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Street Race Truck More... The Toolbox

Replacing a G-Body 7.5" Bolt Rear End

4/3/2020

I’m currently in the middle of doing an LS swap in my 1981 Buick Regal. The G-body platform, for the most part, came from the factory with lackluster power. These cars from the factory were producing between 110-180hp. The exception to that would be the Buick Grand National, which was the fastest production car of 1987 and came from the factory producing 245hp from a Turbocharged V6.

With that being said, GM put 7.5” 10 bolt rear end in the G-body platform and everything, except for the '84-'87 Buick Turbo Regals (Grand Nationals) and the '84-'87 Olds Cutlass 442. These came with an 8.5” 10 bolt rear end. The issue with the 7.5” 10 bolt rear end is that it isn’t made to withstand the power of a 300hp+ motor. I’ve seen many videos of people putting a high hp motor in their G-body while ignoring the rear end. The result is usually a broken axle with the wheel still connected rolling by you down the street.

Knowing that I’m going to be pushing 400hp+, I knew the first upgrade I needed to do was replace the rear end in my car. There are a few different options while doing this. A great option is to order a Ford 9” from Speedway Motors with the correct metric brackets, or some people have been swapping Ford 8.8” into their cars. A 3rd option would be to find an 8.5” 10 bolt from an '84-'87 Olds Cutlass 442 or an '84-'87 Turbo Regal (Grand National).

The going rate for the 8.5” rear end used is usually between $800 - $1200, depending on if it’s an open differential or a posi differential. The 8.5” are becoming harder to find and typically sell fast when someone posts them for sale. If you are fortunate enough, you can find one in a junkyard, which is how I came about mine. I found an '85 Olds 442 in the junkyard and the car still had the rear end in it.

To this day, this rear end is probably the best score I have ever found in the junkyard. The rear end was an 8.5" posi with 3.73 gears. I knew that before I swapped it into my car, I would want to open the cover, change the fluid, put a new gasket on the cover, and make sure everything looked good before I put it into my car.

When I got the rear end home, I set it on jack stands. I set a drain pan under the differential. I loosened the ten bolts holding the cover. I took 9 of them out, leaving one to hold the cover on while the gear oil was draining. There was some old silicone holding the cover on, so I just pried a little till the fluid started draining. After the fluid stopped draining, I took off the cover completely.

After inspecting the gears and posi unit, everything looked pretty good to me, so I was ready to move forward with replacing the gasket with Mr. Gasket 84B Rear End Differential Gasket, putting new gear oil in it. I chose to use Valvoline VV975 Synpower 75W90 Gear Oil.

The first step was to remove the old gasket, silicone off the old cover, and housing. I just used a gasket scraper to do this. When I got all the old stuff off, I used brake cleaner to clean the surfaces. I was ready to reinstall the cover with the new gasket. First, I put some RTV silicone on the outside of the cover. This silicone prevents leaks in case the cover isn’t 100% flat. Next, I sat the gasket on the cover lining up the holes on the gasket and the cover. I put a bolt through the cover to make sure that the cover and gasket lined up correctly. Once I got the gasket lined up on the cover, I bolted the cover to the rear end. I didn’t put RTV on housing, just because it’s already on and machined surface.

With the cover in place, I was now ready to add the fluid. There is a set screw on the side of the housing. With the housing level, I removed the set screw and poured the fluid in. When I purchased the fluid, I got 3 quarts. The capacity is approximately 2 quarts, possibly a little more. I filled the rear end until oil started flowing out of the oil fill screw. After the fluid ran out (telling me that the rear end was full) I put the screw back in. You can also replace the gasket and fluid while the rear end is in the car, I just chose to do it before I installed it into the car.

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