G-body Rearend Swap Guide
Written by Todd Geisler

In this tech section, I will discuss several ways to improve the strength of the rear end in any G-body car. Topics to be covered include: axle housing choices, their strengths and weaknesses, application choices, estimated cost and suspension options. The information here is not to be considered hard fact, only what I have gathered over the past few years. If anyone feels this info to be incorrect, please e-mail me and I will make the appropriate correction(s).This article will be assuming the use of the GM 4-link suspension design.

7.5" GM 10 bolt- stock specs: 7.5" ring gear, 1.438" dia. pinion shaft, 26 spline axles.

This rear is the most common housing to be found under any GM G-body car. The rear is called a 7.5 " in reference to its ring gear diameter. For street usage, this rear will hold up just fine. Also, the 7.5 10 bolt can be used for limited drag racing use. Street tires are recommended to limit stress on the "weak" internals. Slicks can be used with a 7.5, but just don't expect it to last very long. A heavy car and a sticky tire are just asking for breakage problems when using the stock 7.5" 10 bolt. The only major aftermarket parts available for this rear are an Auburn posi unit and a limited selection of gears. For this reason, it makes the most sense to not invest allot of money in this rear. If you plan to visit the drag strip on any regular basis, I would consider upgrading the rear with a heavier duty unit.

Note from Phillip:  A relatively cheap upgrade to the stock 7.5" rearend is to get a F-body "take-off" posi unit from SLP Engineering.  These are 28 spline units that use the 7 5/8" (7.625) ring gear.  They are a direct bolt-in for the 7.5" rear, and you can reportedly get them for only $100 from SLP.  Also, order 28 spline axles from Moser Engineering for a Buick Grand National.  These are a direct bolt-in and cost only $250.  They are MUCH stronger than the wimpy stock 26 spline gears.  You'll end up with a much stronger rear for minimal cash outlay.  Still, this isn't as good as the other options listed below.

8.5" GM 10 bolt- stock specs: 8.5" ring gear, 1.625" dia. pinion shaft, 28 spline axles.

This axle housing is the next step up the ladder in terms of strength and cost. The 8.5 10 bolt was the standard rear in the Buick Grand National , turbo T-types, '84 Hurst Olds, and the 442's of the 80's. The 8.5 identifies the ring gear diameter as it did with the 7.5 10 bolt. This rear is a much stronger piece for drag racing and running around the streets with a healthy motor and sticky tires. The 8.5 is a complete bolt-in operation for any G-body car. There are no modifications to be made to complete the swap. You can plan to spend about $600 and up to find a complete rear. This is your best option as far as low cost is concerned. The only down side to an 8.5 10 bolt is that there is limited parts available for this rear.

Note from Phillip:  If you plan on using the stock TH350 drive shaft, it will need to be shortened.  You need to accurately measure for the drive shaft, or damage may occur to the transmission.  Go here for measuring tips and info:  http://www.dennysdriveshaft.com/html/how_to_measure.html  

Ford 9"- stock specs: 9" ring gear, 1.313" dia. pinion stem, 28 or 31 spline axles.

The Ford 9" is probably the most popular swap for the G-body car. The reasons for this is many. First the housings are plentiful. Second, there are many choices in terms of gear ratios, posi's, lockers, spools, etc. Third, is the ease in which you can change your gear ratio. If you have a spare third member (also called a center section or pumpkin), you simply pull the axles, swap the center section, and reassemble. Probably an hour job for most people. You cannot go to your local salvage yard and expect to find a 9" housing that will bolt into your G-body though. The housing needs to be modified for width, and have the control arm and spring brackets welded on. This is best left to the professionals who have jigs setup to make this operation easier and more accurate. Some of the companies that offer such housings include: Currie Enterprises, Moser Engineering, Chris Alston's Chassisworks, Mark Williams Enterprises and some I know I have missed. Expect to pay $300 and up for the bare housing. You will probably spend $1500 and up for a complete 9" rear drum-to-drum. Don't expect a 9" Ford to be stronger just because it has a bigger ring gear. The main reason a 9" is stronger is due to the beefy pinion gear and the fact that it is supported by three roller bearings. The GM rears only have two bearings supporting the pinion gear. As far as strength goes, the 9" will be strong enough to handle 8 second quarter mile passes when properly equipped.

GM 12 bolt- stock specs: 8.875" ring gear, 1.625" dia. pinion shaft, 30 spline axles.

Swapping a 12 bolt into a G-body is probably the most difficult conversion to complete. The problem lies with the difference in control arm mount angles. You can force the arms to bolt up, but suspension movement will bind up severely. The housings are also different widths which can be corrected with wheel offset (also commonly called backspacing). The housing can also be cut to maintain stock width. The 12 bolt is the route I chose when building my 79 Malibu because there were no conversion 9" housings available when I began my project. I also prefer to keep my GM car ALL GM. I have to admit that the procedure I am about to explain is also one of the most expensive to do.

