Last month, I regaled you with the sorry tale of a magazine cover shoot gone very, very wrong. A heavy right foot, coupled with a quick release of the clutch in an attempt to make lots of pretty white smoke, ended the shoot pronto, thanks to a completely destroyed differential housing.
A costly flatbed ride home did little to ease the sting of what was sure to be a very expensive Sunday afternoon. The next day, a call to the experts at Zip seemingly put things right, as these guys are experts when it comes to rebuilding and fortifying Corvette differentials. Within a few days, they had assembled a much stronger '06-up unit (D6C, you'll remember, is an '05), complete with upgraded axles and a Z06-spec, eight-disc, limited-slip clutch pack.
The '05 diff turns out to be a one-year-only bastard child, as GM revised (read: greatly strengthened) the design for the '06 model year. As we all know, GM doesn't change anything unless it's to make it cheaper-or in an attempt to limit warranty claims on something that was too flimsy to begin with. Guess which was the case here.
While this exploded assembly gives the impression that the Corvette's differential isn't overly complicated, there are some important techniques that must be followed for it to live more than a few miles. Luckily, Zip has the tools and the knowledge to rebuild these things the right way.
So, on the surface, it would appear that an '05 car suffering the same fate as D6C could just have the newer-style diff installed. Luckily, it's almost that simple. The fly in the ointment is that GM changed the bolt pattern on the differential, making an outright swap impossible. This hurdle is easily sidestepped, though, as the intermediate housing between the trans and diff can be swapped as well, making it a true bolt-in. For obvious reasons, Zip recommended going this route.
The swap should have taken less than a day to perform, but alas, there was another spanner in the works. In the process of disintegrating, the differential made an unauthorized modification to the transmission output shaft, which traverses the diff housing on its way to the rear-mounted pinion gear. The output shaft was bent approximately 0.070 inch, which is pretty amazing considering the diameter of the unit. There's a lot of power being channeled through that drivetrain!
This, too, was overcome, thanks to the help of an overnight shipping company and the friendly local GM-parts guy. Unfortunately, the entire transmission needed to be disassembled in order to replace this shaft. We covered this job last month.
This month, we take a look at Zip's differential-assembly procedure and its installation in D6C. As you'll see, the diff is plenty beefy and is holding up to D6C's power just fine. It does cause one to wonder what the next weakest link may be. Let's just hope it's a while before we find out.
Here, you can see the difference between the HD side-gear axle in the '06-'08 (bottom) and the assembly that was used in '97-'05 models. The early axle is a forged part that is undercut where it meets the gear, creating a weak spot. The new one has a splined gear where a splined axle slides into it. Zip says the newer-style units are so much stronger that you will probably never see one broken. | 
The stock axle (top) is shown with Zip's 300M heavy-duty replacement. You can easily see the much larger diameter of the Zip axle. | 
Zip also offers an upgraded differential unit for hard-core road-race applications. The unit (bottom), manufactured by Quaife, is overkill for anything but professional racing, so we passed on it for D6C. |