Building A Currie F9 Axle For Project Storm Trooper

housing build

We all dream about what our perfect project would turn into with a detailed list of parts installed. Everything is perfect, but all we can do is dream, wondering what exactly our vehicle will look like completed.

When one of those dream parts gets talked about we feel like a little kid inside. That was the case with Project Storm Trooper, and the new rearend we always dreamed of having, a Currie Enterprises F9 fabricated housing.

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Not only were we going to be getting an upgraded rearend for the back of our street legal prerunner build, but we were going to be able to watch it get built. We recently upgraded the front suspension adding new Bilstein coilovers, long travel, and Eibach springs, but the time had come to move to the rear of the truck. The rear had new leaf springs, but the factory axle with over 150,000 miles on it was not going to cut it in the desert.


Knowing that we would be getting to spend time with a company that has been in the roots of the off-road world was mind blowing. It was more than just a part, but the family and legacy behind the name.

The Currie family has been doing off-road for what seems like forever and carry a name that is tied to almost all forms of motorsports. “It was built with knowledge gained by the Currie brothers over 45 years of off-roading and off-road desert racing experience,” Brian Shepard, from Currie Enterprises explained. “Everything is purpose built for extremely aggressive, abusive off-road racing applications, while still looking bitchin’ for car guys with bad ass musclecars that use them too.”


From The Ground Up

On the drive up the 15 freeway, we could feel the butterflies in our stomach as it felt like we were about to meet the girl of our dreams. Once at the shop, it felt like being a little school girl at a Justin Beiber concert, the time had come to see a part that we dreamed of.

The first step of the process was to cut down the axle tubes before they were notched to interlock in the center section.

The day started with gathering the materials for the housing. The center section on the F9 housing was preassembled and made from high-form 50 steel. High-form 50 steel is a high strength, low alloy steel that finds its best application where there is a need for more strength per unit of weight. Less of this material is needed to fulfill given strength requirements than is necessary with regular carbon steels.

Our F9 Axle Specs

  • 70 inches WMS to WMS
  • 3.25-inch axle tube
  • 35-spline axles
  • Eaton Truetrac
  • Motive 4.57:1 gears
  • Wilwood brakes
For our application, we went with a 3.25-inch outside diameter, .250-inch wall tubing, but Currie offers five different tube diameters for the F9 housing ranging from 3-inch to 4.5 inches. The axle tubes are made from mild steel.

The center section of this housing is what makes it so strong. “The center section has a 3/8-inch thick third member mounting face,” Shepard explained. “There are two gussets on the top and two gussets on the bottom from the back of the face to the inside of the housing. The tubes get machined on the end so that they key and lock into the bulkhead inside the housing center.”

With the center section and axle tubes tacked together, the back bracing could be custom made to fit the housing.

“We found in housing testing years ago, that a fully back-braced Ford housing would eventually buckle at the shoulders when tested in a 50-ton press,” Shepard said. “Where they would fail is where the bulkhead inside the F9 housing centers are.”

Before the back bracing gets welded, the housing is placed into a hydraulic press. The press puts a bend in the housing the opposite direction that the bracing will pull when welding.

With the general axle shape started it was time to work on the back bracing. Back bracing increases the strength and durability of the axle. The back braces come in standard sizes, but are hand fabricated to fit every axle. Each one gets touched to make sure that the brace fits perfectly.

With the housing getting finished up we headed to a different place in the shop to have our third member built. Our third member would be built with an Eaton Truetrac and 4.57:1 Motive Gears.


We decided to go with the Eaton Truetrac because the truck would see both pavement and dirt. We spoke with Barney Gwozdz of Eaton to find out more on why the Truetrac is beneficial in a prerunner application.

Prerunners are required to quickly negotiate through difficult terrain,” Gwozdz said. “A differential must be smart and efficient to react to the demands of the driver. This scenario is perfect for the automatic torque biasing feature of the Truetrac.”


Bearings are pressed onto the Truetrac.

“The Truetrac’s helical gear design provides optimal torque bias capabilities,” Gwozdz continued. “This efficiently benefits the vehicle while on unequal terrain because of how the torque is directed to the wheel having the most traction. In the scenario of having one wheel on dirt and the other on a solid surface, the majority of the torque will be directed to the wheel on the pavement, while simultaneously driving  the wheel on the dirt – Thus propelling the vehicle as fast as possible.”

