Project Street Reaper: Replacing Second-Gen F-Body Leaf Springs With UMI Performance Hardware
The rear end on Project Street Reaper had been telling me something was wrong for a long time. I had been putting off listening to it. The car tracked sloppily on anything but smooth pavement. You could feel the suspension flexing through corners in a way that had nothing to do with the tires. Leaf spring replacement was overdue before I even bought this car. Once I finally got under there and saw what I was working with, it became clear that overdue was a generous description.
Why The Street Reaper’s Rear Suspension Had To Go
The measurements told the first part of the story. Both sides of the rear bumper sat at 16-5/8 inches off the ground. Springs that had given up long before the car found its way to me were the cause. Then I got my hands on the hardware, and the story got worse fast. Factory rubber isolators between the leaf springs and the rear axle housing were completely mushroomed flat, crushed beyond any reasonable service life. Bushings crumbled apart at the touch. Not soft, not compressed, actually falling apart in my hands.
The Street Reaper sat for approximately 20 years before I got it. That kind of extended dormancy does things to rubber and metal that mileage alone never could. Rubber components that experience regular heat cycling and movement at least remain somewhat pliable during use. Sit a car for two decades, and those same components dry out completely, losing their elasticity. They do this without showing the kind of wear that would flag them as bad during a casual inspection. The isolators looked as if they had just flattened with age. Bushings looked intact until I touched them. Neither was doing anything useful by the time this car moved under its own power again. Because of that, the rear axle had been slowly losing its precise location under the car with every mile since. Leaf spring replacement was the only path forward.
When those isolators give out, the axle starts moving in ways it was never supposed to. The car tells you about it in every corner, every braking zone, and every time you put power down. For a street/strip build, running on degraded suspension hardware leaves traction on the table. It also creates a handling situation that gets worse with every mile.
What UMI Performance Built For This Platform
UMI Performance has been building race-proven suspension components for GM F-bodies for decades. Their leaf spring kits for the 1967-1981 platform reflect everything they have learned from real dirt track and paved track racing applications. That experience shows up directly in how the parts are designed and specified. It is why UMI was the right call for this leaf spring replacement.
Two kits handled this job. The first was the UMI 262025, their 1967-1981 GM F-Body Rear Leaf Spring Kit with poly bushings, running a 175 lb/in spring rate. That rate is a deliberate choice for this platform. According to UMI, 175 lb/in adds meaningful stiffness over the mushy factory units without crossing into race-car territory. The result is a street car suspension that communicates with the road instead of isolating you from it, while remaining livable on imperfect pavement. Their engineers drew on dirt track and paved track racing experience to tune the spring wrap characteristics. The package works well in general street and touring use without requiring a compromise on either end.
Springs come with a pre-installed rubber front eye bushing, which UMI left that way intentionally. Rubber allows the front mount to articulate through suspension travel without binding. That matters on a street car that sees real-world road conditions. The rear eyelet ships bare, so the builder can choose polyurethane or Delrin depending on intended use. For street driving, poly is the correct choice. Delrin is for the guys primarily running at the track, where maximum suspension response is the goal. That additional harshness from the stiffer bushing material is an acceptable trade-off in that application.
The second kit was the UMI 2627, the 1970-1981 GM F-Body Leaf Spring Isolator and Hardware Kit. This replaces the factory T-bolt and single U-bolt combination at the axle perch with a full dual Grade 8 U-bolt setup. Contents include one poly isolator with bushing, four U-bolts at 1/2-13 x 5 inches, eight 7/16-inch thick washers, and eight high hex nuts. Also included are two 1/2-13 nyloc nuts, two flat washers, and two additional 1/2-13 x 5-inch bolts for the front spring mount. UMI designed this upgrade specifically to handle the axle loads that higher horsepower applications put on the rear suspension. On a car like the Street Reaper, that matters.
For the front spring mounting points, I picked up the Screaming Chicken 1970-81 Camaro/Firebird Leaf Spring Mounting Hardware. That turned out to be one of the better decisions made on this entire job. The reason why becomes clear in the removal section below.
Leaf Spring Replacement: Why You Should Not Reuse Old Hardware
Before getting into the removal process, this needs to be said directly. It is one of those things that is easy to rationalize your way around in the middle of a job. Do not reuse the factory U-bolts, T-bolts, or any associated nuts and washers on a car this age. This applies regardless of whether the hardware came off without a fight.
Here is why that matters. U-bolts and fasteners in suspension applications behave as torque-to-yield hardware in practice. Once tightened to spec and subjected to years of heat cycling, vibration, and load cycles, the metal has already stretched and fatigued at the thread level. A bolt that looks fine and came off cleanly still carries microscopic deformation in the shank and threads from decades of those loads. When you reinstall that bolt and torque it back down, you are not starting from a known clamping force. You are adding stress to metal that has already been worked past its original condition. There is no way of knowing how close it is to failure.
