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. |