With the bearings pressed on, the Motive gears could be assembled.

In combination with our differential, we wanted to find out more about the gears we selected to use in our application. Steve Filipiak of Motive Gear was able to help us answer some of the questions we had about selecting the right gear. “The combination of materials, computer controlled heat treating, and tooth cut geometry all go into creating a gear that can withstand the abuse,” Filipiak explained.

With everything assembled, the gears were spun to make sure they meshed correctly.

“Off-road racing and recreational driving deliver extreme abuse to your vehicle’s drivetrain, particularly your ring and pinion gearset,” Filipiak said. “Motive Gear Performance ring and pinions feature first run SAE8620 material. We never use recycled steel. Our gears are designed to withstand high horsepower and survive under extreme conditions. Our ring and pinions are dual-polished to create the smoothest tooth surface assuring a quiet, cool running gear that sets up easily.”


Mocking Up And Prepping For Install

Within a day the axle was taking shape right in front of our eyes, but as we teased before, we took the unfinished housing to do some final mock up. When working with something this custom, making sure the final product fits the first time is crucial.

Measurements and angles were taken from the factory and transferred to the new housing (right). The Bilstein bypass shocks were mocked up to check for any clearancing issues.

We needed to make sure that our spring perches, shock mounts, and pinion angle all worked once everything was finished. We used measurements off of the factory axle to match pinion angle and spring perches, but needed to check clearance on the frame and tire to make sure shocks would clear. From the factory, the shocks are inside the frame and with everything all said and done, they would be moved to the outside for more clearance.


With the help of our Miller welder, we were able to get the brackets tacked to the housing and ready to return to Currie.

A half day on the Bendpak lift and a Miller welder later we were ready to head back to Currie and finalize the reared build. The axle would go back under the knife to have the holes for the U-bolts cut by a plasma cutter before it was finish-welded.

With everything back at Currie, a plasma torch was used to cut holes in the bracing for the U-bolts. With everything completed the housing was able to be completely welded.

With the outside of the axle completed we moved to the interior and the axle shafts. These axle shafts are induction heat treated 1541 forged alloy steel that had 35 splines cut in them. They are the largest diameter shafts Currie makes, but could also be cut into 40-spline axle shafts.

Axle shafts were cut to length and then the correct spline count was added. The studs were also pressed in while our blank rotors were cut with our lug pattern.

Our 6×5.5 lug pattern was cut into the axles and the rotors as they came blank from Wilwood as part of our brake kit. With all the pieces finished it was time to put the whole rearend together.

The light could be seen at the end of the tunnel. Final assembly put everything into its final home.

Bearings were pressed on as well as the wheel studs before being slid into the housing with the third member already installed. The pinion was spun to make sure everything worked without binding. The dawning of the red “C” fill/inspection cap signified the completion of the rearend.



Building A Custom Rearend For Your Application

People can get lost in the terminology and measurements needed to make sure that a product will fit correctly. The best thing to do is ask questions. Whether it is something you do not know or want to know why it needs to be done a certain way, just ask. It will help you better understand the product that is going under your vehicle.

Build for your application, not for what your buddy said. – Brian Shepard, Currie Enterprises

The folks over at Currie are there to help and recommend the correct setup. “Our recommendation is definitely to build for the application,” Shepard said. “Everything can be discussed with a Currie sales and tech staff upon ordering so what is needed is built not, ‘oh my buddy said.’”

One key part is to look at the end goal of the project. It may be far away, but it is better to consider it now than to have to go back and replace parts again. All options should be considered from back bracing and flanged axles to full floating axles.

We would like to thank Currie Enterprises for the opportunity to head to its shop and watch the product get built. It definitely adds a new appreciation for the work they do watching it done step by step.

Up next for Project Storm Trooper is finishing up the rear with a cage, bypass shocks, and a fuel cell. Stay tuned for more updates on the project!


Article Sources

About the author

Steven Olsewski

Steven Olsewski grew up with a true passion for anything with a motor. He loves his wife and kids, and during the year can be found enjoying quality time together. They are a huge part of his life and their passion for God.
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