Corvettes have been around for a long time and there’s the likelihood that they will be around for some time to come. Knowing this there’s the likelihood that the Corvette of your dreams will have “dreams” of its own and they will probably manifest themselves in terms of abuse. Should you find yourself with an older model, like a C2 or C3, here comes the realization that you are dealing with a car that has had lots of usage, and probably abuse, to it. Parts wear out and that’s where this story comes into play.
Both C2 and C3 Corvettes, spanning 1963-’82, share what’s basically the same independent rear suspension and differential package. Though these integral carrier rearends didn’t change much over the C2 and C3’s two-decade production life, there are issues you need to be aware of when it comes to differential rebuilding.
One of the first issues is disassembly. When you disassemble the differential grab your cell phone or digital camera and take lots of pictures and save them. Before removing the side bearing caps, stamp “L” and “R” for “Left” and “Right” on the caps. You don’t want to get these caps reversed. If you’re planning to do any hard driving (dah, it’s a Corvette) we recommend you opt for ARP fasteners throughout. It’s always a good idea to use new fasteners, and the use of ARP fasteners for street use is good insurance.
On a recent visit to Ramjet Rear End Repair we noted they were about to assemble a 3.08:1 Positraction differential from a 1969 390-horse 427 four-speed coupe. These 3.08:1 cogs indicate a Corvette ordered for highway cruising back in the day. Let’s get down to the business of making this rearend more durable.
When you disassemble any differential case, inspect and remove stress risers in the casting. All ragged edges and rough surfaces need to be cleaned up with a die grinder to prevent cracking. Rough cast surfaces may also be ground smooth to allow lube to flow back to the sump quicker. A coat of GE Glyptal on internal surfaces seals the casting and also allows the lube to drain back quicker. Some rebuilders like to install a drain plug to encourage regular periodic maintenance. (Editor’s Note: We highly recommend installing a drain plug.) Always add a friction modifier when you’re doing a Positraction differential. We’ve found AMSOIL Slip Lock Differential Additive works particularly well. One 4-ounce tube of Slip Lock treats differential capacities of 2-4 quarts.
And one more thing, differentials require proper break-in, especially if you have installed new ring-and-pinion gears. New gears require work hardening (break-in), which happens during the first 500 miles, allowing for a cooldown between runs. Work hardening calls for leaning hard on the throttle and backing off hard, which work hardens the gear mating surfaces. Hard acceleration from low speed, then backing off hard work-hardens the backside of the gear teeth. Some rebuilders suggest work hardening while others will tell you to get in, drive and don’t worry about it.
Although we’re showing you how to rebuild your Corvette’s differential this process is best left to a seasoned professional. We’re taking you through the rebuilding process to show you how it’s done and why it can get costly. Eddie Hernandez of Ramjet Rear End Repair shows you the way to success and what you will need to know when it’s time to rebuild your vintage Vette’s rearend.
Tools For The Job
Inch-Pound Torque Wrench
Foot-Pound Torque Wrench
3/8-inch Drive Socket Set
1/2-inch Drive Socket Set
Dial Indicator/Magnetic Base Style
Seal/Bushing Driver Set
Brass Drifts of Different Sizes
Selection of Hammers
Snap Ring Pliers
Loctite Thread Locker
Assembly Lube (Recommended)
Gear Lube (90W)
Friction Modifier Additive (Positraction Only)
Digital or Cell Phone Camera
01. Ramjet Rear End Repair performs a lot of differential rebuilds. Eddie Hernandez has been building rearends for a long time and will take us through the proper assembly and tuning of a C3 Corvette integral carrier rearend. The first thing Eddie does prior to disassembly is lap the gears to see what the current backlash and gear mesh are.
02. Because we have a good set of 3.08:1 ring-and-pinion gears, there’s no need for replacements unless there’s a desired axle ratio change. Eddie has opted for new bearings, seals and clutches from Corvette Central. You’re going to need Loctite, white grease, and marine grease. Use a torque wrench and chase threads to clean them up, enabling an accurate torque reading.
03.The Positraction clutches from Corvette Central should be saturated with Auburn friction modifier prior to assembly. Another option is to soak them in gear lube for good saturation.
04. Clutches are positioned on the side gears as shown for setup. Clutch faces (diamond pattern) should be facing the smooth clutch plate surfaces … never assemble the clutches diamond pattern to diamond pattern.
05. Shims are measured and installed between the clutches and the case. Never install shims between the clutches and the side gears. Shims are typically 0.45-inch in thickness as a baseline. Clutch thickness range is 0.67-0.69-inch, which varies from manufacturer to manufacturer. You may need more or less shim thickness to achieve the proper clearances. Add or subtract thickness .001-inch at a time. Shims are available in a variety of thicknesses to get the appropriate clearances.
06. Set the side gears and thrust washers as shown, with the side gears seated and properly aligned. Apply liberal amounts of lube to the gears, thrusts and clutches. The axleshafts spline into the side gears where highlighted by the arrows.
