Which rearend is better? GM’s time-proven 12-bolt or Ford’s popular 9-inch. Each has its advantages and disadvantages. The Ford 9-inch is heavier and eats up more power due to a disadvantage in pinion-to-ring gear geometry, yet many performance enthusiasts believe it offers unequalled strength. The Ford 9-inch axle was produced for a greater number of years than the original GM 12-bolt and has, therefore, been more available. Hence its popularity when aftermarket parts and axle assemblies were not available.
Chevrolet’s 12-bolt axle, with its 8.875-inch ring gear offers greater durability, less drag, and reduced weight. A 12-bolt assembled properly with the right combination of parts delivers brute strength and durability. Joel Rode of Hot Rod Specialties in Upland, California, has built a pristine 1967 Chevelle hardtop with a Chevrolet Performance crate ZZ454 big-block, Gearstar 700-R4, and a complete QA1 suspension system underneath. The Chevelle has swiftly become what Joel envisioned for his daily driver. It’s fast and reliable.
When Joel was drag and dyno-testing the Chevelle, the Gearstar 700-R4 overdrive automatic worked beautifully, delivering plenty of power from the ZZ454 and impressive elapsed times. Where the car fell short was a failing differential on the track. It just didn’t hook up and Joel was presented with disappointing timeslips.
Determined to improve his elapsed times and commute, Joel contacted Summit Racing Equipment and ordered up an Eaton Detroit Truetrac, bearing and seal kit, and a set of Moser 30-spline axles. Eaton Detroit Truetrac differentials use a cool, patented design of parallel-axis planetary helix gears to provide a smooth division of torque to both rear wheels. The Truetrac works like a limited-slip differential in normal driving during the commute, yet it automatically transfers torque to the wheel offering better traction when driving conditions warrant.
We’re going to show you how to replace the 12-bolt’s differential and set up ring-and-pinion backlash. Proper ring-and-pinion backlash is critical to performance and reliability. Get the backlash too tight and you burn up the gears. Set it too loose and you get wear and tear as a result from the gear teeth beating each other to death. The gear tooth pattern must be dead center on the ring gear with 0.006-0.010-inch of backlash, using a dial indicator. If you have to remove the pinion gear and bearings, preload is 6-8 in-lb used or 14-19 in-lb if the parts all are replaced with new.
Replacing the differential in your GM 12-bolt doesn’t have to be complex or confusing. Take your time and pay close attention to what you’re doing and you’ll be sure to get a strong and reliable rearend, and possibly even better timeslips. CHP
1. The first order of business is to remove the differential cover and drain the lubricant. Examine the lube for contaminants, such as metal shavings and clutch material.
2. The brake drums (or rotors) are removed next to gain access to the axleshafts. This is the time to replace the axleshafts. The factory 12-bolt axles are tapered as you head away from the splines. The Moser axles are of uniform thickness from the splines to the axle flange for added strength.
3. This bolt, which retains the lock pin, is removed at this time to free the lock pin. The lock pin keeps the axleshafts and C-clips seated. A weakness here is the potential for C-clip failure, which can result in the loss of an axleshaft.
4. The lock pin is driven out of the differential, which frees up the axleshafts and C-clips. Resist the temptation to use channel locks or vice grips on this pin, which will score its precision-machined surfaces.
5. Push inboard on the axleshafts, which will make the C-clips visible. Grab the C-clips from the axleshafts with a magnet.
6. The axleshafts are removed at this time and discarded. They will be replaced with Moser 30-spline axleshafts, which are a precision fit in the Eaton Detroit Truetrac differential.
7. Summit Racing has provided everything we’re going to need to get our 12-bolt back in operation. The Eaton Detroit Truetrac differential will connect Joel’s ZZ454 with the pavement, yet provide smooth operation for the street. The Summit cast-aluminum cover offers good heat transfer and structural integrity for the Chevrolet 12-bolt.
