Performance cars have limited-slip differentials, period. The "One Wheel Peel Deal" isn't any more acceptable than running 17s in the quarter-mile. Unfortunately, it seems the only way to get both hoops to hook is by spending a bunch of money. We've been led to believe the solution for a worn-out limited-slip is ordering a new replacement and junking the worn unit.
If just the clutches are worn out, you can replace them for about a tenth of the cost of a new limited-slip unit, while a complete internal kit still saves about $100 over the cost of new. Plus, it's so simple to rebuild a differential, anyone can do it.
Fortunately for Ford enthusiasts, the 9-inch rear was under just about everything, including trucks. While junkyards aren't brimming with muscle cars anymore, they are stuffed with old pickup trucks, many of which have a limited-slip differential, so good buildable cores are common.
A hydraulic press was the only specialized tool we used for the rebuild, though a creative application of C-clamps or pulling the halves together with nuts and bolts in place of the ring gear would also get the job done.
We sourced our clutch kit from Jay Prosch-Jensen at Ultimate Driveline for less than $50, while he has compete internal rebuild kits for around $300. With our kit in hand, we rebuilt a four-pinion, 28-spline Trac-Lok differential in a few hours. And truthfully, if we hadn't been taking pictures, it'd have gone even faster. This is honestly a project even a novice can tackle.
Prosch-Jensen points out that a factory 9-inch differential is good only up to about 350 rwhp under hard shock loads (like dumping the clutch) with wide or sticky tires. More power than that and the differential cover is prone to cracking along the ring gear bolts around the full circumference of the unit. Spending the money on a brand-new differential usually gets you a much stronger billet steel case that can handle more trauma. Upgrading the factory unit to bigger 31-spline axles helps protect against snapped axles or stripped axle splines, but it doesn't address the inherent weakness of the factory cast case. If we were planning on drag slicks or sticky tracks, we'd be concerned. But we doubt a restored car with a warmed-over engine will see that much abuse, making rebuilding our Trac-Lok for under $50 tough to beat.
1 First, break loose the two counter-sunk Phillips screws holding the case halves together. A screwdriver will cam-out the heads; an impact driver prevents that.
2 The case halves are under pressure from the springs inside. A hydraulic press keeps the halves together while removing the two screws, and it controls the separation.
3 The clutches are in the cover. The shim goes against the back of the clutch hub, with the clutch pack being dogged to the cover by ears and half-round locks, and locked to the side gear with teeth. The thickness of that shim is what determines preload and break-away torque (more on that later).
4 Scorched fluid and burned clutches tell us this unit was abused, or at least neglected.
5 The new clutch and steel plate kit from Ultimate Driveline includes four clutch discs, four steel plates, and one "composite" plate: a steel plate with clutch material bonded to it.
6 Soak the new clutches in friction modifier, also called anti-chatter or Posi additive. Ford, GM, and Mopar carry it, as do parts stores. Jay Prosch-Jensen at Ultimate Driveline recommends the OE stuff and prefers Ford's version—high praise from a die-hard Mopar man! The stuff smells horrible. Get it on your hands or clothes, and you'll wear it for days. Coat both sides of the discs liberally and soak them for at least an hour. More is better.
7 Taking apart the differential side, the shim goes between the side gear and the clutch hub's side gear.
8 Under the side gear are the spider gears, the center block, and the preload plate on top of it. This is a four-pinion carrier, which is a bit of a misnomer, as there are really only three pinions: two short and one long.
9 Lift off the plate and we find four preload springs. Some people use these to dial in breakaway torque, but Prosch-Jensen says this isn't the place to do it. The block has two beveled corners and two raw corners: the sides of the plate wrap around the bevels.
10 The pinion shafts are held in place with roll pins, knocked out from behind.
11 Another sign of abuse was needing to hammer the pinions out of the case—the burned fluid made them stick. Use a brass drift or you'll ruin the pinion shafts.
12 After the block and spiders are removed, the side gear is exposed.
13 The side gear comes out, along with a shim that rides between the gear and the case.
14 The shim was nearly a deal-breaker for our rebuild. Note the gouges and burrs, also caused from old or burned fluid and abuse. This one was marginal; Prosch-Jensen gave us a good used one for our rebuild. If there was any damage to any other component, we would have gone with the full rebuild kit, which includes new spiders and shims, and upgraded to 31-spline internals. We took everything apart for inspection and cleaning, then simply reassembled it, using a little gear lube on the pinions to slide them home.
15 This shim goes between the clutch pack and the clutch hub and sets breakaway torque pressure. Shim kits are available, and the master rebuild kit comes with a set. We reused the original. Machining tolerances on the case and other internals, plus the thickness of the clutches, determined the thickness of the shim; with a stock rebuild, the only variance is the new clutches, so the stock shim matched to the original parts is fine in most cases.
16 The new clutches and steel plates get stacked on the toothed clutch hub, alternating clutch and steel. The composite plate goes onto the side gear first; the steel plate rides on the shim.
17 Install the locks, and the clutches are replaced. We hit the heavy wear marks inside the hub with some emery cloth to knock down any burrs or rough spots.
18 The toothed clutch hub is driven by the clutches; the other half of the clutch hub assembly is the side gear. The inside diameter of these two pieces engages the axle splines, and they must be aligned, much like an engine's clutch, before the pressure plate is tightened.
19 You'll need a set of sacrificial axles. One needs flats ground into the end to lock into a vise; the other needs a large nut on the end. Use one to align the clutch hub and end gear while fastening the case halves together. Put it back in the press and seat the case halves together, then snug the two Phillips screws to hold the unit together. Like the old repair manuals say, "Reassembly is reverse of disassembly." No need for thread locker; they can't back out because the ring gear covers them.
20 Once the case is back together, tighten five half-inch nuts and bolts through the ring gear holes, and you're ready to check the breakaway torque—the amount of force needed to break the clutches loose. Do not test breakaway with only the two screws holding the case halves together because you'll get an inaccurate reading at best, or shear the screws at worst.
21 With the unit mounted on a vise, use a torque wrench to determine breakaway torque. A beam-style torque wrench would be better than a click-style, but we don't have one. Factory manuals say that breakaway with used clutches is 40 lb-ft, which Prosch-Jensen said is the same as a tight open differential, while new clutches are between 100 and 250 lb-ft. That's a huge spread, but it covered everything from Cobra Jet Mustangs to grandma's LTD. Prosch-Jensen says 100 lb-ft is barely a limited-slip, while 250 lb-ft will chirp the rear tires going around corners (and is hard on axleshafts). Between 175 and 225 lb-ft is good for a street car. If your spec is too light, add thickness to the shim in the clutch hub; if it's too high, subtract thickness, decreasing preload.
22 Ford differentials have a full case, so you can't see inside to determine if it's open or has a limited-slip. Here's how you tell at a glance: On an open differential (right) the ring gear bolts rest flush on the case, but on a limited-slip (left) the bolt heads ride in recesses.
23 The differences between 28-spline and 31-spline differentials are the side gears and the diameter of the holes for the axles. Large-carrier-bearing differentials can be converted to 31-spline models with a full-internal rebuild kit with new side gears. A machine shop can open the axle holes to allow the larger-diameter 31-spline axle to pass through. If you have the small-bearing carrier, there isn't enough material to machine for the larger axles. MCR