– Building A 8.75 Rearend With Sure-Grip –

Ever since I got my ’72 Fury, I’ve had my eye on upgrading that old 8.25 rear end with something beefier—specifically the Mopar 8.75. The original setup came with those sluggish 2.76 highway gears, which made burnouts nearly impossible. And when I did manage to pull one off, only one wheel spun—far from the tire-shredding action I had in mind.

So, it was time for a serious upgrade. I set out to build a proper 8.75 rear end, complete with a Sure-Grip unit and some higher gears. One of the most appealing things about a Mopar 8.75 rearend, besides its ruggedness, is the drop-out center-section carrier. With this design, ratiochanges can be made with relative ease and speed. And, setting up afresh gearset can be done on the bench rather than under the car.

I tracked down a used 8.75 axle along with the strongest 489 carrier case, then rebuilt it with all-new components: a Sure-Grip differential, fresh 3.23 gears, and new axles.

And to top it all off—disc brakes! With this, my Fury is ready to tear up the road the way it was always meant to.

Part 1:

– The 8.75 Housing –

My New 8.75 Rearend, Sandblasted Already

And The Strongest Of Them All: The 489 Carrier

I started by tracking down the right axle, since C-Body models like my Fury has one of the widest rear axle lengths Mopar has ( 57 7/8 till 59 7/16) . That meant I needed one from another C-Body model, or it wouldn’t fit properly. Luckily, I found the perfect match nearby through a quick search online, along with a 489 carrier.

Once I got my hands on them, I gave both parts a fresh start—sandblasting them down to bare metal and finishing them off with a sleek coat of epoxy primer.

Axle and Carrier Combined. Check Out That Old 8.25 In The Background

Part 2:

– Installing The Sure-Grip Unit –

I recently got my hands on a brand-new Sure Grip unit from Sixpack Speedshop in Holland, along with a complete bearing set, crush sleeve, and a fresh 3.23 gearset to match. As I was working on it, I noticed something important: the bolts attaching the 3.23 gear to the Sure-Grip unit are reverse-threaded, meaning they tighten counter-clockwise.

So, if you’re planning to remove your old ring gear, be cautious! If you turn the bolts the wrong way, you’ll risk snapping them. Definitely something to watch out for.

Sure Grip is the Chrysler name for a limited slip differential

3.23 Gears Installed On The Sure-Grip

The installation process was smooth and straightforward. The only challenge came with the crush sleeve, where precision is key—getting the right balance between too loose and too tight is essential. It’s important to note that the crush sleeve is exclusive to the 489 carrier, while the 741 and 742 carriers rely solely on shims for adjustments.

A crush sleeve gives you just one chance: if you over-crush it, you’ll need to replace it.

A Complete Bearing Set Including The Crush Sleeve

The Pinion Gear Where The Crush Sleeve Will Go

Measuring The Crush Sleeve Pinion Preload

Part 3:

– Dropping The Unit Into The Axle Housing –

A Clean And Empty Axle Housing

The complete unit in the axle housing

With the 3.23 ring gear now secured to the Sure-Grip, it was time to install it into the freshly cleaned axle housing. We carefully fitted brand-new bearings and tightened everything to precise torque specs, ensuring a smooth and reliable assembly.

MOPAR 8 3/4-INCH SETUP SPECS
COMPONENT	SPEC
Ring gear bolts	55 ft-lb
Main cap bolts	90 ft-lb
Pinion preload (89) new bearing	14-19 in-lb
Pinion preload (89) used bearing	6-9 in-lb
Total preload (89) new bearing	30-35 in-lb*
Ring gear backlash	0.006 to 0.010 inch

Getting The Bearing Caps On

Torque Everything To Spec

PART 4:

– DIALING IN THE RING GEAR PATTERNS –

Mastering a differential setup hinges on making precise adjustments. Key parameters like pinion depth, backlash, pinion-bearing preload, and carrier-bearing preload let you fine-tune the alignment of gear teeth, ensuring they mesh perfectly for maximum durability. While it requires precision, it’s entirely manageable if you take your time, and all the necessary values can be found online.

Nailing the setup of your ring and pinion gears is crucial to the performance and longevity of your drivetrain. Get it right, and you’ll enjoy smooth, reliable miles for years to come. Get it wrong, and you could be facing serious issues and worn-out gears in no time.

Gear Marking Compound On The Ring Gear

Measuring Side Lash

Measuring Gear Depth

PART 5:

– INSTALLING NEW AXLES –

I recently bought new axles from Dr. Diff, along with all the required bearings and hardware. When I checked my old axles, I noticed that someone had welded the bearings onto them, which was one of the reasons I decided to go with new ones. I’m not sure why they did that—if anyone knows, feel free to share in the comments!

Another thing that caught my attention was that the new axles have significantly longer spline teeth compared to the old ones. The more the better, right?

My Old Axles: Seemed Someone Did Some Welding On Them

Old vs New Axles

The New Axles Had Much Longer Spline Teeth Than The Old Ones

PART 6:

– 11.25 Inch REAR DISC BRAKES –

With all the upgrades in place, I decided it was time to ditch the drum brakes, so I picked up a 11.25 rear disc brake kit from Dr. Diff. These are likely Ford brakes originally, but they’re designed to bolt directly onto the 8.75 axles, making the swap a breeze.

The kit even includes brake shoes for the handbrake/parking brake, with the disc fitting perfectly over them. After connecting the brake hoses, I welded the hose connectors straight onto the axle, ensuring everything is solid and secure.