On this leaf spring hardware specifically, the stakes are real. These fasteners keep the rear axle attached to the car. A U-bolt that lets go at speed does not announce itself with a warning vibration. The axle moves, and then the car moves in a way you cannot correct for. That is a scenario that can hurt you and everyone around you. The cost of a full hardware replacement from UMI is negligible against that risk. Fresh Grade 8 hardware, torqued to spec, is worth far more than the few dollars saved by reusing what came off the car.
Beyond safety, there is also a reliability argument. Even hardware that is not at immediate risk of failure will micro-shift more under load than fresh hardware will. The clamping force at the joint becomes inconsistent and unpredictable over time. Fresh hardware, properly torqued, gives the axle a solid and repeatable location under the car. That is the baseline everything else in the suspension system depends on. Replace the hardware every time.
Leaf Spring Replacement: Removal Step-By-Step
Getting the car properly supported was the priority before touching anything. Jack stands went under the body seam lines and under the rear axle housing separately, following UMI’s instructions to support both independently. Once the leaf springs unload, the body wants to drop, and the axle wants to droop at the same time. Both need to be held at a working height before any hardware comes off. With everything supported, the shocks came off their lower mounting plates first, relieving tension from the assembly before touching the spring hardware.
Next came the U-bolts and T-bolts at the axle perch plates. Each side has two T-bolts and one U-bolt holding the plate to the spring. UMI’s instructions make clear that both springs need to come off before either one goes back on. This is a hard requirement, not a suggestion. With one spring still installed, the rear axle cannot float freely enough to allow correct positioning of the opposite side. Remove both before reinstalling either, full stop.
The Passenger Side Hardware
On the passenger side, every nut on the axle perch hardware had corroded to the bolts, and none came off with conventional tools. Everyone snapped. With the floor jack under the spring mounting plate, the hardware came off, and the jack lowered slowly until the spring came down. Slowly deserves emphasis here. A leaky floor jack turns this into a real problem. My older jack does not hold pressure, and I learned that firsthand on this car. I walked away from a passenger-side spring that would not release, and when the jack bled down, that spring slammed to the garage floor. The body seam jack stand kept anything serious from happening to the car. A jack that holds pressure is not optional equipment for this job.
On the driver’s side, only one nut snapped while the other three came off without drama. That variation is a good reminder that hardware condition changes by location and exposure, even on the same car.
The J-Nut Problem
With the axle-side hardware handled, the front spring mounts were next. Each spring bolts to the chassis through three fasteners retained by J-nuts, spring-steel clips that hold a nut captive inside the chassis rail. On a car this age, these clips have a well-known failure mode. The nut breaks free from the clip and spins freely inside the rail. There is no access point for a wrench, leaving the bolt unable to be extracted.
On the passenger side, angling a Kobalt impact driver while holding the trigger created enough oscillating pressure on the spinning nut to catch it. That allowed the bolt to thread out. Seven to eight minutes per bolt using that method. The third bolt required a long flat-head Olsa Tools screwdriver wedged between the chassis and the mounting plate, applying downward pressure against the spinning nut. When pressure alone was not enough, it eventually ripped the nut free from the clip entirely. The hole diameter increased slightly. Because the new Screaming Chicken hardware uses integrated threads in the plate, that slightly enlarged hole does not affect reinstallation.
The driver’s-side stuck bolts became the defining obstacle of this entire job. They stretched what should have been a straightforward removal into three days of work. Chiselling did not move them. Drilling was slow because friction from the bit heated the bolt, which then cooled and hardened between attempts, making each pass slower than the last. In the end, a pry bar with a good angle was the answer. With two stuck bolts instead of one, there was enough mechanical advantage to rip them free without damaging the chassis rail. If a rotary cutoff tool is available, cutting the bolt heads is the fastest exit from this situation. Budget time for the extraction fight if one is not available, or commit to the pry bar method early and save yourself days of chiselling.
The Screaming Chicken Save
This is where the Screaming Chicken hardware earns its mention. When it became clear during removal that the factory front-mount hardware was beyond saving, I ordered their 1970-81 Camaro/Firebird Leaf Spring Mounting Hardware. It showed up fast. Screaming Chicken ships, and having those parts in hand when they were needed kept the job moving instead of sitting idle.
Their hardware is also a genuine engineering improvement over what it replaces. Threads are built directly into the J-nut plate rather than retained as a separate captive nut inside a clip. That captive nut failure mode is exactly what turned a bolt removal into three days of work on this car. Screaming Chicken’s design eliminates it. If these springs ever need to come back out, that front-mount hardware will behave the way it is supposed to.
Leaf Spring Replacement: Shackle Removal
With the front mounts freed, the rear shackles came next. The passenger side was corroded and caked in grime, and one nut needed a bolt extractor socket before it would budge. Partway through, dropping the fuel tank made a meaningful difference. Two bolts, three minutes, and the working room in that rear corner doubled immediately. The driver’s side came off easier, though the trunk pan geometry made leverage the main challenge. A wrench extender would have been ideal for the space, but I made do without one.
