I got few years ago a new 3.23 gearset for my 9-bolt rear axle and it's been just lying on the shelf as I've hardly used the car, not to mention that I would have replaced the stock rear with 9-bolt, but for some reason I now "woke up" and decided to rebuild the rear. The gears came with no installation instructions. Now if anyone has done this before, could he/she comment the following, please: -how to determine the correct installation "height" for the pinion gear? Please notice, I don't live in the US and these cars are quite rare here and probably the official GM tools just exist in some bigger cities at least 400km far away from me. I've got the GM camaro shop manual and they describe a complicated method of determining the height by some special tools that I just can't get to my hands. Does anyone know what should be the distance to the pinion top surface from, lets say, the gasket surface of the axle, or the center of the ring gear? Both the old and the new pinion gears have a "0" stamped on the top so they should be installed to the very same height. -can the correct gear lash be determined / decrypted from the numbers stamped on the gears? The new gearsets ring gear has numbers 0578-020 3.23 EP D and 28893. I guess the last set of numbers is the serial # as the pinion has the same numbers. Do the 0578-020 tell something? -if I check out the contact pattern using the color, and get the contact to the middle of the tooth, is the setup then OK or do I still have to worry about the installation height? -how accurate must the setup be to avoid the gears from howling like a bunch of wolves? Must the height and gear lash be like 0.01mm tolerance or is 0.1mm enough? The original gearset has the numbers 0578029009BWCD3.27D. The gear ratio is 3.27 but what do the other numbers mean? Thanks!

First, are you sure that the 3.23 gears are for the 9 bolt? The 3.23 gear is a 10 bolt ratio. I'm not aware of any 3.23 gears for the 9 bolt. The 9 bolt had the 3.27 gears.

As for pinion depth, this should be determined by the contact pattern. Don't worry about a special pinion depth tool. Start with the original pinion shim and set the backlash to .008 inch (don't know what that converts to in mm) and check the contact pattern with the marking compound. You did get an installation kit too didn't you? If not then don't start until you get one. The pinion depth and backlash need to be set to within .001 inch of nominal. Changed in pinion depth are made in .002 to .003 inch amounts at a time until it is close. Then adjustments of .001 and .002 inch can be made to get it just right. Always go by what the contact pattern tells you. Learn to read the pattern and make adjustments by what you see. I've built several hundred rear ends over the years and I don't use a pinion depth tool. I found them to be a waste of time and money.

It was sold as a 9-bolt gearset. At least all the pinion and ring gear dimensions match the old gearset and I already bolted the ring gear on its place. I got the gears from www.9bolt.com and they're of some aftermarket production, not original BW. Anyway, I looked at the old gearset and decided to use that again, and sell the new gears. They don't seem that worn out and the howl they caused obviously just came in because of the polyurethane bushings. At least with them even the original 10-bolt rear axle howled, but when I got back to rubber bushings, the howling stopped.

I used to build a lot of SAAB transmissions and even if FWD, they have the same kind of setup for the final drive, and no matter which gearbox I opened, there were always four shim plates under the pinion of the same thickness. Obviously the manufacturing process is quite accurate, at least in Sweden.

How is the gear backlash measured? Is it measured on the heel of the ring gear tooth? I've got a dial indicator for this purpose and 0,008in converts to 0,2mm

Measure like this. Get the dial indicator as close to perpendicular as possible with the center line of the ring gear.


If the ring gear bolted on then it is a 9 bolt gear. The 9 bolt has 8 ring gear bolts and the 10 bolt has 10, so a 10 bolt gear will not bolt up to a 9 bolt differential.

I bolted on the old gears as a closer look to the new gearset revealed a very crappy finishing on the tooth surfaces. They're probably some Chinese crap even if bought from a US company. I put in the original shim thickness (0.4mm) and used the original shims also for the carrier. The clearance seems to be quite OK. I attached two pics of the contact pattern and at least to me it looks that the contact on the drive side focuses to the toe end of the ring gear tooth. What should I do to move the contact closer to the center? I've been looking at different contact pattern interpretation pictures but found the information to have some deviations depending on where it's printed on.


Another question, about the GM cars in general. DO they balance the cranks and flywheels separately so that they can be changed individually, or must they be replaced as matched pairs? I changed a 3.1L crank to my 2.8L motor (of course the pistons, too) and got annoying rumble at the range from 2000 to 2500rpm that didn't exist with the old crank. What is the probability that the rumble is caused by the clutch assembly that hasn't worked properly even for a day since its installation?

The pinion is too deep. Use a thinner pinion shim and try again.

I have the 9 bolt rear with 3.27 gears with the letters r.a.t. on the diff tag, what does that stand for? and do I have to change out the pumpkin for a 10 bolt from a 88 camaro or firebird, to install 3:73 gears?

You can put 3.70 gears in your 9 bolt.

New attempt... Is this of any better? I moved the pinion farther away from the ring gear by 0.1mm and also moved the ring gear leftwards (farther away from the pinion) the same amount. The backlash is now 0.15mm (earlier was only 0.05mm, far too tight).

Better, but looks like the pinion is still just a little too deep. I don't have a metric conversion handy, but backlash on a used gear should be about .010 inch.

0.01" converts to 0.25mm. I could make the shim even thinner under the pinion bearing cup but then it's only 0.1mm. Is it possible that the axle would have been assembled at the factory with no shims at all? I doubt 0.1mm movement would bring the contact pattern to optimal.

It might run fine there, but if you can get about .002 inch more out of the pinion shim I think it will be good.

When you run your pattern you want to run it on three consecutive teeth and need to see your cost side of the gear also that is where most howl comes from. On that rear you want between 6 to 10 thousands backlash. And from what it looks like to me the pinion is too far away due to its coming of the crown of the gear. And when you run your pattern apply some preasure to the ring gear as you turn the pinion it will result in a clearer pattern. And a trick is to thin out the pattern compound with some atf it will also make alot clearer pattern.

Thanks for the help, do you have a link to were I might get them?

Thanks for the help, do you have a link to were I might get them?

Theres some on ebay right now.

You can get them from me. Send an emal if you are interested. biggearhead@bellsouth.net