8. Do you see the difference between the GM axleshaft (left) and the Moser axle on the right? The Moser axle isn’t tapered like the original GM shaft. Strength is lost via the tapered shaft and excessive porosity.
9. The Chevelle’s factory axleshaft exhibits excessive wear at the bearing contact surface, which indicates the need for replacement.
10. The axleshaft rides on these roller bearings. When you remove the old axleshaft and install the new shaft, take extra care not to damage the axle seal. Lift the axleshaft during removal to make sure it doesn’t tear the seal or eject the garter spring inside. While you’re in here, pack the roller bearings with wheel bearing grease for a good start-up.
11. The differential cap bolts are removed next. Hold onto the differential, which can fall out and smash your toes.
12. The damaged differential is removed and thrown on the shelf for safe keeping. Unless there is significant damage to the clutch-style differential, it can always be rebuilt and used again.
13. Shims are added or subtracted from each side to achieve proper backlash between the pinion and ring gears.
14. As a matter of good practice, the differential caps are loosely reinstalled in the same position they were removed from for proper reference during assembly. Because these caps are machined to match the centersection, they must go back to exactly the same location.
15. The axlehousing is thoroughly cleaned with brake cleaner, with all lubricant and debris removed from the sump.
16. Because Joel recently rebuilt the axle, it has a new ring-and-pinion set, bearings, and seals. Joel is transferring the current ring gear to the Eaton Detroit Truetrac using a mallet for removal. Never use a hammer for ring gear removal.
17. The new differential carrier bearings are pressed onto the Detroit Truetrac differential as shown. You want the bearing’s inner race completely seated onto the differential. Confirm it is seated before going any further. It is always good to examine the bearing rollers for scoring and nicks, even if the bearing is new.
18. Joel has opted for ARP fasteners to secure the ring gear because it is unwise to take any chances.
19. Joel has opted for ARP fasteners to secure the ring gear because it is unwise to take any chances.
20. The ring gear bolts are torqued to 55 ft-lb in a star pattern as shown. You can also torque these bolts to the proper specifications once the differential is installed in the housing, which makes tightening easier.
21. These side shims are employed to correctly center the ring gear and differential. The ring-to-pinion backlash is adjusted by adding and subtracting shims from each side until you achieve the proper backlash.
22. The differential is loaded into the case and centered along with the bearing caps on each side.
23. With the bearing caps secured and torqued to 60 ft-lb, the ring gear bolts are torqued in a star pattern to achieve uniform torque and ring gear seating.
24. Joel checks the ring-and-pinion backlash using gear-lapping paste and a dial indicator, which should get the ring and pinion marriage where it belongs. Joel did not remove the pinion gear because he was confident backlash would be close to where it was prior to disassembly. Backlash should be between 0.006-0.010-inch. Work the ring gear back and forth via the yoke or pinion flange.
25. The ring gear tooth pattern should look like this once you have run the gears through several times. The pinion depth looks good, as does the backlash.
26. The axleshafts have been installed. This is what you want to see when the axles are seated. The C-clips fit into the groove shown, then the axles and C-clips are seated into the differential.
27. With the axles and C-clips seated, this foolproof axle block in the Truetrac differential takes the place of the lock pin Joel removed from the failed differential.
28. This round block is seated against the inner axle block which virtually eliminates the risk of C-clip failure. A snap ring keeps the round block seated.
29. Summit’s 12-bolt cast-aluminum differential cover is engineered to get rid of destructive heat and secure the carrier bearing caps to reduce the risk of failure.
30. These adjustment studs in the Summit Racing cover provide a preload against the carrier bearing caps inside, which provides support. The cover stud preload against the bearing caps is 10 in-lb. Once the preload is set, tighten the nuts. The magnetic drain plug catches ferrous metal debris. We’ll fill the rearend with Torco SGO 75W90 synthetic gear oil and their Type G limited-slip additive.
Photos by Jim Smart