For those who dig '71-'73 Mustang performance, here's a couple of handling solutions from Mustangs Plus and Bill Thomas Enterprises. Mustangs Plus gets it right with a selection of cool suspension components. Bill Thomas Enterprises has the talent and goods to build everything from a stock differential to a brute chunk that can take 700 hp. Randy's Ring & Pinion is setting us up with the hard parts necessary to make it happen.
Fourth-generation '71-'73 Mustangs get a lot of criticism for their size and weight, but they aren't much different from '65-'70 Mustangs. Up front, the '71-'73 models employ a Maverick/ Torino/Fairlane unit body structure intended to accommodate big horsepower planned for these cars during the late '60s. Open the hood on a '71-'73 and you'll see big shock towers and a large engine compartment. Insurance rates and government regulations killed the big engines and horsepower that Ford had planned for the Mustang beyond 1971. After the '71 429 Cobra Jet and Boss 351, the horsepower wars came to an end. That doesn't mean you can't get more out of a non-Boss or CJ '71-'73 Mustang. These wide-track models are long on potential and make great road cars.
At the rear, you can ditch the worn-out elliptical four-leaf springs and get into stiffer five-leaf, mideye, down-low performance. Opt for rubber or urethane bushings, depending on the caliber of performance wanted. What's more, choose the right shock absorber for the desired balance of ride and handling. Mustangs Plus offers standard gas shocks, which yield a smooth ride with good handling. KYB gas adjustable shocks from Mustangs Plus offer a stiffer ride with better handling.
When it's time to fit a '71-'73 Mustang with a new rear suspension system, it's wise to consider Ford's heavy-duty 9-inch rearend if you're planning big horsepower and torque gains. The standard 8-inch rearend can stand up to 350 hp, but not continuously without giving up the ghost. For street and weekend strip use, you'll want a four-pinion 9-inch limited-slip for durability and to get as much torque to the ground as possible. Randy's Ring & Pinion offers the 28-spline (PN YDGF-28-SM) or 31-spline (PN YDGF-31-SM) Trac Loc differential for street and strip use. Because it's made from machined billet steel, it's much stronger than a comparable four-pinion Ford unit.
We're fitting a '71 Mustang SportsRoof with stronger underpinnings in front and back. In front, we've already installed a Grab-A-Trak suspension with 620 coils and gas shocks. In back, we're installing five-leaf, mideye springs with gas shocks for an exceptional ride and moderate handling. As we said earlier, shock dampening determines ride. Because a smooth ride was priority, we opted for standard Grab-A-Trak gas shocks instead of stiffer KYBs. The result will be improved handling and a nice ride.
We began our efforts at Bill Thomas Enterprises, building an N-case nodular iron 9-inch Ford differential with 3.55:1 gears from Randy's Ring & Pinion. We've chosen 3.55:1 gears because the owner is going to install an automatic overdrive transmission behind his gutsy 302ci V-8 mill from Trans Am Racing.
Here's everything needed to...
Here's everything needed to build a bulletproof, four-pinion Traction-Lok 9-inch differential. Bill Thomas Enterprises is providing our '70 vintage, N-case axle housing and axles. Randy's Ring & Pinion has provided the 3.55:1 gears and Traction-Lok unit.
There really is a difference...
There really is a difference in differentials. On the right is a Ford four-pinion Traction-Lok unit. On the left is a Yukon Trac-Loc with billet construction for 31-spline axles (YDGF-31-SM) from Randy's Ring & Pinion.
Aside from its nodular iron...
Aside from its nodular iron construction, the Ford N-case sports a beefy pinion support, which is a key failure point in 8- and 9-inch differentials. If an N-case is outside your budget, try the C7AW-E case. Stay away from WAR and WAB waffle cases, as they aren't as strong and are known for cracking.
When Bill Thomas builds a...
When Bill Thomas builds a differential, he uses only genuine Timken bearings and seals. He examines bearing races and rollers closely for scoring. During assembly, the bearing gets a liberal amount of assembly lube.
