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9-inch Ford-style Rearends - Anatomy of a 9-inchWhat Goes Into Building Hot Rodding’s Most Popular Rearend? From the August, 2011 issue of Rod & Custom By Kev Elliott Photography by Rob Fortier
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When it comes to aftermarket 9-inch Ford-style rearends, we can pretty much guarantee the first company name that comes to mind for any hot rodder is Currie Enterprises. For good reason too, as the company has built tens of thousands of rearends over the past 50-plus years, and especially since their axles became synonymous with the performance aftermarket in the ’80s. Today Currie can supply an all-new 8-, 9- or 8.8-inch rearend as well as 12-bolt GM and Dana 44 and 60 rearends for hot rods, performance applications, or for off-road use. We were keen to find out exactly what goes into building a 9-inch rearend, given that it’s possible to purchase a 9-inch from Currie that uses absolutely no original Ford parts with the exception of the brake components, if so desired. Here are the component parts... Here are the component parts for the axle housing, a new round back, early style centersection, new tubes, and large Torino-style bearing housings. Partly driven by supply and demand (when was the last time you saw a wrecking yard full of 9-inch rearends?) the company undertook to produce its own axle center housings, gear cases, yokes, and differential carriers. Sold under the name 9-Plus, these products are brand-new manufactured components and assemblies, meeting or exceeding OEM specifications. The housings and covers are made from high tensile steel, almost 40 percent stronger than conventional 1010 low carbon steel. The rearend we’ll follow from component parts to ready-to-ship item uses a new hot rod early style round back housing, with 3-inch-diameter tubes (0.188-inch wall thickness), Torino-style bearing housings, and a new nodular 9-Plus Sportsman gear case, capable of withstanding around 600 hp. Building the housing to a customer’s required width (which previously would have been referred to as narrowing an axle casing before new parts were used throughout) is the “easy” part, as assembling the differential is somewhat more time-consuming and technical, yet the team at Currie make it look simple. Building several every day will do that though! Incidentally, all the component parts are available individually, should you want to build your own axle, though we’d be inclined to wait for the UPS driver to show up with a crate rearend strapped to a pallet, ready to go!  The axle housing is set in...  The axle housing is set in the jig and a gear case installed with collars in place of the bearings. With a bar placed through the collars, the axle tubes and bearing housings, also with collars in place of the bearings, can be installed on the bar, which aligns all four bearings. The gear case is set level, as are the bearing housings.  With everything leveled, the...  With everything leveled, the axle tubes are tack-welded to the center housing and the bearing housings.  The entire assembly is then...  The entire assembly is then moved to a slowly rotating jig for final welding.  This heavy-duty press is used...  This heavy-duty press is used to perfectly align the completed housing should any warpage occur during the welding process. It may look simple, but experience counts for a lot here, knowing just how much and where to apply pressure.  The completed housing, now...  The completed housing, now media blasted, is drilled for a vent.  Here are the all-new component...  Here are the all-new component parts for the third member laid out on the bench. The 9-plus Sportsman gear case will soon be home to a 4.11:1 ring-and-pinion on a 9-Plus open carrier and a new yoke.  Assembly begins with the tapered...  Assembly begins with the tapered bearings pressed onto the carrier.  The ring gear is bolted to...  The ring gear is bolted to the carrier and torqued to 65 ft-lb.  With the bearing pressed onto...  With the bearing pressed onto the pinion …  … it’s ready to have the pinion...  … it’s ready to have the pinion support installed. This is done using one of a number of specially fabricated drifts found throughout the Currie assembly area.  The pocket bearing is installed...  The pocket bearing is installed in the gear case.  The 9-Plus carriers are supplied...  The 9-Plus carriers are supplied with caps, cap bolts, billet spanner nuts, and spanner nut retainers. The bearing cap surfaces are dressed with a fine file prior to assembly to ensure a perfectly flat surface.  After installing the bearings,...  After installing the bearings, caps, and spanner nuts, the cap bolts are torqued down to 85 ft-lb.  Backlash is set using a dial...  Backlash is set using a dial indicator gauge. This particular 9-inch is set to 10/1000 to 12/1000 of an inch, but backlash depends on the gear manufacturer’s recommendations.  Gear marking compound is applied...  Gear marking compound is applied to the ring gear in preparation for establishing the contact pattern between it and the pinion gear.  Spinning the pinion using...  Spinning the pinion using an electric motor, and hence meshing with the ring gear, will indicate the contact pattern.  When the pattern is set correctly,...  When the pattern is set correctly, the ring gear will look like this. Adjustments are made by re-adjusting the backlash, using the spanner nuts, or in some cases, by shimming the pinion in the support. The pattern should be central on the ring gear teeth.  The yoke is installed on the...  The yoke is installed on the pinion to complete the third member assembly, and tightened with an impact wrench.  While the third member is...  While the third member is being assembled, the casing is placed in this jig, a fixture dropped onto the carrier bolts, and the axle shafts checked for length. They should butt up to the fixture. This axle uses Currie’s new billet 31-spline shafts.  Once happy with the length...  Once happy with the length of the shafts, the wheel studs are pressed into place.  Another dedicated drift is...  Another dedicated drift is used to install the seals in the bearing housings.  With a bead of RTV sealer...  With a bead of RTV sealer on the housing, the third member gasket is installed, followed by another bead of RTV.  Third members are heavy, and...  Third members are heavy, and with the number Currie installs, this hoist is a back saver!  All-new brake assemblies are...  All-new brake assemblies are installed next. These are 11 x 2 1/4-inch drums, the assemblies sourced directly from Ford, though Currie manufactures their own drums. The brakes use the Aftermarket Brake Configuration Standard (ABCS), which was developed to provide interchangeability, meaning 11-inch Explorer and 11 5/8-inch Mustang discs, as well as Baer, Wilwood, and SSBC discs can all be directly swapped for the drums.  Collets and bearings are pressed...  Collets and bearings are pressed onto the shafts, but not before the retaining plates are installed!  With a little grease on the...  With a little grease on the bearings, the shafts can be slid into place, and the brake backing plates bolted up. The retaining plates use the same studs.  Occasionally a shaft may be...  Occasionally a shaft may be a whisker too long, in which case it’s shaved slightly as shown.  Ta-da! The completed rearend...  Ta-da! The completed rearend on its shipping pallet, ready for collection.  In addition to the round back...  In addition to the round back hot rod housing, Currie also supplies heavy-duty housings, fabricated housings in steel or chromoly, as well as fabricated housings for independent rearends using 9-inch third members, and aluminum center housings. Rear braces, bracket kits, axle pads, and more are also available to fit 9-inch axles in non-Ford applications.
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