Complete Rear bushing and 3.91 gear swap DIY

[jeremywes]

On to the 3.91 gear swap.

I was impressed by this little cordless impact gun. It handled 90% of the work, however I did have to use a more powerful corded impact gun for some of the heavy lifting



This puller was too small so I had to open it up with a grinder but I was able to make it work. a 3 arm puller would have worked better.


Here is a picture after the yoke is removed


Next I marked the bearing caps left and right with a small chisel


Here you can see that I have layed everything out exactly how it came out of the housing so I can ensure it goes back in the same way.


To remove the pinion I hammered it over a piece of wood so it had something to fall on.


Here is the pinion removed from the housing


Next remove the bolts that attach the ring.
NOTE: These are reverse thread bolts (lefty tighty, Righty Loosey)


Once the bolts have been removed tap the ring off, I had to gently pry to get it started. Again I used a piece of wood so it could fall safely


I placed the carrier in the freezer and the ring on a propane BBQ. They went together very easily after that. Otherwise they would need to be pressed on.







Next using the new bolts from the GM kit I attached the ring


I removed and reused the old pinion bearing using a friends press. I would have just used a new one but the car only has 3k miles on it and the local dealer didn't carry it in stock.

Now same operation for the pinion bearing.


Here you can see the old pinion, shim, bearing, crush sleeve. Make sure it goes back on the pinion the exact same way.


Here is the old crush sleeve compared to the new one


I made a little tool out of 2" PVC pipe to tap the bearing on. It didn't take much pressure to tap it on once the pinion had been in the freezer and the bearing was heated.


Next CLEAN the pinion yoke by scraping all of the white factory paste out of the splines. Grease the inner bearing and crush sleeve to make sure it doesn't bind when you begin re-assembly.

Here I am using a pry bar and two of the yoke bolts to get some leverage as I tighten the pinion nut down to crush the pinion crush sleeve. Here is where a strong impact gun will come in handy


This is the tricky part. Tighten the pinion nut until you achieve 18 - 25 inch pounds of rotational torque. Work in slow bursts and take your time because you can't un-crush if you go past the torque range of bearing preload.


Once you have the proper rotational torque pin the pinion nut by punching it into the pinion recess


Now insert the carrier into the housing keeping all of the bearing races and shims on the same sides that they came from.


Here I used a 2x4 to tap the left side shim into place.


Now install the bearing caps and torque to spec. I used 77ftlbs

NOTE: GM makes 3 different rear ends so make sure you know what you have. I am installing this into a 2012 SS with the 218mm axles

Remember to use anti-seize anytime a steel bolt goes into an aluminum housing


Here are some GM spec that a friend printed out for me


I made a steel plate so I could get the magnetic base dial indicator to attach to the housing.


Here is the zeroed dial indicator


Here the backlash is .003" GM spec is .002 - .005




Here you can see the pattern on the DRIVE side of the ring gear teeth


Here you can see the pattern on the COAST side of the ring gear teeth


Next reinstall the case cover. I used 55lbft

Remember to use anti-seize anytime a steel bolt goes into an aluminum housing


Remember the gear lube WITH a friction modifier


Here are some GM specs


Here is where you can see what rear end your car has. You can see mine is a 218mm axle

[jeremywes]

Update- I corrected the Axle ratio with a diablo tunner so now the TC is happy and speed is registering correctly. I do notice a very subtle vibration from the diff bushings but no normal person could tell the difference. I finished my initial break-in on the gears and tried a few 75% 0-60 pulls and you can really notice the torque down low.

[IPT]

excellent write up and great skills you got, awesome. I can only imagine the pull with those gears!

[driyac]

Originally Posted by IPT

excellent write up and great skills you got, awesome. I can only imagine the pull with those gears!

Times a billion! You should open up a shop ( just kidding). You have been very detailed and meticulous with your build.

[jeremywes]

Originally Posted by IPT

excellent write up and great skills you got, awesome. I can only imagine the pull with those gears!

They do pull hard, even when cruising at higher speeds. 1st gear is a neck snapper!

Originally Posted by driyac

Times a billion! You should open up a shop ( just kidding). You have been very detailed and meticulous with your build.

Thanks guys! I hope this build helps anyone wanting to do it on their own.

[silverz28]

Great "How To"! Although I would probably not do this myself on my daily driver. I love the freezer and BBQ. Great way to expand and contract metal.

[hairzee]

WOW. You da man!

I have started my front bushing replacement and have one done.... what a pain in the ass this is going to be.

I had the gear swap done at a shop for couple hundred... small price to pay for me not screwing that up.

thanks for your time and knowledge.... VERY much appreciated.

[jeremywes]

Originally Posted by hairzee

WOW. You da man!

I have started my front bushing replacement and have one done.... what a pain in the ass this is going to be.

I had the gear swap done at a shop for couple hundred... small price to pay for me not screwing that up.

thanks for your time and knowledge.... VERY much appreciated.

It is a pain in the ass for sure! But well worth it. If you can do it over several days it makes it easier. The whole project took me a little over 10 hours over the course of 2 weekends and a few beers

Good luck

[PfadtRacing]

Great write-up!

[hairzee]

How do i grt the new bushings in. I dont see anyone else having a problem with this step. So what am I missing here.

[jeremywes]

Originally Posted by hairzee

How do i grt the new bushings in. I dont see anyone else having a problem with this step. So what am I missing here.

I'm not sure about this one. Post as a new thread and I'm sure you'll get a solution from someone who has already done it! Good luck

[hairzee]

thanks Jeremy, will do

[2012-2SS]

Great write-up! Subscribing for future reference.

Sent from my Galaxy S3

[jeremywes]

Update - After a gear break-in period consisting of roughly 10 progressive (20 minute) heat cycle sessions followed by a complete cool down and approximately a thousand total miles since I installed the 3.91's, I replaced the fluid in the rear end. I found the same heavy micro metallic build up on the mag plug that I had after breaking in the original gears. Another quart of royal purple and she is good to go.