[Julianskylark]

Hey guys! So as you can tell, I got my first swap meet this weekend. I'm going there specifically to try and find a GM 8.5 rear end to bolt into my 1970 skylark. I don't care if its a regular or a BOP (though I will certainly jump for a BOP if I see one).

My question is this: I know how to tell the differences between a 7.5, and 8.2, and an 8.5. That's easy. My concern is finding the right year of 8.5, between the years of 1970-1972. What measurements will I be looking for? between the mounts? Between the "ears" on top? Number codes?

I would really like to find the proper one for me. I have already bought the 8.5 ring, posi, and pinion (yes, I know you shouldn't buy before you can put it together, I've already learned my lesson on that).

also are there any aftermarket 8.5 housings for sale in the aftermarket? after hearing they were the most common swap from an 8.2, I found it hard to believe that no company had ever looked into recreating them, though searching has shown that none have.

Thank you for all the help!

 

[RWENUTS]

Yup!
Shouldn’t buy stuff like that.
An 8.5 won’t work with your intended motor. Well it might work for a little while.
You need to step up to a 12 bolt or 9 inch.

 

[Julianskylark]

Yup!
Shouldn’t buy stuff like that.
An 8.5 won’t work with your intended motor. Well it might work for a little while.
You need to step up to a 12 bolt or 9 inch.

So do you think I should sell the current gears and stuff I already have? any idea what I should look for instead?

 

[RWENUTS]

Yes!
Like I said.
You need a 12 bolt or a ford 9 inch.
68-72 chevelle 12 bolt will bolt in.
9 inch available from places like Currie that will also bolt in.
With your engine choice you need to beef up your complete drivetrain.

https://www.currieenterprises.com/gm-vehicles-bolt-in-crate-rearends

 

[big gear head]

The only 8.5 that will work in your car is from the '71 and '72 Cutlass and Skylark. These all had bolt in axles instead of c clip axles. The over all width is 61 inches.

 

[ericnova72]

I would agree with RWENUTS.... a BBc /turbo application is going to be stretching the limits of a full tilt prep 8.5", needing tubes welded to center section, C-clip eliminators or 9" Ford style housing ends, upgraded to 30 spline axles(and that is still really too small, IDK if a 33 spline is available for the 8.5"), rear girdle cover or steel carrier caps.

Add that to the fact you've got to find a housing that was only used for 1-1/2 to 2 years(mid 1971 and 1972) from a Buick or Oldsmobile A-body.

Even a stock 12-bolt is marginal, since stock 30 spline axles are nothing great.

Smarter to just buy an already modified aftermarket 12-bolt, or a 9" ford or Dana 60 made just for this A-body swap.

Moser Engineering, Strange Engineering, and Quick Performance are other choices for rear swap housings and assembly's.

The reason you don't find anyone making aftermarket 8.5" swap assemblys is that there is already a better alternative in the market...12-bolt, 9" Ford, Dana 60.....no market for a weaker alternative that would cost the same for the aftermarket to make as the stuff they already make..

 

[cerial]

Find yourself a "work" 2500/3500 van with a 5.3 or 6.0

They go for cheaper then the trucks. Your going to play with diffrent intakes and acessories. But this will give you a good block, fuel system, hydroboost, dbw pedal, often a 2wd 4l80e, and a strong rear end.

The engine can easily be built for boost with both the transmission and rear being up to the task with a wide range of aftermarket support.

 

[timothale]

go for a 9

The 9 in design was based on what was happening in semi trucks in the 50's, the "Straddle mounted pinion" with bearings fore and aft to support the pinion. the after market has all you need to be strong enough and housings and wheel pattern, and brakes you need. spend the bucks to only buy one once.

 

[64nailhead]

I agree with everyone here.

IMO, the rear end is a skill tester. If you have the skill to measure, modify, weld, adapt brake parts, etc, then putting in a rear from a different model is the cheapest way out (and really not to difficult). Not the quickest and easiest, but definitely can save some money. On the other hand, if you want to bolt something in over the course of weekend that will take all of HP you throw at, then buying a prefabbed housing for your particular car, Dana 60, 12 bolt GM or Ford 9 inch, is the way to go. The prefabbed units can be bought complete with brakes, brake lines and parking brake cables, or you can piece them together to suit your needs. Definitely quicker and will cost some more.

Quicks Performance, Strange and Moser are great sources for prefabbed units. If you want to go the the 'build your own' route from boneyard parts, then the infamous Explorer 8.8 swap will handle most anything when in good working condition with stock parts.


Something else to keep in mind when looking at rear end durability, most everyone talks about the amount of HP/torque a rear end can handle. Most of these references are looking at NA and/or nitrous builds. But a turbo car is much easier on the rear due to how they operate. Putting 700-900 hp through a rear 2-3 seconds into the WOT pull from a standing start is WAAAAAAY easier on the drive line (rear) than a nitrous or NA build that hits like a sledgehammer in the 1st 1/2 second of a WOT hit from a standstill.

Lastly, whatever rear you have will live for quite a bit of normal, maybe mildly spirited driving with a turbo car. A 7.5 will take quite a bit of abuse for a long time until you start launching with sticky tires. My point being, the rear end can be upgraded at most any time of a build - it's not one of the 1st items that need to be addressed as compared to items like your fuel or ignition system, or transmission. And it might not need to be upgraded at all if your build is not much more than a dyno queen or for appearance only.