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Rear Axle Swap

Magnums typically came with 8" Peg-leg (only one driving wheel) rear ends with a highway gear in the 2.9X:1 ratio. If you ordered the trailer tow group -- you might get a 9" axle with a 3.55:1 Suregrip.

 

The Mopar 7", 8" and 9" were of the same design -- with a back cover for access, like a Dana -- instead of a front 3rd member like a Ford 9" or Mopar 8".

 

If you're going to add some serious power to your Magnum -- the 8" isn't going to hold up. If you already have a 9" in your car -- you're probably going to be alright -- but an 8" is the best of the three for durability and rear drive gear selection. The 8" came with three different center sections (also called pigs or chunks). They're referred to as the 741, 742, and 489 because of their casting numbers. There is a lot of debate between the pluses and minuses of each -- but truthfully any will be just fine. Racers prefer the 742 and 489, and so they're harder to find. I have a 741 in mine and there's nothing wrong with it. One of the main raps is that it doesn't have as good of a gear selection -- but it will have the popular ones. I run a 3.55:1 in my car. I'm at about 3,000 RPM at 70MPH with 235X60X15 tires. I foresee going to a Gear Vendors overdrive and maybe a 3.70-3.90 gear in the near future -- but 3.55s are fine for a non-over-drive transmission car with the tires I have.

 

Setting up a 8" 3rd member properly is beyond the scope of this paper. It takes some practice -- and I'd suggest observing someone who knows what they're doing for your first one -- and be supervised on your second one. There are a lot of people who know what they're doing -- and you can check the technical site at MoparStyle for more details on Mopar axles.

The scope of this article is to tell you what axle housing to look for -- for fit and ease of installation. The above links will be of further help.

 

The last year of the 8" was 1974. The 1971-1974 B-Body 8" are a perfect fit for the 1975-1979 Forgotten Bs -- but the 1973-1974 are a more perfect fit. The reason is that the spring perch mounting pin is larger in the 1971-1972 than in the 1973-1974 -- but the hole in the spring perch can be drilled out to work if you can only find a 1971-1972. So 1973-1974 8" with a Suregrip should be your 1st choice -- but a 1971-1972 will work with the mounting holes drilled out bigger. From there it is a simple job of swapping in a properly setup axle with just a pair of jack stands, hand tools, and a good floor jack for lowering out the old and raising up the new.

 

This is a good time to change your shocks, upgrade to larger brakes, and do a little rear end detailing.

 

Out with the old (the greasier one)

 

 

Raising up the new

Good time to go to the biggest brakes you can find for an 8"

 

If you are going to be putting a lot of power to the pavement and/or using slicks -- this is the time to upgrade the actual axles to aftermarket alloys from Strange of Moser. It might also be the time to upgrade the U-bolt yoke set up on the pinion gear to a stronger one that both companies sell for about $100.


Epilog

 

A couple of members posted on the MagnumGT board:

Don1

I would add that the 1967/68 and possibly 69 C body IE: I used a Fury 8 3/4) worked great in the 75 Cordoba. Driveshaft bolted right in too.

dusterdood

The 8 3/4 from a '72 polara custom wagon bolted right under my brothers' '73 satelite. Aren't they the same width/leaf spacing as the mid-70s B-bodies?

 

 

 

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