Bill stresses proper treatment...
Bill stresses proper treatment of seals and gaskets to prevent leaks. Surfaces must be clean and free from scoring. The O-ring seal has axle lube and is ready. Pinion depth has been checked.
Bill installs the yoke and...
Bill installs the yoke and runs the locknut down. Loctite was used on the pinion carrier bolts, which are torqued to 30-45 lb-ft.
When building a rearend, always...
When building a rearend, always use a thread locker for added security. Loctite 262 is a high-temperature thread locker to use in conjunction with proper bolt torque.
Make sure surfaces between...
Make sure surfaces between the ring gear and differential are free of debris and scoring. Any irregularity will cause ring gear misalignment and failure.
Bill seats the differential...
Bill seats the differential and ring-gear assembly in the carrier for setup. Bearings must be lubricated at this point with assembly lube.
Differential side bearing...
Differential side bearing retainers are installed, then screwed down to secure bearings and mate the ring and pinion. These adjusting nuts call for a special spanner wrench. One of them is left-handed thread, which enables the ring gear to be moved left or right to adjust gear backlash.
Bill sets the backlash, which...
Bill sets the backlash, which is 0.008-0.012 inch via a dial indicator.
Backlash and gear tooth depth...
Backlash and gear tooth depth is checked via this yellow compound designed specifically for gear lapping. This is how the gear tooth pattern should look, right smack in the middle of each tooth. Contact should be dead center in pull and coast.
Once backlash and gear depth...
Once backlash and gear depth are set, side-bearing retaining nuts should be torqued to 12-25 lb-ft. Here, Bill installs side bearing adjustment nut locks. Don't forget Loctite on these.
Our N-case Randy's Ring &...
Our N-case Randy's Ring & Pinion 9-inch Trac-Loc differential is ready for installation.
Our '71 Mustang SportsRoof...
Our '71 Mustang SportsRoof came from the factory with a 302 two-barrel V-8 and 8-inch peg leg rearend with 3.00 gears. We're going to fit it with a Bill Thomas 9-inch housing, N-case Trac Loc, 31-spline axles, and new drum brakes.
First, disconnect both rear...
First, disconnect both rear shackles and support the 8-inch axle with a floor jack. Our Mustang is already supported with jackstands in the interest of safety.
Disconnect both shock absorbers...
Disconnect both shock absorbers at the leaf-spring brackets.
You'll have to remove the...
You'll have to remove the rear seat to get at the top mounts, which are also stud mounts.
With the rear axle properly...
With the rear axle properly supported...
...remove both leaf-spring...
...remove both leaf-spring brackets.
Both front leaf-spring mounts...
Both front leaf-spring mounts are removed with a 3/4-inch socket and box-end wrench. It's a good idea to spray all hardware with a penetrating lubricant such as WD-40 before you begin work to break down corrosion.
With the rear axle properly...
With the rear axle properly balanced, roll the entire assembly from underneath the body. This means you never have to lift anything. It's always good to have a buddy's help with this
Leaf-spring installation begins...
Leaf-spring installation begins at the forward mounts. Use liberal amounts of chassis lube on the bolts and bushings to reduce corrosion risk and provide lubrication.
We're installing the new Bill...
We're installing the new Bill Thomas 9-inch the same way we removed the 8-inch. Use a floor jack and roll the axle into position. Elevate the axle higher than the leaf springs, relying on a buddy to maintain balance.
When installing shackles,...
When installing shackles, use chassis lube or white grease on the bushings for ease of installation.
Lower the rear axle onto the...
Lower the rear axle onto the leaf mounting points.
After you secure the leaf...
After you secure the leaf springs and torque the plates to 30-45 lb-ft, install both Grab-A-Trak gas shocks. Remember to turn them upside-down and bleed them before installation. Tighten shock mounts to where rubber bushings expand to washer circumference. Don't overtighten.
Install the parking brake...
Install the parking brake cables and brake lines. Now is the best time to flush and bleed your Mustang's brake system.