How to build a 9-inch Ford Rear Differential
When it comes to rear differentials, the 9-inch Ford reigns supreme. The 9-inch Ford is not perfect; the pinion gear rides a little low on the ring gear, which eats up a little more power, but the drop-out style third member that houses the gears and heavy-duty nature make it perfect for high-performance use. The aftermarket for the 9-inch is thriving, there is nothing that you can’t buy to build one, which is where we are at, building a 9-inch for a 10-second drag car.
We knew the housing and axles for a 9-inch would hold up to the 600-horsepower big block, but a stock unit is only good for about 400-500 ponies. Building on a budget meant we couldn’t just go out and buy a ready-made aftermarket unit, we needed to build it ourselves, using a combination of used and new parts. We sourced a stock big-bearing housing on Craigslist for $75. The housing came with a stock iron third member which we considered using, for a minute.
The biggest problem with the stock iron third-member is that the castings are thin, so when you put more than 400 horsepower to it, they start to tweak and flex, eventually, they break and that is bad news on the track. The pinion support is pretty weak too, so we opted for an aftermarket third-member, but to keep it budget friendly, we stayed with a cast-iron unit from Yukon Gear. We had the housing narrowed by our local machinist, Harold Evans, who also narrowed the axles using his custom-made spline jig. Harold is the only person in Oklahoma that splines axles, nobody else does it anymore. Having a local shop that can do this type of work can save you a lot of dough, just in shipping alone. Harold does this type of work the right way, removing the entire axle tube replacing them with new ones cut to spec; instead of just cutting the tubes in half and sectioning them back together. The sectioning-method can lead to off-set and weaker axle tubes.
For the internal components, we went to Randy’s Ring and Pinion. We chose a set of 4.11 Yukon gears with a Yukon install kit that feature Timken bearings. The bearings in a differential are critical, and Timken are considered the very best. The Yukon cast-iron third member came from Randy’s, as well as a new billet aluminum pinion support for a solid, flex-free differential. The cast iron third member is quite heavy but it is rated up to 1200 horsepower, so it should hold our 600 without a problem. To finish it off, we picked up a 31-spline Detroit Locker to make sure we can leave a pair of stripes with ease and still take corners without chirping. Lockers are little noisy on the street, but this is for a drag car that will spend most it’s time on the track. Since we didn’t have all the specs when we ordered the parts, we went with a set of axle blanks. These are full-size axles that have not been drilled for lugs or splined. In most cases, you would have the housing narrowed before you order axles, but sometimes you have to do things a little backwards, and we have the ability to custom spline our axles anyway. In the end, having Harold cut and spline our blanks was cheaper than ordering a set of custom-cut axles from the factory.
Assembling and adjusting the Ford 9” is easier than non-dropout rears, like the 8.8, because you can build it on the bench. That is one of the most endearing qualities of the 9-inch. You will only need one special tool to set up the gears; a dial indicator with a magnetic base. These can be found on the cheap at Summitracing.com. In the end, we spent $1701 for a new rear end that will hold up to just about anything we can throw at it.
Right off the bat you can see big differences between the stock dropout and the new Yukon unit. There are a couple of extra webs and all of the webs are thicker.
The back side is even more dramatic, note the flange on the new dropout is almost twice as thick. The Yukon unit has heavy-duty machined caps with cap-screw bolts for a precision fit.
The bearings for the locker and the pinion must be pressed on. We took them to the local NAPA and had it done for about $5 bucks. Unlike most other rears, the pinion bearing does not require shims, so you can don’t have to do all the trial and error fitment under the car. All of the new parts were cleaned up before assembly.
First, we marked the caps on the dropout. Since these are precision machined, they need to stay on the correct side. Make sure you use a permanent mark like a stamping number, otherwise it could wipe off.
The pinion support was dropped over the pinion, and then a new crush collar and forward bearing installed.
The pinion nut was installed with light pressure, just enough to seat the bearings, but not enough to crush the collar. You don’t need the seal yet.
