From http://craig.backfire.ca/pages/autos/boxchevy/10-bolt
10-Bolt Axle Rebuild
The axle that the Caprice came with was a 7.5-inch "10 bolt" axle. They are not known to be very strong, and mine had over 200,000 km on it, did not have a limited slip carrier, and was very rusty. To make things worse, it had a 2.41:1 gear ratio which was passable with the 3-speed automatic transmission, but had no place behind a 5-speed manual with overdrive. Had I stuck with this axle, the engine would only be turning at 1200 RPM at 100 km/h, which is too low to be in the power band. Finally, the axle was equipped with small drum brakes, and disc brakes for this axle are limited, if they even exist.
To remedy all of the aforementioned problems in one fell swoop, I bought a 10-bolt axle with an 8.5-inch ring gear from a 1992 Buick Roadmaster. This axle is a drop-in replacement for my car, other than having a longer pinion which necessitates a shorter driveshaft.
Cleaning and Painting the Axle
The first task, which was definitely the easiest, was to clean, strip, and paint the new axle. This processes only took a few hours, and was made simpler by mounting it to an engine stand with a custom bracket and threaded rods.
Removal and Installation of Bearings
The axle has a total of six bearings. There is one on each end of the housing which bears almost all of the weight of the vehicle put through the axles, and then two which support the carrier in the housing. The final two support the pinion, with one at the front of the housing, and one near the pinion gear itself.
One difficult task with the disassembly process was removal of the bearings from the carrier. There are many bearing pullers, but I could not find one that would grip the bearing properly. Instead, I used an angle grinder and very carefully cut the bearings off. It took a very long time and a steady hand to avoid damaging the carrier.
The bearing races inside the axle housing were knocked out with punches, and new ones were installed with a mallet. To confirm that the races had seated completely in the housing, I used the thinnest feeler gauge I have to make sure there was no clearance of any kind.
The bearings at the ends of the housing can only be removed with a slide hammer and a great deal of force. To install the new bearings, I used the old ones as shields and drove them into the housing with a sledgehammer.
The bearings that go on either side of the carrier must be installed with a very powerful hydraulic press. My press at home was not up to the job, so I brought it to a nearby garage to have the task completed.
The bearing that goes on the gear end of the pinion must also be pressed on, but goes on much more easily. There is a thin shim that sits between this bearing and the pinion gear, and directly affects the gear mesh pattern. Unfortunately, this bearing must be removed every time the shim needs to be adjusted.
Setting Up the Gears
With the axle stripped down and cleaned, it was now possible to install the new gears. I chose Richmond Sportsman gears with a ratio of 3.42:1 to match the rest of the car's configuration. This is a significant difference to the 2.41:1 ratio it is replacing (about 40%).
There are many variables to be considered when setting up the gears in a differential. The gears must mesh with one another properly, or they could end up being noisy, or fail completely. To illustrate the complexity of setting up the gears, I have made a gear mesh dependency graph, as seen below.
Of the many variables to deal with, many have strict specifications. The pinion bearing and carrier bearing preloads are important as they relate to bearing wear. Too much preload would cause premature failure of the bearings, while too little would allow parts to flex, possibly to the point of failure. The pinion bearings have an especially narrow range of tolerance for preload.
Backlash is the amount of play between the pinion gear and ring gear. If there is too little, the gears may bind when they reach operating temperature. If there is too much backlash, the teeth could snap when the gears are suddenly loaded, such as when accelerating from rest.
New gear sets normally come with markings which indicate the pinion depth and backlash necessary to have proper gear mesh. Unfortunately, this requires the use of a special tool to measure the pinion depth, which is difficult to obtain. Instead, a trial-and-error method can be used.
On a GM 8.5-inch 10 bolt axle, the starting point for establishing gear mesh is to use a 0.035" pinion bearing shim, and setting the backlash to within specifications with the carrier bearing shims. The pinion nut must be torqued to prevent the pinion gear from wobbling when looking at the gear mesh pattern. At this stage, pinion bearing preload is not important, but it should not be overdone.
Measuring Backlash
To measure backlash, a dial indicator with a magnetic base is used. The pinion must be locked to prevent it from rotating. The ring gear can then be turned by hand. The free play (backlash) will appear as a maximum on the dial indicator. It is important to setup the dial indicator to be perpendicular to the tooth face to get an accurate reading.
Reading the Gear Mesh Pattern
To read the gear mesh pattern, a white marking paste is put on some of the ring gear teeth. The ring gear (not the pinion) is then turned by hand a few times. The pinion teeth will strip off some of the marking paste, and leave a clear pattern. This pattern can then be used to interpret whether the the pinion depth is too large or too small, or whether the backlash is too large or too small.
The pinion depth and backlash are the two direct means to change the gear mesh pattern. The most important consideration with respect to gear mesh is the location of the contact point relative to the root (base) and the top land (peak) of each tooth. The location of the contact point with respect to the toe (inward edge) and the heel (outward edge) is also important, but much harder to change. Generally speaking, if the heel-toe contact points of the contact point is acceptable, then it should be left as-is. More attention should go to the root and top land relationship. It is also important to check both the drive side and coast side of each tooth on the ring gear.
When I first set up the gears, I got the following pattern, seen below.
Gear mesh pattern from attempt number one.
The pattern was close to being adequate, but the pattern was a bit too close to the top land on both sides. To remedy this, I changed the pinion shim, reset the backlash (as they are dependent on one another) and tried again. The second time, I got the pattern seen below. The pattern barely looks different from the first, but in real life, the difference was clear.
Gear mesh pattern attempt number two. The pattern appears to be closer to the top of the teeth than the first time, but are actually farther in. The marking paste was starting to smear from having turned it so many times.
Pinion and Carrier Bearing Preload
Once a proper gear mesh has been established, the number and overall size of the shims used on each side of the carrier bearings had to be recorded. This ensures that the proper gear mesh pattern will be maintained after final assembly.
The carrier bearing preload should be set first, with the pinion removed from the housing completely so that the carrier can be spun freely. The carrier must be reinstalled with the shims that were used to setup the gear pattern. Then, the carrier is spun by hand and watched for resistance. The carrier should spin freely, but not continuously. It should slow down after a few seconds. If it spins very freely, shims of equal thickness need to be added to both sides until it spins correctly. If the carrier does not spin nicely but rather slows down and stops very quickly, shims of equal size must be removed from both sides.
Once the carrier preload is set, the number and overall size of the shims must be recorded again, and then removed from the housing along with the carrier. The pinion is then installed into the housing, along with the crush sleeve that goes between the pinion yoke and the front pinion bearing. Also, the splines on the pinion shaft need to have RTV sealer put on them to prevent oil from leaking out.
Pinion bearing preload is established by tightening the large nut which crushes the crush sleeve and presses on the bearing. The actual preload is determined by using a very small torque wrench on the pinion nut to determine the pinion shaft's resistance to rotation.
Normally, it takes many turns on the pinion nut with a large breaker bar to get the crush sleeve started, but the bearing preload then comes up very rapidly. I went through two nuts and crush sleeves before finally getting it right on the third try. A breaker bar with a very large handle can improve control over the nut, but brings the risk of overtightening.
Final Assembly
With the pinion installed and set with proper preload, the carrier can be reinstalled with the same number of shims, and torqued down. From here, the brakes and other components can be installed.
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