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Discussion Starter · #1 ·
Hey fellas!

So apparently i've done the impossible and tracked myself down an 8.5 rear end from a 71 skylark for my 70 Buick Skylark! With the bolt in axles!

Needless to say I'm overjoyed at my find, but I just wanted to get some clarity: How much horsepower will this hold? A regular 8.5 is said to hold around 500 tops if kept stock, but I have heard that the BOP rear ends can hold a fair amount more due to their construction. By the end of my build, I am planning to make 600 horsepower (gonna make 400 at first but in a few years I'm planning to upgrade to twin turbos. just doing the proper bits like piston ring gap and keeping the compression near stock until I get them). Will this differential hold up through my journey? I am throwing a posi from yukon gear into it as well.

best regards!
 

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More for Less Racer
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20,378 Posts
I don't see how anyone figures the Buick version is any stronger, it is still a 8.5" 10-bolt. The bolt-in axles give a slight better safety margin as far as keepng the wheel under the car when an axle breaks, but aren't any stronger as far as power capacity, they will break just like a c-clip version will.

To beef it up you've got 3 places to adress beyond the aftermarket positraction you've already got planned.
#1, a rear girdle cover, to support the cast carrier bearing caps and prevent blowing the drivers side cap off and destroying the entire assembly.
#2, aftermarket axles, which will be non-tapered behind the spline and made of a better material and have a better heat treat spec. Increase to 30 or 33 spline at the same time, don't stay with 28
#3, welding the axle tubes to the cast centersection.

A solid pinion spacer instead of a crush sleeve is also a good recommendation.

With that done, it will handle 700 Hp without a problem. I've heard more with housing braces, into the 800-850 Hp area. Guys are doing pretty much the same with the near identical 8.8" Ford.

If you want a helical gear differential rather than a clutch pack differential check out the AAM(American Axle) unit, Denny's Driveshaft is one source.
4 pinion Helical diff called a Trac-Rite, 30 spline, 8.5/8.6" rear
 

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Discussion Starter · #4 ·
Much appreciated! are there any aftermarket axles you would recommend? and what do you mean by welding the axle tubes? I've never heard of it before and would like to know more
 

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High Performance Rear Ends
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3,778 Posts
Moser may be your only source for axles for this rear end. If it had c clip axles then there would be other options, but because it has press on bearings there aren't any other companies that I know of that are going to make axles for it.

As for welding the tubes, the axle tubes are pressed into the center casting and plug welded in 3 places. Under high stress these plug welds can break and allow the axle tubes to rotate in the center casting. This can cause the pinion to rotate up and put the drive shaft into the floor. It will also pull the drive shaft out of the transmission and allow it to flop around at high speed. This can destroy a lot of stuff. Welding the tubes all the way around makes it stronger, but it must be done correctly. You are welding steel to cast iron. You must use a rod or filler material that is compatible with cast iron and steel or the weld will not hold. You also need to follow preheat procedures and allow it to cool slowly to prevent cracks.
 

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Race it, Don't rice it!
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Gear Head,
Can he drill 1/2" holes through the center casting into the axle, thread the holes and use good bolts? I've done that a few times and all quick changes come that way already.
 

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Drilling and bolting is better than nothng, but it isn't even close to as good as welding the tubes to the center.

Another tip on the welding is it needs to be ground very clean where the weld will take place....you actually need to carefully grind into the press-fit intersection between tube and casting .030" or so to make sure you've gotten all rust out, plus clean the 1" or so of tube outside the press-fit intersection and the face of the casting boss on the housing.

The welding needs to be done with a high nicle content rod....your typical wire feeder ER-70S mild steel wire is NOT good and the weld will crack loose from the casting.
 

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welding is not as critical in this 4-link setup as the top links keep the center casting from getting to far.

Most of the welded housings I have ever seen are cracked all the way around after heavy use. Moser and Strange well their new housings and they seem to hold up.
 

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High Performance Rear Ends
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Camaro and other leaf spring rear ends, and rear ends with ladder bars or drag race 4 link need to have the tubes welded. Chevelle and other triangulated 4 ling rear ends are not as critical as the others due to what Imsport said. I've welded quite a few of these and I TIG weld them with N99 wire, which is 99% nickel. I grind the casting through the skin and clean it well. Then use a torch to boil out any oil that is trapped between the tube and casting and clean it again. Then preheat and weld and allow it to cool in still air. Peening is recommended while it's hot. Welding cast iron is not a simple process like welding steel.
 

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Hey fellas!

So apparently i've done the impossible and tracked myself down an 8.5 rear end from a 71 skylark for my 70 Buick Skylark! With the bolt in axles!

Needless to say I'm overjoyed at my find, but I just wanted to get some clarity: How much horsepower will this hold? A regular 8.5 is said to hold around 500 tops if kept stock, but I have heard that the BOP rear ends can hold a fair amount more due to their construction. By the end of my build, I am planning to make 600 horsepower (gonna make 400 at first but in a few years I'm planning to upgrade to twin turbos. just doing the proper bits like piston ring gap and keeping the compression near stock until I get them). Will this differential hold up through my journey? I am throwing a posi from yukon gear into it as well.

best regards!

600hp will be no issue - upgrade the axles and get it setup correctly.

Regarding welding the tubes, what is being described is not beginner welding work. Pay someone to do it is my best advise unless you're willing to put in the time to learn to weld properly and buy the correct equipment. Also, it can be done later - it's not hard to pull a rear and make this into a weekend project.
 
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