07. Eddie applies this wedge to seat the side gears and clutches. He then checks clutch clearances. If he can turn the carrier, the clearances are excessive. To achieve accurate clearances, the carrier should be turned several revolutions (ideally no less than 50) to seat the clutches if you’re doing it right. If the carrier requires excessive effort to turn, the clearances are too tight … a moderate effort should be needed to turn the carrier. This step is performed without the preload springs installed, but with the support pin installed to keep the side gears positioned.
08. The preload springs are installed next. There should be enough clearance to seat the preload springs.
09. The pin is installed next along with the lock bolt. Use Loctite 271 on the bolt threads.
10. Eddie checks the clutch clearances with a torque wrench a second time to ascertain proper clearance. The carrier should turn with some resistance, but not extreme effort. The resistance shouldn’t be any greater than 45-50 ft-lb as a rule.
11. The differential side bearings are installed next.
12. The ring gear installation calls for substantial prep. The ring and carrier mating surfaces must be dressed with a stone to remove all high spots or nicks. If neglected, and high spots have been missed, the ring gear will distort and you can expect a short differential life. The ring and carrier contact surfaces must be perfect. Lube the contact surfaces and slowly run the ring gear down as shown. Take your time and do not force the ring gear. Use Loctite #271 on the bolt threads and torque in a crisscross fashion to 55 ft-lb.
13. Differential side bearings and races seat against spacers like this one on each side. Shims/spacers of varying thicknesses are used to adjust ring gear positioning and backlash.
14. Pinion bearing races have been methodically inspected and pressed into the housing. Eddie closely inspects the bearing rollers and cages for scoring. The bearings are thoroughly lubricated with assembly lube to ensure a well-lubed startup.
15. With the pinion bearing lubed and seated in the race, install the pinion seal using Permatex The Right Stuff around the perimeter as shown. The seal lip is generously lubricated with assembly lube. If you’re working with a garter spring style seal, pack the spring pocket with assembly grease to prevent spring loss during installation.
16. The pinion depth is determined by shim placement between the pinion bearing and gear. Initial setup begins with the number of shims the pinion had during disassembly. Start there then add or subtract shims as necessary to achieve the proper pinion depth.
17. The pinion and yoke are installed as shown going into the case with a crush sleeve fitted, making sure the bearings and races are generously lubed for safe startup and break-in. The preload on the pinion bearings should be just enough to call for 15-20 in-lb to turn the pinion. There should be no resistance or binding when you turn the pinion. Snug, but not tight. Final torque on the pinion nut is necessary to compress the crush sleeve. Some builders prefer to use spacers instead of crush sleeves.
18. During the setup phase, use white grease on the pinion yoke for ease of assembly and disassembly. Chances are good you will have to disassemble the pinion and yoke several times to get the backlash right. Clean the yoke and pinion gear with solvent to remove the white grease before final assembly. During final assembly, apply Permatex The Right Stuff to the pinion yoke spline to prevent leakage.
19. The ring and carrier are positioned in the case for setup and backlash adjustment.
20. The bearing caps are seated and snugged, not torqued, for ring-and-pinion setup.
21. The ring-and-pinion backlash is checked using a dial indicator. Backlash is the amount of “play” between the ring and pinion. You’re going to want 0.006-0.010-inch backlash depending upon your gear manufacturer’s recommendations and how you intend to drive.
22. Gear marking paste or white grease is used to “paint” the ring gear teeth to establish gear mesh. The tooth pattern should be centered across each tooth indicating a good ring and pinion marriage. Seasoned technicians know what to listen for when they run the pattern. This is all about proper pinion and pinion gear depth.
23. When you examine the pinion gear, which has been “painted” for the setup, it is clear that the wear pattern is normal and it’s ready for a return to service.
24. The differential carrier is permanently installed. The bearing caps are seated and the bolts, coated with Loctite #271, are torqued to 60 ft-lb. Torque the cap bolts in one-third values crisscross to seat the caps, then, apply maximum torque value.
25. Torrington axleshaft side bearings and seals have been installed in the case. The seals and bearings have been dressed with assembly lube.
26. The axleshafts have been lubed at the seal contact area and are inserted into the case. The C-clips (white arrow) are installed on each axleshaft. You want a minimum of 0.005-inch clearance between the shaft tip and the differential pin (black arrow).
27. Modest amounts of sealer are used around the differential cover rail. The cover bolts are torqued to 60 ft-lb in a crisscross fashion. You can use a gasket or opt for sealer.
28. One 4-ounce tube of AMSOIL Slip Lock treats differential capacities of 2-4 quarts.
29. And finally, add 90w gear lube for a return to service. Not all rearend builders like synthetic lubrication for these classic differentials. Synthetic lube offers better wear and tear qualities than conventional lube.
30. The Ramjet Rear Ends rebuild has been reinstalled in a 1969 390-horse 427 Corvette being restored by California Muscle Cars. The owner wanted a nice weekend driver sporting classic big-block power and performance.