12 bolt rear end

Has anyone put a GM 12 bolt in their 90's caprice? I want to put one in my 94 caprice. ive seen alot out there used, rebuilt but dont know if they are all relatively close in length. most are from 65-70 gm passenger cars but i dont know if i can make that work. i just dont want to get one that is too wide. i plan on redoing/moving/replacing the mounts for the shocks and springs but the width is worrying me. any advise would be greatly appreciated.

I have found swapping stock rears to be an exercise in frustration. A custom-made swap or a properly-engineered kit to easily weld-on the mounts on to a housing of your choice is the way to go.

I have had great success with the Ford 9-inch rear. Its easy to set gears up, easy to swap ratios and is lighter to boot, than many GM rears.

Here are some suppliers which may be good for info or parts/kits.


Art Morrison Enterprises
5301 8th Street East
Fife, WA 98424
(800)929-7188
www.artmorrison.com

Hotchkis Performance
12035 Burke St. Suite 13
Santa Fe Springs, CA 90670
(562)907-7757
www.hotchkis.net


Auburn Gear Inc.
400 E. Auburn Drive
Auburn, IN 46706
(219)925-3200
www.auburngear.com

Mark Williams Enterprises
765 South Pierce Avenue
Louisville, CO 80027
(800)525-1963
www.markwilliams.com
Chassis Engineering
1500 Avenue R
Riviera Beach, FL 33404
(561)863-2188
www.chassisengineering.com

Moser Engineering
102 Performance Drive
Portland, IN 47371
(219)726-6689
www.moserengineering.com


Chris Alston's Chassisworks
8661 Younger Creek Drive
Sacramento, CA 95828
(800)722-2269
www.cachassisworks.com

S&W Race Cars
11 Mennonite Church Road
Spring City, PA 19475
(800)523-3353
www.swracecars.com

Currie Enterprises
1480 B North Tustin Avenue
Anaheim, CA 92807
(714)528-6957
www.currieenterprises.com

Tom's Differentials
14709 Paramount Blvd.
Paramount, CA 90723
(562)634-8431
www.tomsdifferentials.com

If this is a track car a 9" is O.K. If it's a DD I'd stick with a GM 8½" 10-bolt. They are about 4-6% more efficient and will save you fuel and give better trap speed. The 9" is stout if built right but pretty inefficient because of the poor pinion location. In otherwise equal cars the one with a 9" will lose because of the HP wasted in the axle.

Its not a track car but also is not a Daily Driver (only driven in summer). Car has a 5.7 350 in it but i want to change to possibly a 383 stroker or something else down the road. I only want to do the rear one time so i want to do it right and im thinking 9 inch is the way to go but not sure what gears i should do or if i should put a posi in it?

Its not a track car but also is not a Daily Driver (only driven in summer). Car has a 5.7 350 in it but i want to change to possibly a 383 stroker or something else down the road. I only want to do the rear one time so i want to do it right and im thinking 9 inch is the way to go but not sure what gears i should do or if i should put a posi in it?


with rear disc brakes use the 8.5 inch ring gear limited slip axle code 2LZ 3.23 axle gear ratio. this was used in the police cars. axle RPO GU5

with the 9.5 inch rear drum brakes use the limited slip axle code 2LU . this is also the 3.23 ratio diff. axle RPO GU5

my 1996 impala has the 8.5inch ring gear diff. 3.08 axle ratio. working great since 1995 when I purchaed it. with these gears you do have a great top end if you like going at 100 mph . fuel MPG is still about 17 MPG all around driving 25 mpg on hywy 75MPH.

3.23 ratio would be good for lower speed acceleration. though the 3.08 with the wide tires do provide great traction ONLY ON DRY PAVEMENT.

IF YOU USE SOME OTHER DIFF REMEMBER THIS HAS A SPEED SENSOR IN THE DIFF. AT LEAST IN MY 1996 IT DOES. 1994 MAY NOT BE AN ISSUE.

if you now have the 7 5/8 inch GW9 , 2.93 axle ratio open diff I would replace it.

axle code is on the forward facing axle tube passenger side .

ya i have the stock 7.5 axle with 2.56's i believe. What is the difference between limited slip and posi? Are they different or are they the same thing just two different names? im very confused on this and getting different answers when looking online??

ya i have the stock 7.5 axle with 2.56's i believe. What is the difference between limited slip and posi? Are they different or are they the same thing just two different names? im very confused on this and getting different answers when looking online??

these vehicles with the limited slip diff is just that a very limited amount of both wheels providing traction. this is always working . always providing both wheels traction.

with one rear wheel on the ground the other off the ground vehicle in neutral and the front wheels blocked so it will not roll it takes about 50-60 ft lbs of torque to rotate the rear wheel off the ground.


with this limited slip both tires must be matched for wear and tire pressure. if one tire is worn the other new or one tire has low pressure this causes the diff to rapidly wear out. also I have found the diff lube needs replacing at about 6,ooomles when first in use. then every 20-30,ooo mi especially with intown driving ..more turns more wear.

on the gm trucks you have the locking diff. this is the eaton locker. this is not always working or locked. only when one wheel rotates more than the other does the rear DIFF lock as one axle, no slip . this continues for several rotations then releases. when making turns this does not engage . when you get on an ice patch the wheel on solid pavement with quickly provide traction with no slippage. this is because the rear axle is locked solid. trucks need this type of diff. cars only need the limited type.

plowing / pulling a boat out at low tide on a steep ramp you need that locking diff..

the term positraction has been around for a very long time . never understood what it is refering to ...gm calls the caprice/impala diffs limited slip. on the trucks its called the locking diff.

8.5" 10bolt is plenty of axle in '89 they upped the spline count to 30 and it is pretty much as strong as a 12-bolt and certainly stronger than some 70s era junk.

Only reason to consider a 9" would be for gear ratio fine tuning or if you were looking to hit the axle from idle with enough power to turn 9 second timeslips or hitting it soft with enough power to run 8s.

The 8.5" is easy to set gears up in if you have the tools and take your time. I have done 3.42s, 3.73s, 4.10s and 4.56s in this axle, actually I have setup 3 sets of 3.73s in 8.5" axles and another set of 4.10s in a 7.5" under a Camaro. Only time I ever had to do any rework on any of these was the Camaro axle because the guy wanted me to use a solid spacer instead of a crush sleeve and I did not get that set right the first time so I had to redo it.

I am just a dumb carpenter though.

You asked in a PM if I had any rear suspension work. I have aftermarket rear lower control arms, have BMR but would do UMI if I were buying again today. Uppers NEED to be able to rotate, the stock ones are flimsy with rubber so they can allow that, many aftermarket options will not unless they have some variation of a heim or johnny joint in them. Heim will wear out and possibly make noise, johnny joints are expensive so stock uppers do nicely.