The first thing that needs to be done is to fabricate a jig fixture to locate the lower control arm mounts and stock pinion location. At this point you need to remove the lower control arm mounts and spring pads from the 12 bolt housing. You also need to remove the same pieces from the 7.5 10 bolt. Next, install the 12 bolt housing in the jig and also 10 bolt control arm mounts and spring pads. After double checking your measurements, you can weld everything up. Now the lower control arms (of the 12 bolt) will be at the same angle as the 10 bolt was. The upper control arms are a different story. Since they are part of the cast center section, they cannot be modified. The solution is to use an upper control arm that has spherical rod ends to allow for the mis-alignment of the angles. At this time it would be wise to add "no-hop" bars to allow adjusting of the instant center. The control arms and no-hop bars I run are available through Art Morrison Enterprises. The rod end also allows for pinion angle adjustments, which comes in handy when you need to compensate for track changes. Being able to make such adjustments will make your car be able to hook even when track conditions change. Very important in my opinion!

NOTE: Careful measurements will need to be taken to allow for proper wheel/tire fitment. The 12 bolt housing is wider than the stock 7.5 rear. I am still looking for the stock 7.5 rear width (drum-to-drum), but will provide the 12 bolts width for your comparison. The 1964-67 Chevelle housings measure 60.5 inches drum-to-drum. The 1968-72 housing measures 2 inches wider which would be 62.5 inches drum-to-drum.

Suspension Options

In addition to the set-up I just described, another company makes a kit to help the 12 bolt be a bolt-in operation. The company is Southside Machine and the kit includes 4 control arms, and the needed spring pads and shock mounts. These arms are made to combine the angles of both rears. I believe this kit sells for about $250. The down side is that this set-up lacks the ability to be adjusted.

For more street oriented cars, Hotchkis makes a real nice set of control arms that are made from heavy box steel. The have polyurethane bushings with grease zerk fittings. These will eliminate any flex associated with the flimsy stock stamped pieces.

Any car that is raced, should consider adding a rear sway bar. These G-body cars will twist quite a bit even when a cage is implemented top stop the body from flexing. The sway bar will help to take the dreaded "G-body twist". One other tuning tool that may help is a set of air bags inside the rear coil springs. The passenger side is the only one that is really needed. The air pressure can be adjusted to tune the launch characteristics of most cars.

Also, drag race G-bodies should seriously consider adding some type of bracing for the upper control arm mounts. This piece of metal is simply press formed sheet steel barely 1/8" thick. Under hard launches, this crossmember can flex, and eventually crack. There are two main ways to strengthen this mount. The simplest is to add the 4-link diagonal brace kit sold my Hotchkis and other suspension companies. the other way is to run diagonal bars from the main hoop of the roll bar/cage. The second way is probably the stronger of the two choices. I also recommend adding a diagonal brace off the roll bar/cage main hoop/seat bar intersection down to the lower control arm mount. This will strengthen the frame and lower arm mounts to add consistency in traction and eliminate cracking of the factory welds. This brings up another topic in which I will briefly touch on. Any G-body that is raced regularly should have some sort of roll bar/cage. Putting the safety concerns aside, the G-body platform needs stiffening. Both E.T. and consistency will improve when the chassis is stiffened and flex eliminated or reduced.

On my car however, the launch has been getting more aggressive as my quarter mile times have dropped.  I feel my car has overpowered what the factory sway bar/air bag combination can do. I plan to install a "chassis car" style anti-roll bar to help tame the twisting launch. I'll let everyone know how this works when I get a chance to install/test the new set-up.

Here is a list of manufacturers of suspension and rear end housing parts. If anyone has any companies that I should include in this list, please let me know.

Art Morrison Enterprises
5301 8th Street East
Fife, WA 98424
Hotchkis Performance
12035 Burke St. Suite 13
Santa Fe Springs, CA 90670
Auburn Gear Inc.
400 E. Auburn Drive
Auburn, IN 46706
Mark Williams Enterprises
765 South Pierce Avenue
Louisville, CO 80027
Chassis Engineering
1500 Avenue R
Riviera Beach, FL 33404
Moser Engineering
102 Performance Drive
Portland, IN 47371
Chris Alston's Chassisworks
8661 Younger Creek Drive
Sacramento, CA 95828
S&W Race Cars
11 Mennonite Church Road
Spring City, PA 19475
Currie Enterprises
1480 B North Tustin Avenue
Anaheim, CA 92807
Tom's Differentials
14709 Paramount Blvd.
Paramount, CA 90723

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