Zip's rebuild process begins...
Zip's rebuild process begins with a customer-supplied differential core such as this one.
One of the downsides of upgrading a car incrementally is the tendency of such an approach to highlight existing weaknesses in the driveline. In the case of performance-modded fourth-generation Corvettes equipped with an automatic transmission, it's the stock rearend that usually fails first. We'll get to the reasons why in a moment, but first, a little history.
In one of those mystifying engineering decisions that could only make sense to a GM accountant, all auto-trans C4s (and all '84 models, regardless of transmission) were factory equipped with the none-too-sturdy Dana 36 differential. Manual cars from '85 through '96, meanwhile, got the significantly brawnier Dana 44.
Place the two units side by side, and the differences are easy to see. In addition to its larger gears and thicker housing, the 44 is equipped with a cover that incorporates a pair of bearing-support blocks. Without descending into engineer-speak, these blocks "preload" the carrier-bearing caps, preventing them from flexing or even cracking under load. Crack the caps, and the unsecured differential will attempt to exit through the back of the housing, often in fiscally ruinous fashion.
Recognizing the limitations of the Dana 36, many a performance-minded automatic-C4 owner has mulled the idea of swapping out the stock differential for its stouter sibling. Unfortunately, the two diffs don't directly interchange, instead requiring a hodgepodge of ancillary parts from both units to create a workable hybrid. And with the demand for Vette-spec 44s trending upward as the cars age, used-parts vendors are asking top dollar for rebuildable cores.
Since our '96 C4 project is all about affordability and ease of installation, we decided to look for another option when upgrading the car's rear. After riffling though our imposing library of Vette-parts catalogs, we found what appeared to be a workable solution in the form of a heavy-duty rebuilt Dana 36 from Zip Products.
Zip's technicians begin each rebuild by degreasing and shot-peening the differential housing to relieve stress in the metal and provide a like-new appearance. A pair of thick aluminum blocks are then welded into the unit's cover to provide support for the carrier-bearing caps, la the Dana 44. Finally, the entire unit is overhauled with quality U.S.-made internals and set up to the same precise tolerances you'd expect from an assembly-line piece.
The result is a better-than-factory-spec Dana 36 with a durability quotient approaching that of a 44. (Author's note: For extreme-horsepower automatic C4s equipped with drag slicks, Zip recommends stepping up to a full-on 44 swap. To ease the job, the company offers an all-encompassing swap kit that comes with the necessary driveshaft, cover, and other supporting bits.) Throw in a 3-year, unlimited-mileage warranty, and the $1,395 price tag (plus a $525 refundable core charge) looks like quite a bargain.
In addition to the aforementioned fortifications, Zip loaded our diff with a set of 3.54 gears in place of the stock 3.07s. (The company uses Richmond or U.S. Gear sets in all of its rebuilt units.) This change alone should noticeably improve the car's acceleration on the street and show up in the form of quicker e.t.'s at the dragstrip. We'll be back to report on that in an upcoming issue, but for now, let's take a closer look at how Zip's rebuilt Dana 36 differential earns its "heavy duty" designation.
Most fourth-gen devotees consider...
Most fourth-gen devotees consider the durability of the factory Dana 36 differential to be marginal, even on stock C4s. Adding power, mounting gluey rear tires, and driving like this story's author are all surefire ways to hasten its demise.
Once the differential is disassembled,...
Once the differential is disassembled, the parts are submerged in a tank filled with a water/detergent mixture. The mixture is then heated to 170 degrees (F) to remove any oil and grease from the parts.
After their dip in the hot...
After their dip in the hot tank, the differential components are visually inspected for cracks or other imperfections. The carrier is also Magnafluxed to uncover any flaws that might otherwise have escaped notice.
If the parts pass inspection,...
If the parts pass inspection, they're placed in a cabinet and blasted with millions of fine steel particles, or shot. The process--known as shot-peening--cleanses the components' surfaces and stress-relieves the metal.
Fresh from the cabinet, the...
Fresh from the cabinet, the differential components look as good as new.
Assembly begins with the clutch....
Assembly begins with the clutch. Here, Zip technician Keith Wilhelm applies Posi lube to a clutch plate before installing it on one of the spider gears.
Next, the gears are maneuvered...
Next, the gears are maneuvered into place in the carrier.
Wilhelm then installs a washer...
Wilhelm then installs a washer on the gears using a special task-specific tool.
With that done, the finished...
With that done, the finished carrier is ready to install in the differential.
The next step involves bolting...
The next step involves bolting the diff housing to a stand in preparation for the gear-setup process.
Zip uses special setup bearings...
Zip uses special setup bearings that are machined so that they can be slipped--rather than pressed--on and off, eliminating the potential for damage.
Once the setup is done, the...
Once the setup is done, the bearings and races are swapped out for new ones, and everything gets checked one final time.
Wilhelm places shims on either...
Wilhelm places shims on either side of the carrier to achieve the desired backlash reading.
The ring gear is then "painted"...
The ring gear is then "painted" with a pigment-based grease. When the ring gear and pinion mesh, the grease pattern left behind indicates whether the pinion has been set at the proper depth. A pinion-depth gauge is also used when needed.
With the setup process complete,...
With the setup process complete, it's time to press on the new bearings.
A special torque wrench is...
A special torque wrench is then used to set the pinion's rotating torque. The differential assembly is now complete.
Here's a close-up look at...
Here's a close-up look at the finished product.
As mentioned earlier, our...
As mentioned earlier, our differential also received one of Zip's heavy-duty covers. This shot of the HD cover shows the welded-in bearing-support blocks at the 3 and 9 o'clock positions.
The blocks preload the carrier-bearing...
The blocks preload the carrier-bearing caps, preventing them from flexing or failing under heavy acceleration. The beefier Dana 44 diff used in '85-up manual-trans C4s is equipped with a similar feature from the factory.
This look inside the differential...
This look inside the differential provides a good view of one of the carrier-bearing caps.
Zip builds each of its differentials...
Zip builds each of its differentials to precisely match a specific diff cover, thus ensuring just the right amount of bearing preload.
The HD cover is also available...
The HD cover is also available as a stand-alone piece, but some machining will be required to match it to your stock diff. Here, a Bridgeport mill is used to achieve precisely 0.003-inch interference between the carrier bearings and the support blocks in the cover.
Our differential came loaded...
Our differential came loaded with 3.54 gears in place of the factory-issue 3.07s, a switch that should noticeably improve our '96 coupe's performance at the dragstrip. Check back next month, when we cover the installation process.