Bryan Blocker began the disassembly...
Bryan Blocker began the disassembly by unbolting the rear stabilizer bar from the down links and swinging it out of the way. Then we unbolted the differential cover and drained the case. While removing the 51/416 pinion-shaft retaining bolt, we used caution since it's easy to round off the head if it's lodged in place.
A call was made to the Precision Gear tech line, and the company agreed that our math was correct. The tech also recommended we stick with a regular gear set since the intended use was primarily street. Their Ultra-Lite series of racing gears would have been overkill in this application. We ordered the 3.42:1 gear for a GM 10-bolt differential (PN GM 10/342, $155.95)
A second call to Summit Racing netted a Ratech (PN RAT-310K, $89.99) installation kit that included all the needed bearings, shims, marking compound, seals, and gaskets. A local parts store provided the requisite gear oil and limited-slip additive for our upgraded rearend.
With the gear set selected, we wondered what kind of performance gain we might expect from changing to the deeper gears. Data for the T/A's performance in the eighth-mile was available. It had run a previous best of 8.13 at Centerville Dragway in Russelville, Arkansas. Would we see any additional performance by simply swapping gears?
Follow along as we perform the install in a day's time and return to the track for follow-up testing.
We positioned the differential...
We positioned the differential for access to remove the bolt, pinion shaft, and C-clips without disturbing the unit. If we were to rotate the differential without the pinion shaft in place, the spider gears could walk and make reassembling the unit a nightmare. With the bolt removed, our pinion shaft slid out easily. If yours doesn't, a few gentle taps with a hammer should dislodge it.
We removed the carrier once again to facilitate the final assembly of the rearend. To install the crush collar, Bryan had to-of course-take out the pinion. He then reinstalled it with the outer pinion bearing, pinion seal, yoke, washer, and nut, torqueing the latter to 300-400 ft-lb in order to place the proper preload on the pinion bearings. Lastly, Bryan checked the backlash and contact pattern once again before bolting on the cover with its gasket and refilling the unit with fluid and limited-slip additive.
It should be noted that any new gear set requires a break-in procedure. Consult your installation instructions for the exact methods recommended by the manufacturer of your gears. Most require the car to be driven 15-20 miles, and then allowing the new gears to cool completely before driving it again. Many also caution against towing or spirited use during the first 500 miles. Improper break-in can lead to overheated and damaged gears.
See the "Drag Testing" sidebar to learn how the small detail of swapping 3.42s in place of 3.08s paid dividends on the strip after proper break-in.
Then we pushed each axle inward...
Then we pushed each axle inward with the hope the C-clips would simply fall into the differential case. They didn't, so we used long-nosed pliers (a magnet will work, too) to remove them.
Reading Patterns And Gear TermsToe: Inner end of the gearHeel: Outer end of the gearPitch Line: Halfway across the toothRoot: Bottom of the toothFace: Area above the pitch lineFlank: Area below the pitch lineConvex: Drive side of toothConcave: Coast side of tooth
The pattern should be in the middle of the tooth (pitch line), splitting the face and the flank. In theory, it should be even on the drive side and coast side, and even in width and diameter across the tooth with feathered edges.
Pinion depth affects the pattern. While it will change toe and heel a bit, it mostly moves the pattern up or down relative to the pitch line. Increasing pinion depth moves the pattern up toward the face, while decreasing it moves it down toward the flank. The coast and drive sides move opposite each other with changes in pinion depth.
Backlash affects the pattern. A decrease moves the pattern from heel toward toe; an increase does just the opposite. The drive and coast sides move evenly in both cases. A pattern toward the heel is quieter than one toward the toe.-Tom DeMauro