Ford nine-inch diffs have long been the go-to third member for street machiners. Back in the day, if you wanted to put a nine-inch under your car, you had to source a complete diff from a production car and modify it to suit your application.

First published in the September 2022 issue of Street Machine

More often than not, that involved having a fab shop shorten the housing, cut and re-spline the axles and remove the stock bracketry, before hacking the brackets off your OEM diff and fitting them to your used nine-inch housing. Once that was done, you’d end up with whatever gearing, centre section and brakes happened to come with your donor diff – along with whatever wear and tear it had incurred in its life thus far.

With horsepower now cheaper and easier to come by, demand for nine-inch diffs is greater than ever, but second-hand examples are getting thin on the ground. The good news is that the aftermarket is now meeting the demand, and these days you can have a nine-inch built to your specs from brand-new parts and shipped to your door.

For the past 10 years, I’ve had a shortened ex-Commodore 28-spline BorgWarner diff fitted to my LC Torana. It has served me well, but with a new engine in the works aiming for more grunt and lower ETs, I had some concerns about its longevity, and began looking into nine-inch conversions.

My search led me to Castlemaine Rod Shop’s 31-spline nine-inch, which is offered to suit most Holdens, Aussie Fords and Valiants, plus a handful of American models. It is supplied as a bolt-in unit built to order, meaning you can customise the length to suit your application, choose from a full spool or a torque-biasing gear-type limited-slip centre, pick your optimal gear ratio, and select from a range of brake options.

I specced my new diff on The Rod Shop’s website (rodshop.com.au), and SM staffer Kian Heagney then headed along to The Rod Shop to see what’s involved when the team builds a custom, bolt-in nine-inch from scratch using brand-new bits.

STEP 1. ORDERING

Before any fabrication kicks off, you’ll need to let The Rod Shop know exactly what specs you’d like your diff built to. They have an order sheet that breaks down all the juicy options for each diff combo, which they can take you through, or you can order online.

Variables such as diff length, mounting brackets, gear ratio, diff centre type, brakes and even stud pattern are all individualised with each order. In our case, we were after a diff to suit an LC Torana at 1259mm total length (much shorter than standard, requiring relocated shock absorber mounts), with a limited-slip centre, 3.25:1 gears and Wilwood 280mm disc brakes in HQ/Chev pattern.

We’ll explain why we selected those options as we go along.

STEP 2. FABRICATION

Each diff housing is hand-built to order, and The Rod Shop team starts off with nothing more than a bare centre shell.

First, the axle tubes are cut to length to arrive at the customer’s specified overall diff length. Our required length of 1259mm is equal to the shortened BorgWarner diff the car was already running, which had been set up for 15x7in wheels with 33/8in backspace, 225/50/15 tyres and standard wheel tubs.

With the axle tubes cut, the whole shebang is mounted to a diff jig, which in our case is for the factory LC-LJ trailing arm set-up. The Rod Shop has one of these for each make and model of diff package it offers, so the team can accurately build a bolt-in diff in their dedicated diff fab area in around a day.

With pre-made trailing arm brackets on hand, the fabricator skilfully MIG/TIG-welds the brackets and axle retainers.

STEP 3. STRAIGHTENING

Even though Matthew takes his time when welding onto the housing to allow for cooldown, pouring that much heat into the axle tubing can still lead to some warping.

For that reason, after the housing is complete, he runs it over to a massive press where the axle tubes are checked for any warping and straightened where required.

STEP 4. PAINTING

The Rod Shop churns out a huge range of handmade parts and kits each day, so there is an on-site spray booth and painter, both of which are kept plenty busy.

This is where all the diff housings are taken to have a fresh coat of GM-H chassis black applied after they’re done with fabrication. From there, it’s off to final assembly.

STEP 5. AXLES

All Rod Shop rear ends come with 31-spline axles, which are cut to length and have the desired stud pattern drilled during the assembly stage of the build.

The axles are fitted to the housing and the overhang is measured to determine the required axle length; the axles are then cut to size. The edges of the splines are then lightly ground down to remove any burrs, and the bearings are pressed on.

The wheel studs are also pressed in – Ford, HQ/Chev, Commodore and early Holden are the available PCD options. Broads runs Center Line Convo Pro wheels in HQ/Chev pattern on the Torana, so we ticked that box.

STEP 6. CENTRE

The centre sections for the diffs come to The Rod Shop as a complete, drop-in unit for their handmade housings. You can choose either a full spool or the torque-biasing, gear-type LSD centre we opted for in the LC.

It’s the most common choice for street cars, while the spool is better suited to drag and burnout cars. There are plenty of gear ratios to choose from, too: 3.0:1. 3.25:1, 3.5:1, 3.7:1, 3.89:1 and 4.11:1.

When deciding on a ratio for the LC, Broads pulled out the trusty Moroso Power-Speed calculator and found that 3.25:1 gears would allow enough headroom for roughly 137mph at the track on 225/50/15 (24-inch) tyres at an anticipated redline of 6500rpm – which will be ample. If you don’t have a Moroso slide handy, there are plenty of online calculators to help you select a suitable ratio.

The diff centre is installed in the freshly built housing using copper crush washers and nyloc nuts, all torqued to spec.

STEP 7. BRAKES

Brakes are the last piece to find their way onto the diffs before shipping. Customers are given the choice of either drum brakes or Wilwood discs, which use four-piston calipers and can be had with either a 280mm or 320mm rotor.

Broads opted for the smaller of the two disc options, because the front discs installed on the car are 290mm in diameter. He also went with black calipers, but customers can opt for red (pictured) if they prefer. Disc-brake packages include a smaller drum-style handbrake mechanism inside the disc, but twin calipers can also be ordered.

STEP 8. FINISHING & SHIPPING

With brakes done, the final touches like bushes and stickers are fitted to the diffs before shipping. The only thing customers have to do is fill the diffs with oil, as shipping regulations mean they have to be freighted dry.

On a good day, a team of two at The Rod Shop can finish three to four diffs between them with all the parts on hand, and even with all the shortages going on right now, they’re still pumping them out.

“We did 300-odd last year, and we’re projecting to do 400 this year,” says The Rod Shop’s Heath Waddington. “Sometimes the wait times drag out due to parts and material shortages, but we build them as fast as we can get the bits, and we’re always flooded with orders.”

STEP 9. FITTING

The neatly packaged nine-inch was shipped to MPW Performance, where the latest round of upgrades to Broads’s Torana were being carried out.

Basic fitment was as simple as unbolting the existing diff from the factory trailing arms and bolting the nine-inch into place. Everything lined up perfectly, with the only jobs left to sort being the brake lines, handbrake cable (Wilwood cables are available from The Rod Shop) and of course the tailshaft.

Due to the diff being significantly shorter than standard, our application also called for the shock mounts to be moved inboard to avoid caliper-to-shock clearance issues. It’s something Broads was able to avoid by running drum brakes on the previous diff, but was happy to deal with this time in order to accommodate discs.

The Rod Shop will be able to tell you at the time of ordering if your desired diff length will require any such mods.

NICK OF TIME

It’s a good thing that Broads made the call to upgrade to the nine-inch from his old 28-spline BorgWarner when he did.

While removing the Borgy to fit the new diff, he popped the passenger-side axle to see what shape it was in. Fairly twisted, as it turns out! The car had been powered by a stock Holden 5.0L with a Harrop blower making 300rwhp on pump fuel, and had a PB of 11.4@116mph. With a built engine, more boost, a switch to E85 and a transbrake-equipped Powerglide in the pipeline as we speak, there’s no doubt that the Borgy was living on borrowed time!