The dropout has a snap ring that retains the pinion pilot bearing. The ring comes out and then the new bearing can be pressed in and the ring reinstalled.
Each differential bearing cap has a threaded adjustment ring. The ring provides the side-to-side adjustment and the bearing preload for the locker.
We installed the 4.11 ring gear using an impact wrench. The trick here is to gradually tighten the bolts in a criss-cross manner (like you would torque lug nuts). This keeps the ring gear from warping. Once the gear is installed, each bolt is torque to spec at 60 ft lbs.
A little Royal Purple Max-Tuff assembly lube was added to the bearings before dropping the differential into the dropout case.
This old bell housing became quite useful; we used it to hold the third member while the adjustments were made.
With the locker in the dropout, the caps were tightened, but not torqued. We made this adjuster tool from some scrap shelving and a couple of ¼”-20 bolts. This makes it easy to thread the adjusters. Once the backlash is set, the caps get torqued to 80 ft lbs (but not yet!).
The pinion was bolted into the housing without any shims and a dial-indicator was set up to set the backlash. Backlash is the amount of space between the ring and the pinion, and is adjusted by moving the ring gear towards (less backlash) or away (more backlash) from the pinion. You are looking for .0007-.0010” of backlash for a new set of gears. You check this by rotating the ring gear until it touches the pinion gear. It takes a second to figure out, but once you have it, it is easy. We sourced our indicator from SummitRacing.com.
Next a section of 5 or 6 teeth get painted with yellow grease and the ring gear rotated several revolutions. This transfer the contact pattern for the gears. If the pinion gear is too deep, the gears will grind, too shallow and you don’t have enough contact area. Either situation will greatly reduce the life of the gears. This pattern shown here is too close.
When changing the shims, you want to start big and work your way down. We measured each of the supplied shims and moved the pinion out .030” to start.
It took a couple of tries, but we eventually arrived at a nice even pattern that matched one of the acceptable patterns referenced in the supplied guide book.
With the adjustments set, the pinion came out of the pinion support, a new crush sleeve installed and then the pinion assembly was put back together.
This time, the new pinion seal was installed with a little RTV silicone on the outside edge. The inner rubber was lubed up with some Royal Purple Max-Tuff so that the pinion shaft wouldn’t tear it.
This time, we used the impact to tighten the pinion nut to seat the bearings, and then we rotated the pinion support as we continued tightening the nut. Once you feel a little resistance, you are done. You can use an inch-pound torque wrench to check the resistance on the bearing, but none of our friends own one and you can’t rent them. This is how most of the builders we talked to do it.
Next, the new o-ring was placed on the pinion support and the assembly was installed into the third member case.
The pinion support bolts were torqued to 40 ft lbs. We also used a punch to lock the pinion nut in place.
At this point, the narrowed housing has been installed under the car. We could pull it out to install the third member, but we didn’t. A new paper gasket was placed on the housing after a final wipe-down was done to make sure any debris or dust was removed from the inside.
The third member weighs close to 70 pounds, so don’t drop it on your fingers or head while getting it in position, a second set of hands is really good to have. We used our new Ingersoll Rand Hammerhead impact wrench to tighten the nuts. Then we hand torque them in a criss-cross pattern to 40 ft lbs.
Moving to the outside, the new bearing races were tapped in place with a dead-blow hammer. The races should slide all the way in just past the edge of the housing, leaving a small lip.
We hit the backing plates with some Rust Encapsulator from Eastwood and placed them on the housing.
Then the new axles were installed and the bolted on with some new grade-8 hardware. Yes, those studs are huge and may be a little bit of overkill, but that’s OK. We used the small Ford bolt pattern, 5 on 4.5” for the rear, to match the front. All we have to do now is install the brakes.
To secure the Centerline Fuel drag wheels, a set of Mcgard Tough Nuts Drag Race lug nuts were used. These lugs feature an open side (a threaded cap is included) for safety inspection. When slicks are used, the studs have to extend into the lug nuts by at least the diameter of the stud itself; these accommodate that.