Swappin' And Droppin' | Early Classic Enterprises


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Dear Early Classic Enterprises Customers,

Early Classic is excited to announce we have joined the Holley family of brands. To all of our customers, we thank you tremendously for your continued patronage over the last 24 years and moving forward. You will soon be able to find all of your favorite Early Classic products, as well as new ones, at www.holley.com. Keep on Truckin’

Your friends at Early Classic Enterprises

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Swappin' And Droppin'

Publication Name: 
Classic Trucks
Publication Date: 
April, 2003
Swapping Rearends

One of the few drawbacks when building a '60s-era GM truck is the limited amount of choices when it comes to updating the rear axle assembly. Since the majority of these trucks came from the factory with manual transmissions, they usually are found to have a rather deep rear gear ratio.

Now if you're a stop light racer or use your truck to haul or tow heavy loads, then a low gear ratio is generally a good thing. Those of us who like to drive our hotrods at highway speeds desire a higher ratio to lower the engine RPM. This not only saves wear on your engine, but usually results in better fuel mileage.

Many truck owners change the front suspension from the earlier six-lug to the later 70's style five-lug bolt-pattern disc-brake setup, Although there are six-lug front-brake rotors available in the aftermarket, many rodders choose to swap out the rear axle assembly to a later five-lug model.

Starting in 1960 and running until the 1972 model year, GM utilized a trailing arm/coil spring rear suspension design on the Chevrolet trucks, and a rear leaf spring arrangement on the GMC models. Either brand could be special ordered from the factory with the opposite suspension, which explains why you'll see a Chevy with rear leafs and a GMC with rear coils.

For those truck owners with rear coil suspension, it's possible to install virtually any rear axle assembly under your truck by changing the mounting saddles. The pre-'70 rearends 1 1/2 inches narrower than the later '70-87 units.

But as long as a rearend with 3-inch axle tubes (take a look at Ford 9-inch as well) is used, it can be swapped in without much hassle. A new bolt pattern will require new wheels so a change in track width is not a problem as it can be made up in wheel offset.

Replacement axle saddles are available from Early Classic Enterprises that can be welded to a donor rearend at the correct angle and position. The only other dilemma has been the lack of a mounting provision for a panhard bar on the replacement housing. Early Classic solves this problem with our Super Track Bar kit, which replaces the original panhard bar design.

Our kit is designed to mount in the original driver's side frame bracket, but instead of the opposite end attaching to the axle housing, it runs all the way over to the passenger side trailing arm. Their 12-inch longer bar design reduces the radial arc, which lessens the side to side movement of the rearend as the suspension moves up and down.

Follow along as we show the installation of a later model rearend into a '70 Suburban, along with an ECE 5-inch rear drop kit to change from the original six-lug to newer five-lug bolt pattern wheels.

How To Steps and Images: 
Typical of many '60s GM trucks, this '70 Suburban came with a manual transmission and very low rearend gears. Since the owner is converting the front end over to a '71-and-newer style five-lug disc brake front suspension, he wanted to update the rearend to five-lug to match. If you can locate an original '71-72 rear axle, it already has a five-lug pattern and the necessary mounting brackets to directly replace the original assembly. The owner could not locate such a unit, but did acquire a newer model leaf spring axle that will be modified to fit.
Along with updating the rearend, the owner chose to install an Early Classic five-inch Rear Drop kit. This is the opportune time to install this kit as the rearend will be out of the way. The factory upper shock mounts needed to be chiseled off and replaced by the new ECE relocated brackets. The lower mounts are held in place by the U-bolts, which will also be replaced.
The coil spring rear suspension is supported laterally by the panhard bar, which runs between the driver's-side framerail and the axle housing. The lack of a panhard bar mounting point on the axle housing has prevented installing anything but an original rearend in these trucks, without fabricating some type of bracket. Early Classic solves this dilemma with their Super Track Bar kit, which will replace the original completely.
After disconnecting the driveshaft, parking brake cables and brake hose, the old rearend assembly was hauled out. Shown here is the comparison between the old unit and the newer model five-lug leaf spring setup.
The leaf spring pads and shock mounts need to be cut off and the axle tubes ground smooth. A plasma cutter or oxy-acetylene torch will make short work of this task, but be careful not to burn through the axle tube. An electric grinder with a flexible wheel was used to smooth out the tubes in preparation for the new mounts.
Early Classic stamps new rear axle saddles for the trailing arm suspension, which saves a tremendous amount of time. Although it is possible to cut the original saddles from the old housing, there usually isn’t much left when you’re through. After the new rearend is installed and centered up, the pinion angle will be determined and the new saddles welded in place.
he owner of this Suburban wanted a nice low stance, so an ECE five-inch drop rear kit was also added. The kit consisted of heavy duty five-inch drop coils, KYB shocks, shock mount relocater kit and Super Track Bar kit. This setup will properly lower and correct the geometry of the rear suspension.
Early Classic achieves their lowered stance with computer-designed lowered ride height coil springs that maintain the stock factory characteristics. ECE’s Stan Hammond highly recommends using the proper rated spring to ensure that your vehicle’s handling and ride are not compromised.
When dropping the rear suspension, the shock angle changes drastically. In order for the shocks to work properly, ECE designed a new upper and lower mount that stand the shock back up to it’s factory angle. This allows the shock to actuate properly and dramatically improves the ride.
The new rear end was set in place on the trailing arms and located side to side. The new U-bolt slides down through the saddle to attach the rearend housing to the trailing arm, with the Super Track Bar mount captured in between.
The new relocated lower shock bracket mounts onto the bottom of the trailing arm, and is held in place by the U-bolt.
The new relocated lower shock bracket mounts onto the bottom of the trailing arm, and is held in place by the U-bolt.
Probably the most important step in the installation is setting the pinion angle. After making certain the rear axle housing was centered, the pinion angle was adjusted up or down to match the transmission tailshaft angle using a degree wheel. It is imperative to degree the pinion angle with the chassis at ride height to achieve an accurate dimension.
After everything is aligned and in place, the new saddles are welded onto the axle tubes. This is a critical step in the procedure, and failure to set the pinion angle correctly will result in a vibration and potential driveline failure. If you are unsure of your work, take it to a professional!
The remaining items left to finish were the brake hose, driveshaft and parking brake cables. A new rear brake hose was bought at the local parts house and installed, and the u-joints mated exactly to the original rearend. The only dilemma was sorting out the brake cables, but a simple solution was to reuse the original factory units with the new rearend. Ultimately, the intermediate cable can be lengthened or shortened for a custom fit, if needed.
With a set of stock ralley wheels and 275-60-15 tires, this project is back on the ground and ready to hit the street. The lowered stance improves both the look and driveability of this Urban Lurkmobile, and the reduced gear ratio is certain to improve the gas mileage. We will cover the front suspension upgrade in a future issue. Stay tuned!
Technical Articles
Company Name: 
Early Classic Enterprises
--, CA --
United States
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