THE WHOLE NINE YARDS

BUILDING A CUSTOM BOLT-IN NINE-INCH DIFF FROM SCRATCH WITH BRAND-NEW COMPONENTS

-  ANDREW BROADLEY & KIAN HEAGNEY

FORD nine-inch diffs have long been the go-to third member for street machines. 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. 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 geartype 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.