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I had a couple of reasons for doing the swap.
A lot of people don't mind gear noise in the differential in the pursuit of power and acceleration. I really did not want to sacrifice longevity and quietness for price. Drag racing gears are often softer to handle the shock of launching, but at the expense of longevity and sometimes are noticeably louder than stock gears. Some brands just have the reputation of being loud, Richmond for one.
I really did not want to sacrifice gas mileage and since 1st and 6th gears were a little too tall for my tastes, this seemed like a good choice.
I wound up doing this job completely, twice. For my first try I went to a 3.73:1 ratio. I searched around and decided on AAM gears. From what I have read, American Axle and Manufacturing took over gear production and supply for GM a number of years ago. These are the gears that go into new GM vehicles. They should be more finely lapped and run-in before you get them as compared to other brands. Why does this matter? Gears are manufactured with a theoretical spacing and alignment. Since these are hypoid gears there is an inherent amount of rolling/sliding contact between the gear teeth while turning. These gears should produce less heat on initial break-in and have a better finish than other gears.
I made several mistakes learning to read patterns and as a result I never was happy with my AAM gears. I had to set the gears up multiple times. However, they were also poorly toleranced and made installation and removal of the pinion bearings extremely difficult. I ran the 3.73s for a season and then switched to Motive 4.10 gears. Knowing a lot more this time the 4.10 gear install went really well and they are really quiet.
I will try to show what not to do as well as what works in the following pages.
Start by lifting the car and removing the wheels. Remove the brake calipers and the disc rotors. Hang the calipers so as to not damage the flexible brake lines.
Just to be careful, I marked the companion flange and drive shaft orientation so that everything would go back together the same way when it was all back together.
After removing the drive shaft from the flange, I simply lifted it up and placed some wood blocks on top of the exhaust and torque arm to hold it up. This meant I never had to pull the shaft out of the transmission and never had to worry about storing the shaft anywhere while working on the axle.
Remove the cover bolts. I left the top one in loosely to act as a hinge as I pried the lid open. The fluid globs out everywhere once the lid is loose. Check the magnet on the inside of the lid for unusual build of of metallic debris. Mine looked normal with just a bit of fuzz on it.
After hosing the housing out with brake cleaner, I took a reading off the stock gears just to see if anything was wrong with them. The factory set up measured out at .013" of back lash where the service manual tolerance is .005" - .009" total. So for a still unknown reason, the factory backlash was too loose. I checked the factory pattern as well as shown in the next photos. To measure the back lash, align the dial indicator as shown, close to tangential to the gear outer diameter. Clunk the ring gear back and forth with out letting the pinion move. The difference in the two measurements on your dial indicator is your back lash measurement.
To measure a pattern, put gear marking compound on the teeth and turn the gear through the pinion. Do this while applying drag torque on the ring gear. I use a heavy work glove and jammed my hand into the housing at the top of the ring gear to make it hard to turn the pinion. Do this in both directions to make a pattern on both the drive and coast sides of the gear. It is a good ideas to do this in two places on the ring gear since there is always some backlash variation.
The drive side pattern shows the gear somewhat too far out to the heel.
The coast side shows the opposite with the pattern somewhat shifted to the toe. Between the two it looks like the pattern would be pretty good if the excessive backlash were taken up a bit.
Spin the differential around to get at the bolt holding the 'paddle' in the middle of the carrier (visible in picture). Remove it with an 8 mm socket (it seemed to fit better than the 5/16"). Slide the paddle out, shove each axle in towards the center of the car and the 'C' clips holding the axles will slip out, remove them. Now pull the axles out of the housing from each side.
The fluid will drip out forever, I took this time to clean the gasket surface. It takes a lot of brake cleaner to hose out all the fluid, it all drools out the bottom.
In this closer view you can see that once the retaining bolt is removed, the paddle slides out and just misses the ring gear teeth. Because I was changing to a numerically higher ratio (3.73:1 vs 3.42:1) The pinion gear is therefore smaller and has fewer teeth (11 vs. 12). The ring gear thickness and tooth height is therefore taller in order to reach the pinion (the ring gear teeth tips extend a little more to the right in this picture). This crowds the path of the paddle into the carrier.
Once the paddle is out, push the axle shafts inward. In the picture above, the shaft tips are in as far as they go. The C clips have been taken out. They just drop out once you push the axle in, fish them out with a magnet.
The C clips are really simple little parts and they don't have much to do but hang on to the end of the axle. These clips are the only thing keeping the wheel and axle from leaving out the side of the car.
