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Street Race Truck More... The Toolbox

Chevy Trucks: Rear DJM Suspension Drop

6/22/2020
Where we left off.

After getting Jonny’s truck squared away mechanically, I started driving it to run errands and do chores. After all, he still owed money on it, and we had agreed that he couldn’t start driving it until it was paid-in-full.

Then I blew up the rear-end. No, not the way you think. No giant smoky burnouts and fractured spider gears. That would’ve at least been a good story. Nope, it ate a pinion bearing/race. Fortunately, it would still move but it was very noisy. Especially on coast.

After all the other repairs and checking we did, we’d just assumed that the differential was full of gear lube. Or any lube for that matter. As it was, the pinion seal on the factory rear end leaked horribly. When we got the truck, it was already dry and probably had been for a long time. Lesson learned on long-stored vehicles. If it’s not leaking, it’s probably empty.

We found a direct match differential from a truck that a friend had acquired to part out. It was decent timing because we were also ready to lower the truck and take it to the next stage of Jonny’s plan.

We found a direct match differential from a truck that a friend had acquired to part out. It was decent timing because we were also ready to lower the truck and take it to the next stage of Jonny’s plan.

We used a DJM Lowering Kit that included everything that we needed to flip the rear axle from below the springs to above. This change, in conjunction with a lowering block incorporated into the housing mounts drops the rear 6” from stock. This change necessitates a C-notch in the rear frame rails above the housing. These are also included in the kit.

This kit also includes a pair of dropped spindles for the front and a set of lowering coil springs for the front. We also opted to use the DJM Calmax Lowering Shocks all the way around. The reduced travel and compressed length make them the perfect solution for drastically modified ride height trucks.

During the operation, we also cleaned up and painted parts that were making their way back onto the truck in an effort to tidy up at least the areas we worked on. The first order of business was the “new” differential. It’s a good thing Jonny worked at a car wash. Those tokens came in handy.

After he had it mostly de-greased, he went to work on it with a wire cup on an angle grinder to strip all the rust and scale from drum to drum. The housing was then painted charcoal grey and received a chrome cover. While we had the cover off, we discovered a nice surprise. His truck had originally been equipped with a 3.08 gear ratio open diff. The replacement was equipped with 3.42 gears. Still a one wheel wonder but with a bit more low-end grunt. This would prove to be very bad for the health of one of my 275/60 tires that had been loaned for the project.

After we had the new rear all ready to go, we jacked the whole truck up as high as we could get it. Which, laughably, was not much higher than the stock ride height.

Taking the old rear end out was pretty straightforward. Since it rides below the springs, you just cut the u-bolts, driveshaft and brake attachments loose and you could just roll it out the back. Well, you could’ve, had we not put jack stands directly in the way. So once we had it dropped, we had to drag it out from the side. Then we dropped the front of the leaf springs to allow access to the top side of the springs where the new rear would mount. (Please note: We did this because the entire rear end was being replaced. For your project, you will likely take the approach of supporting the housing and removing the springs to relocate them under the housing.)

Again, we spent some extra time cleaning and painting.

Slightly less icky

We prepped the new housing and rolled it under the truck in front of the springs on Rolling Car Dollies. Once the housing was up on the cradles we loosely bolted the assembly together and raised the rearend and springs into position so they could be bolted back in. Only snug the rear end U-bolts enough to eliminate any gap between the components.

This is the point in the project where things deviate a bit from the typical bolt-in installation. The C-Notch supplied with this kit consists of a weldment with an arch and a reinforcement plate that bolts to the outside of the frame rail.

DO NOT NOTCH YOUR FRAME BEFORE YOU RAISE AND LOCATE THE REAREND HOUSING.

You need to raise the rear end to ride height and mark the center of the axle tube on the frame rail. Then mark and cut your frame. Don’t guess. The rear end moves rearward as the spring compresses. Also be sure that your truck it supported well in the front, middle and rear of the frame. Six jack stands, plus support under the center of the rear end. This helps reduce the possibility of any sag.

Once you’ve got the C-Notch completed according to the DJM instruction, you can reconnect all the brake lines and bleed the brakes. We replaced the wheel cylinders and rerouted the brake hose to add clearance to the box floor while we were there. Install the shocks, reconnect your driveshaft and bolt the wheels back on. Then use ramps under the rear tires to support the rear weight of the truck.

Once you have the rear completely reassembled, you need to adjust the pinion angle so the angle of your differential matches the angle of the transmission tail shaft. The mounts in this kit have this adjustability built-in. Once the angle is set, then tighten down all the u-bolts equally to securely fasten the housing to the springs.

In the next article we’ll compete the install with the front portion of the kit.

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