Leaf Spring Replacement: Installation
Front Mount First
With both sides stripped down to bare chassis mounts and shackle perches, the UMI 262025 leaf springs went in starting at the front mount on each side. The pre-installed rubber front eye bushing is already in place on each spring. So the mounting plate slips directly over it and bolts to the chassis using the new Screaming Chicken J-nut hardware. No drama, no fight, clean installation where the old mount had been a nightmare.
For the rear eyelets, the polyurethane bushings from the UMI 2625 shackle kit went in before swinging each spring rearward toward the shackle location. UMI includes grease with the bushing kit, and it gets used. Poly bushings installed dry will squeak and wear prematurely. A coat of the supplied grease on the bushing bore before assembly takes care of both problems from day one.
Swing, Shackle, And Plate
With the front of each spring mounted to the chassis, the rear swings up to the shackle location. The UMI 2625 shackle kit includes four shackle plates, eight poly bushings, four 1/2-13 nyloc nuts, four 1/2 flat washers, and four 1/2-13 x 5-inch bolts. Rear shackle hardware gets torqued to 50 ft-lbs, but not until the car is back on the ground at ride height with weight on the suspension. For now, hand tight only.
Next, the UMI 2627 hardware replaces the factory T-bolt and U-bolt combination at the axle perch. The poly isolator goes between the axle housing and the spring, with the alignment pin seated into the locating hole on the isolator face. From there, the mounting plate goes on the floor jack, and the jack raises the plate into position until the U-bolt holes align. This part takes patience. Pump the jack and check the alignment. Lower slightly, adjust, and pump again until the threads line up and the U-bolts can be started by hand. On the passenger side, a brake line tab was in the path of the new U-bolt and needed to be moved before the plate could seat correctly.
Torque Specs And Final Assembly
With both sides in place, the spring plate nuts on the U-bolt hardware were torqued to 45 ft-lbs in a crisscross pattern, per UMI’s instructions. Front through-bolts at the chassis mount torqued to 50 ft-lbs. Shocks went back onto their lower mounts, wheels back on, and the car came down onto its tires. With the suspension loaded at ride height, the rear shackle bolts were torqued to 50 ft-lbs. Fuel tank went back up, two bolts, done.
Leaf Spring Replacement Results: What Changed
Before the install, both sides of the rear bumper measured 16-5/8 inches off the ground. After one week of settling, both sides measure approximately 15 inches, a drop of roughly 1-5/8 inches from the sagged factory position. UMI notes that leaf springs typically settle an additional 1/4 to 3/8 inch after the first drive. So the final ride height will come in slightly lower once the car has been on the road.
Street Reaper has not made its maiden voyage on the new suspension yet, because the car is currently running without a hood and the weather has not cooperated. The stance alone tells a clear story about how much the old springs had given up. A follow-up article and video will cover the first drive. That video is in final editing now and will be posted to our YouTube channel shortly, so subscribe and turn on notifications so you do not miss it. We will embed it in this article as soon as it goes live.
What To Check After The First 500 Miles
UMI recommends a post-installation inspection after the first 500 miles to verify that everything has settled correctly. Check for any unusual tire contact or rubbing against the wheel wells, body, or brake lines as the springs find their final position. Go back over every nut and bolt in the installation and confirm that the torque values have held. This includes the U-bolt plate nuts, the front through-bolt, and the rear shackle hardware. Fresh suspension components can micro-settle in the first few hundred miles. Catching any movement at the 500-mile mark is far easier than addressing it after the hardware has been running at an incorrect clamp load for a full season.
Frequently Asked Questions
The UMI 262025 leaf spring kit is rated at 175 lb/in. UMI chose this rate to add meaningful stiffness over worn factory units while maintaining a street-friendly touring feel, without going to a full race-oriented spring rate that would compromise ride quality on the street.
Per UMI’s installation instructions, the spring plate nuts on the U-bolt hardware torque to 45 ft-lbs in a crisscross pattern. The front leaf spring through-bolt at the chassis mount torques to 50 ft-lbs. Rear shackle bolts also torque to 50 ft-lbs, and these should be tightened with the car on the ground at ride height, not with the suspension hanging free.
Yes. UMI’s instructions require removing both leaf springs before reinstalling either one. With one spring still installed, the rear axle cannot float freely enough to allow correct positioning on the opposite side. Attempting a one-side-at-a-time install will make the job nearly impossible.
For street use, polyurethane is the correct choice for the rear leaf spring eyelet. Poly provides a good balance of compliance and axle location for daily and street driving. Delrin bushings are available for racing applications where maximum suspension response is the priority, but the added harshness makes them a poor choice for street-driven builds.
Factory U-bolts and T-bolts on these cars are 45 to 50 years old. Even hardware that removes without snapping has experienced decades of torque-to-yield stress, heat cycling, and fatigue at the thread level. Reinstalling old fasteners in a suspension application means torquing down metal that has already been stretched beyond its original condition, with no way to verify the actual clamping force at the joint. UMI’s 2627 hardware kit replaces the factory T-bolt and single U-bolt arrangement with stronger Grade 8 dual U-bolts that provide consistent, known clamping force and better handle the axle loads on higher horsepower vehicles.