Remove the two main caps and pry the center section out of the housing. I found that a crow bar fit nicely in from the left side into the webbing of the face behind the ring gear. I didn't like sticking the bar into the Torsen's internal gear region. There is no way to pry heavily without pressing on the spiral gears pretty heavily. I did not want to mar them in any way. The housing is really in there tightly. The factory uses a case spreader that pries the whole case open using the two holes in the rear cover flange. The center section plus its shims is approximately .006"-.008" larger than the empty case. At the factory, the case is pried open, the center section and shims are dropped in and the spreader is removed. The case puts a huge preload on the center section and this is what you are fighting when you pry things out.
Just grab the yoke with one hand and using an impact wrench, spin the nut off. Pull the nut and thick washer off. Using a large hammer, tap the end of the pinion. The yoke will pop off very easily, you are mostly pushing the pinion through the front smaller pinion bearing. It is important to keep the pinion pushed back to seat the bearing in its seat when hitting with the hammer. You do not want to be slamming the bearing into its seat over and over. Once the pinion is out, the bearing is trapped loosely inside until you remove the seal from the housing.
There are two seal styles. The one on the right is what was in my car, the left one was the replacement that came with the install kit.
A piece is pressed on over the yoke to mate up properly with the larger style seal. I do not know if this was a factory item or a warranty change. GM warranty records for my car showed that the pinion seal had been changed twice.
With the pinion now removed you can now pop out the pinion bearing races. Here you can see both races. There are 2 access windows cast into the housing. You can see them at the sides of the forward bearing in this picture. Reach in with a large drift and tap alternately on each side and push the race out of the housing. Come in through the front to remove the rear larger bearing.
The bearings show wear from 50,000 km of use. The clouded appearance of the races seems normal, but there is also some shiny pits in the races. This seems to be normal wear.
On this bearing, the cloudy areas are not uniform around the bearing race, it has a couple of dips for some reason, something was not lined up or evenly supported.
Next step was to put the new pinion bearing cups into the axle housing. This bearing and seal driver kit is from Lisle tools. The nice thing is it included an air hammer fitting (black piece). This saves a lot of hassle when there is no room to swing a hammer under a car. On the front pinion bearing seat, the lower right piece was the best fit but it would not seat in the bearing cup the way it should. A friend with a lathe had to turn it down slightly to make it fit. Its best not to mess around with the bearing cups, any burrs or dents and the bearing will not last long.
Time to replace the bearings.
These side bearings were not too hard to remove even with this light duty puller. The new ones were installed using a press.
I started by pulling the ring gear off and trial fit the new one on. Remember that the ring gear bolts are left hand threaded. In my case, I pushed the paddle into place with the new ring gear installed and found it got stuck just as the last of it was about to get past the ring gear teeth. I ground small chamfers where it scraped on the ring gear teeth as seen above on the right end of the paddle. 4.10:1 gears would require more grinding on the paddle since the teeth are even taller.
New bearings are on and the carrier is ready for the new ring gear. The new one is at the left.
Torqueing of the ring gear. I torqued the bolts in a criss-cross pattern in 3 stages. I torqued the bolts all to 30 ft.lb, 70 ft.lbs, and finally 89 ft.lbs. One point about fasteners, the rebuild kit I bought had much smaller ring gear bolts included in it. The OEM bolts had much larger heads and therefore a much bigger bearing area. I re-used the stock bolts.
This is a Motive 4.10 gear. Unlike the OEM AAM gear, the tips of the teeth are already chamfered. This lets the paddle slip in much easier. The grinding done on the paddle for the 3.73 gear I did first was enough for this chamfered 4.10 gear.
First step was to pull the large pinion bearing from the pinion. This bearing separators was needed to pull the pinion bearing off of the pinion to recover the pinion shim. I placed the separator under the bearing, snugged it up, and used a press to push the pinion down, using metal blocks to hold the separator up allowing room for the pinion head to slide down. Once the old pinion bearing was off I hogged out the inner race out little with a die grinder to make it a slip fit over the new pinion. This let me make quick shim changes while doing gear pattern checks.
To help with pinion shim swapping I did not want to fight with the self locking pinion nut. I took the old nut and ground off the self-locking portion so that it would just spin on and off easily.
The shim is in place, the bearing stops at this point and needs to be pressed down. The pipe is used to help press down on the bearing.
The old pinion bearing race flipped over makes a good pressing tool along with the pipe safely pushes the bearing in place. Make sure the bearing is pressed down really well. Clean the parts carefully before assembly. Even a small piece of debris would prevent the parts from evenly seating.
Goto 3.73:1 AAM Gear Pattern Setting