Rear Axle Swap
Revised September 12, 2011
Magnums typically came with 8¼” 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 9¼” axle with a 3.55:1 Suregrip.
The Mopar 7¼”, 8¼” and 9¼” 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 8¾”.
If you’re going to add some serious power to your Magnum — the 8¼” isn’t going to hold up. If you already have a 9¼” in your car — you’re probably going to be alright — but an 8¾” is the best of the three for durability and rear drive gear selection. The 8¾” 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 8¾” 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. I would suggest you visit the technical forums at MoparStyle.com for more help on the topic.
The last year of the 8¾” was 1974. The 1971-1974 B-Body 8¾” 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 8¾” 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 8¼”
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.