

If you’re looking for a cost-effective way to end your 10-bolt woes, we would highly recommend checking out Strange’s S60.

Article Sources

About the author

Chase Christensen

Chase Christensen hails from Salt Lake City, and grew up around high-performance GM vehicles. He took possession of his very first F-body— an ’86 Trans Am— at the age of 13 and has been wrenching ever since.
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