Finding a solid ford 9 inch pass through axle for a tandem setup or a specialized off-road rig can be a bit of a hunt if you don't know exactly what to look for. Most people are familiar with the standard Ford 9-inch—it's arguably the most famous rear end in the history of hot rodding and racing. But when you start talking about "pass through" versions, you're entering a whole different world of custom fabrication and heavy-duty engineering.
Usually, you see these setups in 6x6 builds, heavy-duty haulers, or specialized overland vehicles where you need power to go to more than one rear axle. If you're trying to build something with six-wheel drive, you can't just slap two regular axles under the back and call it a day. You need a way to get the power through the first differential and back to the second one. That's where the pass-through concept comes into play.
Why Use the 9-Inch Platform?
You might wonder why anyone would go through the trouble of modifying a Ford 9-inch for this instead of just grabbing a heavy commercial tandem axle. The answer is pretty simple: versatility and strength. The 9-inch is famous for its "drop-out" third member. This makes it incredibly easy to work on compared to an axle where the gears are shimmed inside the housing itself.
For a pass-through setup, that removable center section is a lifesaver. It gives fabricators a much better starting point to create a custom input and output path for the driveshaft. Plus, the aftermarket support for the 9-inch is insane. You can find any gear ratio under the sun, upgraded lockers, and beefy axleshafts that can handle way more abuse than the factory ever intended.
How a Pass Through Axle Actually Works
In a standard rear end, the driveshaft spins the pinion gear, which turns the ring gear, and that's the end of the line for the power—it goes out to the wheels. In a ford 9 inch pass through axle, the setup is tweaked so that the power doesn't stop there.
Typically, this involves a specialized third member or a custom-machined housing that allows a "thru-shaft" to connect to the pinion and then continue out the back of the differential housing. This second output then connects to a short intermediate driveshaft that powers the rear-most axle. It's a clever bit of packaging, but it requires some serious precision. If your alignment is off by even a tiny fraction, you're going to deal with vibrations that feel like the truck is trying to shake itself apart at 40 miles per hour.
The Challenge of the Thru-Shaft
The biggest hurdle with a pass-through design is the physical space. You're trying to cram an extra shaft through a space that was originally designed just for one set of gears. This often means using a high-pinion setup or a custom-cast third member that has an extra set of bearings to support that through-shaft.
Some builders use a "power divider" or a small transfer case-like box mounted directly to the axle, while others go for the more integrated approach. The integrated approach is cleaner and saves space, which is a huge plus if you're building a rock crawler or a desert racer where ground clearance is everything.
Lubrication and Cooling
Another thing people often overlook is how the oil moves inside the case. A standard 9-inch relies on the ring gear flinging oil around to keep everything cool and lubricated. When you add a pass-through shaft and extra bearings, you're adding more heat and more parts that need oil.
Most high-end ford 9 inch pass through axle setups will incorporate some kind of internal oil pump or at least a high-capacity cover with extra cooling fins. If you're planning on long-distance hauling or high-speed off-roading, you really can't skip on the lubrication side of things. Burnt bearings are a quick way to turn an expensive custom axle into a heavy paperweight.
Sourcing and Fabrication
So, where do you actually get one? You aren't going to find these sitting on the shelf at your local auto parts store. Most of the time, these are built by specialized off-road shops or companies that focus on 6x6 conversions.
Custom Third Members
The heart of the beast is that third member. Companies like Currie or Strange Engineering make some of the strongest 9-inch components in the world, and while they might not sell a "pass through kit" as a standard catalog item, their parts are usually the foundation for these builds. You'll likely need a custom-machined case that allows for a rear exit point.
Housing Modifications
Then there's the housing itself. You can't just use a stock Ford housing from a 1970s F-150. You'll need a fabricated housing—often called a "sheet metal" housing—that is reinforced to handle the extra torque of driving four wheels instead of two. These housings are usually braced with extra steel plates to prevent the tubes from bending under the weight of a 6x6 rig.
The rear of the housing has to be modified with a seal and a flange for the trailing driveshaft. This part is tricky because it has to be perfectly centered with the internal shaft. If you're doing this yourself, you're going to spend a lot of time with a dial indicator and a welder.
Practical Applications for a 6x6 Build
Why go through all this effort? Honestly, most of the time it's for the "cool factor," but there are real practical reasons too. In deep mud, sand, or snow, having that extra axle means more surface area and more traction.
Overland builds are probably the biggest market for a ford 9 inch pass through axle right now. People want to take heavy campers into the middle of nowhere, and a 6x6 setup handles the weight much better than a traditional 4x4. By using 9-inch components, these builders ensure that if they do break something in the middle of a desert, there's at least a decent chance of finding replacement gears or seals without needing a helicopter to fly in parts from another continent.
Common Mistakes to Avoid
If you're diving into the world of multi-axle drivetrains, don't cut corners on the intermediate driveshaft. Because the distance between the two rear axles is usually pretty short, the angles on that driveshaft can get aggressive. If you don't use high-quality U-joints and make sure your pinion angles are synced up, you'll be snapping parts every time you hit a bump.
Also, think about your gear ratios. Both rear axles must have the exact same ratio. It sounds obvious, but you'd be surprised how many people grab a deal on a front-rear axle only to realize later that one is a 4.10 and the other is a 4.11. On a 6x6, that tiny difference will cause binding and eventually blow out your transfer case or the pass-through shaft itself.
Wrapping It Up
Building or installing a ford 9 inch pass through axle isn't a project for the faint of heart. It's a complex piece of machinery that sits at the intersection of custom hot rodding and heavy industrial engineering. However, if you want the ultimate in traction and strength for a custom multi-axle project, it's hard to beat the 9-inch platform.
It gives you the peace of mind that comes with "bulletproof" parts, combined with the flexibility to tune your gear ratios and lockers exactly how you want them. Just be prepared for a bit of a learning curve—and maybe a few late nights in the garage with a welder—to get it all dialed in perfectly. But once you feel that six-wheel grip for the first time, you'll know it was worth the extra grease and sweat.