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No matter how much money you put into it, the rear suspension on your 70-73 F-body has one glaring shortcoming: It’s got a solid axle. That means that when one side of your car reacts to road conditions, the other side also reacts, even if it isn’t experiencing the same thing. For example, let’s say that you’re in a hard turn at speed and there’s a slight depression in the road on the driver side. The passenger side, because it’s solidly tied to the driver side, is also going to react to that slight depression.
There’s a way to overcome that, though. It’s called an independent rear suspension (IRS). That’s why every performance car on the market today comes stock with an IRS. Heidts makes an IRS kit that I believe is the best and easiest to install on the market. Although you can weld it to make it stronger, the Heidts Pro G is a completely bolt-in independent rear suspension system. I’m going to walk you through the installation of the basic Pro G system without the parking brake option.
Caption: A good dead blow hammer like this one is vital to this installation. Image credit: Armstrong Tools.
A good torque wrench calibrated in inch-pounds like this one is also vital to this installation. Image credit: Youtube.
You’ll need a complete set of wrenches and sockets for this installation. You’ll also need a jack and at least two jack stands. On top of these, you’ll need some tools that you might not have, but can easily rent or borrow from such places as Autozone,/a> O’Reilly Auto Parts and Pep Boys. These include:
• A torque wrench calibrated in foot pounds that goes up to 150 lb-ft.
• A torque wrench calibrated in inch pounds up to 150 lb-in.
• A dead blow hammer
• A level
• A 36 mm socket
• Red or Green Threadlocker
• Stepper bit/Unibit or three drill bits up to ½ inch
• RTV Blue or Black sealant
Heidts went with a Ford 9-inch rear end for their Pro G IRS kit for a few of reasons, but the two main reasons are that it’s an excellent rear end that is tried and true, and that this type of rear end is basically ubiquitous in the automotive performance aftermarket.
If you don’t insert the 6 ½ inch long bolts into the shock mount bosses now, you’ll have to either lower the whole assembly or cut holes in your trunk later.
The fit between the hubs and the outer uprights should be quite tight. Remove only enough material from the uprights to allow the socket head bolts to draw the hubs flush into the uprights without exceeding their torque value.
When removing the adjuster knob, don’t lose the gaskets. These are known to builders as “Jesus gaskets” because if you do lose one (or both), you’ll be saying “Jesus! Now where am I supposed to find new ones?” Be sure to reinstall the adjusters and gaskets once you’ve got the springs on the shocks.
Double-check that you’re using the torque wrench calibrated in inch-pounds and not the larger one, as you can destroy the adapters and button head bolts.
Remove the Existing Rear Suspension
The whole rear suspension currently under your car has to come out. This process is almost identical to that of removing the rear suspension from a 67 – 69 F-body, so check out the detailed description in my other article here and then come on back.
Prep the Frame Rails for Installation of the Saddles
Remove the stock bump stops on the left and right rear frame rails. Use a wire brush or similar abrasive to clean road grime and grit from the rails. Now inspect the rails for distortions and rust and repair as necessary. If the areas around the mounting holes for the bump stops are rusted, you might want to give serious consideration to welding the rear saddles in place after they’ve been bolted in.
Use the plastic or rubber-coated dead blow hammer to seat the saddles. Using a metal hammer such as a carpenter’s or ball peen hammer will damage the saddles. Leave the two front bolts on both saddles loose to install the pinion support in a later step.
If you can’t find a friend to help you with this step, use your floor jack with a block of wood on top to protect the aluminum differential housing. You may need to use a prybar to “convince” the two mounting holes to line up properly.
Most Heidts Pro G kits come complete with a third member. However, if you supply your own, you must make sure that it’s a 31-spline unit, as the stub axles with the Pro G kit are 31-spline units.
This is where the majority of the strength of this kit comes from.
Remember that the longer stub axle goes in the passenger side of the housing. The image above shows installation without the parking brake kit. If the parking brake kit is used, the front plate shown is replaced with the plate that comes with that kit. Rotate the stub axles to locate the hub cutout over the studs and nuts to tighten the nuts.
If you haven’t already, I recommend using string or wire to tie the outer uprights to the lower control arms while you mount the control arms. This will keep the uprights from flopping around and possibly pinching your hand(s) or finger(s). Again, don’t tighten these bolts yet as they have to be left loose in order to adjust the alignment once the installation is complete.
Double check that the outer CV joint bolts are also torqued to the proper spec. If not, torque them to what’s specified on the label.
It must be stated again that you must not use an impact gun either to run the axle nut down or to tighten it. It must be torqued with a torque wrench. Using an impact gun can cause damage to the bearing and the axle shaft.
Be sure the bend in the upper links is closer to the outer upright and oriented down. This bend gives the clearance required for the suspension to operate properly. Incorrect installation will damage the link and the frame.
Make sure that the adjusting knobs are angled inwards prior to final torqueing of the lower shock mount bolts.
The strut rods allow for minor adjustments to both toe and caster in the rear end. They also help stabilize the suspension under extreme loads.
The Pro G kit comes with a number of shims to help you align the calipers correctly. The diverter block (the three port block that sends brake fluid to the calipers from the master cylinder) will now mount to the car’s floorpan. If you have a pipe/tubing bender and double flare tool and are good with them, bend and double flare up some 3/8 inch brake tubing and connect to the block and both calipers. Pop open the bleeder screws on both calipers, make sure the master cylinder is filled with clean fluid and bleed the brakes. Be sure to spread the cotter pins on the calipers.
Reinstall the driveshaft and slap the tires on. Lower the car and torque the wheels.
The lower control arms and axle shafts should be as close to horizontal as you can get them.
