7 1/4 to 8 1/4 Swap

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HTMLmopars

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I'm planning on doing a rear end swap over my Christmas break this year, I already have an 8 1/4 from a 76 F-body (Aspen I believe), its got 3.21s and sure grip, so it's a step up from my peg leg 2.76 7 1/4. I know that the F-body axle is wider than the A, and the bolt pattern is BBP anyways, so I'll need new wheels, I'm going to measure offset on my Cragars and find some cheap steelies that won't hit the fenders. The spring perches are also different on the F-bodies, too, right? I'm planning on getting a relocation kit for those, and while I'm at it, find some Durango springs in a junkyard (or my driveway) and add redo the springs with those. From what I've seen the pinion length shouldn't cause problems with the driveshaft, but on that note, is this axle a bigger u joint, and is there an adapter ujoint that will work for me? For those of you that have done the swap, what do I need to be careful about, mistakes to avoid making? Just trying to plan ahead enough to get the project done within budget and time constraints.
 
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The drive shaft will need to be shortened. The differential yoke is further forward on the 8 1/4" axle.
 
The drive shaft will need to be shortened. The differential yoke is further forward on the 8 1/4" axle.

Did this swap a couple years ago. You will also need to get new shock plates since the axel tubes are bigger as well as new brakes if your rear end does not have them. I had never swapped a rear end in my life and a buddy and I had it knocked out in under an hour I believe.
 
The drive shaft will need to be shortened. The differential yoke is further forward on the 8 1/4" axle.
How much further forward, I know I've got some extra engagement in the yoke on the trans side. If its inevitable, how much does shortening a driveshaft normally cost?
 
The place I use in Sacramento charges $100 to shorten, balance and install 2 new U joints.
 
They have been the same on the ones that I have seen. 7260 series I recall.
 
You can oblong the holes in the 7 1/4 shock plates to work
Can you actually oblong the holes? With the way that the shock mount is set up it looks like it might require some grinding or a lot of shims to be able to make that work. A guy at my local machine shop (who now that I think about it, might not actually work there) said that he doesn't know of a way to oblong those holes besides a CNC machine. I should've just ignored him and talked to the guy that was actually going to do the machining. But y'all are saying its possible to get through the 1/4 inch thick steel?
 
Yep,just oblong the holes. I would set the ends of the u bolts where they should be on the plate and mark the location then drill the holes and use a saw to cut out the metal between the old and new holes,no CNC machine needed.
 
Yep,just oblong the holes. I would set the ends of the u bolts where they should be on the plate and mark the location then drill the holes and use a saw to cut out the metal between the old and new holes,no CNC machine needed.
So if I drill them to full size, I'm going to get nowhere, because they overlap. But I can do that if I move one hole and reuse the other, but that would probably mess up geometry with the shock.


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If I use smaller holes and cut to those I could have a chance, like drill a couple 1/4" holes where the slots need to go, then cut between to make a slot. That would keep the geometry centered.
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Its called a die grinder,with a carbide burr. Will hog out that 1/2 of a hole in seconds. Theres all kinds of shapes and sizes of burrs as well as die grinders,air and electric.
Very handy.
 
Its called a die grinder,with a carbide burr. Will hog out that 1/2 of a hole in seconds. Theres all kinds of shapes and sizes of burrs as well as die grinders,air and electric.
Very handy.
So a Dremel with carbide should do it? Glad it sounds like I can get this done at home, I was getting frustrated with all of the complications
 
Looking at harbor freight, I should be able to find it in my budget to get a cheapo die grinder and some carbide. Should be getting this car down to ride height and get the driveshaft measured up soon!
 
Dremel is nice, but a little light.
The 1/4” shank carbide burrs need a little more oomph. I use air powered,then upgraded to better air powered. Big difference in tool used a makita electric once,was clunky but had power to spare.
 
Finally got all the holes bored out on the shock plates, and got the axle bolted up, and weight onto the springs to get the driveshaft measured up, got it out to get cut yesterday, hoping to get it back by Monday. Hopefully wheels come tomorrow, and hopefully my measurements were accurate, should be able to get her on the road by Tuesday at the latest
 
Finally got all the holes bored out on the shock plates, and got the axle bolted up, and weight onto the springs to get the driveshaft measured up, got it out to get cut yesterday, hoping to get it back by Monday. Hopefully wheels come tomorrow, and hopefully my measurements were accurate, should be able to get her on the road by Tuesday at the latest
Didnt mention how you enlarged holes....
 
I always look for a reason to buy a tool or two. Die grinder is versatile. Add a cutoff wheel and some buffers. Wont regret it.
 
Nice. I have the tools. Sonny dont want my help,would rather screw up the tool or the part then ask.
Was that way for a long time.
Thats why i wont let him near my lathe.
Spend the time with the old man, think about the question before asking, never argue. Will enhance your quality time together. Keep in mind,your time together is limited,take all you can get.
I lost my dad at 19.
The tools are a bonus.
 
I'm definitely realizing the importance of the time with him, since I don't live at home all year any more, I definitely try to spend time with him, in the garage is just extra productive
 
Finally got the driveshaft back from being shortened! Got the car on the road yesterday, still ironing out some parts, but I finally have it on the road. Definitely feels quicker, but I haven't given it too much throttle yet.
 
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