How to put a 9-inch diff into your overpowered nugget

While it is rad to go to a junkyard today and get an engine which you can then turbocharge using Electro Space Wizard powers to make tar-shredding performance, there are some areas where you can't escape spending money. A wrecking yard diff will handle abuse up to a certain point, but when you start making four-digit power figures you really need to build a differential to suit your car.  

In this case we follow Matt Dietrich from Geelong Diff Services (in Geelong, Victoria, in a shocking revelation) as he walks us through how to set-up a bulletproof live-axle rear-end for my '64 Pontiac packing a Harrop-supercharged 6L LS (SEE VIDEO HERE).

The new diff build actually starts with a bare housing modelled on a Ford nine-inch. The old diff is physically larger, but doesn't have good aftermarket support for new parts, so building something fresh makes more sense. 

The Ford 9in is used in countless cars, having originally been fitted to trucks (like F100s) and muscle cars back in the day. There are alternative diffs available, but for ease of sourcing parts in Australia it is hard to go past a 9in. 

“The days of the F100 nine-inch conversion are pretty much over, with the aftermarket being so large for these assemblies," offers Matt. "You can buy everything brand new, so you don’t need to go through rebuilding a tired old F-truck diff to suit your car."

Once all the brackets are measured, and tacked in place with the right pinion angle (2-degrees up in this case) we're ready to build the diff. To set the pinion angle you get the diff to ride height and use a magnetic degree wheel to show when it is angled correctly - this is important as the gearbox and diff need to match these angles. 

It is important to note you can't go hog wild and start gluing everything on because you need a jig to hold the diff so it doesn't warp when you weld it up.

When it comes to providing a diff builder with all the info he needs, Matty just needed to know where the brackets had to sit, and the distance from the mounting surface on the inside of each rear wheel. You can't set the diff up without knowing what size wheels and tyres you're going to run, and what offset/backspace wheels, especially.

Once that was all confirmed Matt TIG-welded all the brackets on, as well as the axle flanges. The diff was then given a lick of fresh black paint, though some people powdercoat their housings these days as well.

"The only time a diff assembly will fight you is if you use poor quality parts,” says Matt. “We’re using good gear in this diff, like the Strange Engineering carrier, Wilwood disc brakes, HARROP TrueTrac LSD, 35-spline billet axles, and Italian Motive gears, and this is really the tried-and-true combo for a 1000hp street car today.” 

"I have spent time mocking everything up and checking many of the individual components, but I start building the diff with the centre section," Matt explains. Any diff build comes down to three basic areas: the housing, the axles, and the centre section. 

The front of the centre section features a pinion gear (shaped a bit like a pine cone), which converts the spinning force of the tailshaft onto the large crown wheel (the big round gear), and the crown wheel is responsible for turning that drive sideways through the Truetrac LSD, onto the axles, which turn the wheels.

In the same way that most of the work in an engine build is in setting tolerances, clearances and checking alignments of spinning components, this is where much of the science of diff building comes in. Ensuring all the gears mesh correctly and the bearings have the right amount of preload on them is paramount to having a diff that will live a long life and work how it is intended.  

Aftermarket centre members like this Strange Engineering nodular iron unit have a few benefits, like larger bearings for the pinion to ride in compared to a factory 9in from a production car. Before the pinion gear is fitted into the centre Matty checks the preload on the bearings, before the upgraded 1350 uni is torqued down onto the pinion. 

Typically, most old stock diffs come fitted with 1310 yokes. However, many street machine diffs are upgraded to 1350 yokes, which are the largest able to be fitted to a nine-inch. Matty says they’ll be perfect for 1000hp street cars, though a 1410-series yoke from trucks as well as different coupling formats, are available for those wanting to get even beefier.

Before the pinion assembly is fitted for the final time a metal depth-shim is fitted between the pinion and the case, as this sets the correct location of the pinion gear. Before this can be ticked off the crown wheel assembly needs to be fitted and the gear mesh checked. 

“Once the pinion gear is assembled in the pinion support, it is time to look at the carrier assembly, which includes the limited-slip centre and crown wheel. This diff uses an Eaton TrueTrac from Harrop Engineering, which is a helically-geared torque-biasing LSD. With equal grip and under power the helical gears bind together and lock solid, but when you’re off the power it drives like a stock open-wheeler.”

The crown wheel is bolted to the TrueTrac using a minimum Grade 8 bolt. Matty also only uses Timkin bearings in his diffs due to the standard of quality from them.

“One of the benefits to running a nine-inch diff is it has third bearing, a pinion support bearing, in the centre housing," he says. 

Screw adjusters set eight-to-12-thou of backlash of the crown wheel, which is the fore-and-aft rocking in the housing. Matt dry assembles the diff in stages, which allows him to double and triple-check clearances as he goes and spot any issues early.

The crown wheel and pinion are responsible for setting the vehicle’s final drive ratio, and this 9in will have a 3.25 ratio for good highway cruising and low-end torque. A car being used at the drags would typically run a more aggressive 3.9 or 4.11 gear to get a better launch.

Gear marking paste is then used to check engagement between the pinion and crown wheel gears. After turning the diff by hand the pattern left on the teeth tells Matt whether he needs a different shim in the pinion (to move the pinion forward or back), or to adjust the backlash on the crown wheel (moving it left or right).

After checking the pattern left by the paste the pinion shim is swapped from a 28-thou item to a 31-thou item, and then it is rechecked. The backlash is then triple-checked with a dial indicator to ensure it only reads 10-thou, before the carrier bolts are torqued-up and locking plates fitted, completing the centre section.

“This diff will run 35-spline axles made out of 4140-grade steel, as anything running more than 700hp at the crank should go to a 35-spline axle because you can’t beat the diameter of the spline itself,” offers Matt.

As the axles come pre-drilled with Ford and Chev pattern, so Matty has to make sure he gets the studs in the right holes. Once the axles are cut to the correct length and the wheel bearings have been pressed on, they’re treated to a lick of spray lubricant before they are slid into the housing.

Before the axles are slotted in Matty fits the centre section to the housing, and this is generally done at the end of a work day to give the spooge time to set.  

“Original nine-inch diffs run a 20-thou gasket between the centre and the housing, but most aftermarket housings are set-up to simply run a thin bead of silicon, which I find is better,” Matty says. “The centre can then be fitted and torqued down.”

“This diff will also use a competition-grade, Strange Engineering screw-in half-inch wheel stud, which is a decent upgrade over the standard 7/16” wheel studs many old cars use,” Matty says.

Finally, once the axles are sitting in the housing the Wilwood disc brakes can be fitted. These items are the model with in-built handbrake assemblies, while race cars can afford to use kits which do away with the parking brakes.

All up the process took a few hours to assemble over two days, but this follows quite a lot of set-up time and it is this preparation that Matt finds is key to getting a good result. 

“Diffs will be set-up for different power levels and different uses of the car,” explains Matt. “Someone building a Sunday cruiser compared to a person wanting to drag race and this affects the way we set the hardware up."