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8 1/4 swap to 8 3/4

Magnes

Magnes

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I've got a 72 SSP that had the small block, 904, and 8 1/4 originally. I've swapped to a 440/727 and had my drive shaft shortened to work. I'm at the point where I want to swap out that 8 1/4 rear for a 8 3/4 because I'm tired of babying the car around out of fear that I'm going to blow that 8 1/4 to bits if I get on it too much. Not to mention the fact that I'd love to have a rear that will actually spin both wheels, haha.

Now before you ask, yes I used the search function here but I keep coming up empty for an actual answer...

Some guys say that any 8 3/4 out of any 71-74 B body will bolt right up and other guys have said that for a 71 or 72 you have to use a 71 or 72 rear end because the 73 - 74 8 3/4 housing is different.

What's the story on that?
 
Well, they dropped the 8 3/4 at the end of 73, so that narrows it down a bit. I believe the 73 would technically fit but they started using the 'iso-clamp' system in 73 so the actual mounts get a little weird (rear sits on a steel box that holds rubber pads that isolate the leaf springs-a horrible design IMHO). The spring perches have a larger centering hole to accommodate the iso-clamp design. To mount on 'regular' leaf springs requires a spacer that resembles a large thick washer. Anyone with an Aspen/Volare/Diplomat will know all about that stupid design. When I had a 74 Road Runner, I bolted in an 8 3/4 from a 68-70 B body. I think it was slightly narrower but I can't recall anymore (wish I still had that car).
 
Welcome to the realm of the mysterious rear end. :) Let this link be your guide as you start down the road...

http://www.moparts.org/Tech/Archive/axle/16.html

As you can see, 71-74 widths are all the same. That's the good news. The bad news is 71-74 rears are hard to find. You can usually find the housings, but the axles are gone and the housing isn't worth beans without them. What I find myself tripping over are E-body rears, and I got one of these from YY1. These are about 1.5" narrower than a 71-74 B body, but that's not really a bad thing especially if you're going to be running wider tires. You just need to cutoff the old mounts and get new ones welded on in the correct location.

What's gonna suck for you is I believe you're going to have to get your driveshaft worked on again as the 8.75 center is longer than the 8.25. This is why I've held off putting my 8.75 in my 74 until I get the 440/727 in. That way I only have to mod the driveshaft once.

By the way, I had a 73 Roadrunner with an 8.25 that I put a 440 putting about 450HP into, and I raced the crap out of that car without the 8.25 having any issues. I think a lot of the talk about needing an 8.75 is BS unless you're dropping some 700HP beast in there and doing a lot of racing.
 
I have a 8 3/4 Out of a 71 Charger and it has the Axels with it. I don't know if your interested.
 
Yeah this subject has been confusing for me in the past but with what has just been posted above, you've already shed some light on it for me.

I have 3 72's right now, all of them have the 8 1/4. I bought a 74 Charger SE with a factory 400 4 barrel and even THAT car has an 8 1/4. I've got these things running out my ears.

I've talked to a couple guys that said "If you don't care about being "correct", and you want a good, strong, cheap rear end, then you can get a 9 1/4 rear out of an old C body or pick up truck and have it narrowed". I'll be honest and admit that I really know nada about rear ends. I was really just wanting a bolt in swap because I have no idea where to go to have a rear modified like that nor what measurements they'd need to get the job done.

And I'm not that worried about the drive-shaft, I knew going in that I'd have to use a different one once I swapped rears. I have several others laying around here. I'll keep this one and have one of the others worked on.

I might be better off just waiting a while longer and seeing if I can find another 71-72 B body that has the right rear and buy the whole car since people want so much for just the rear ends.



Habib: I will send you a PM but I'm afraid shipping from up where you are and down to Houston would kill me on it.
 
Well, if you're going to have a 9.25 put in, there's really no point in having it narrowed. If it comes out of a 70-71 C Body, it'll be the same width as the B body rears. If it comes out of a 72-74, it's 1.5" wider. Is anyone really going to notice a .75 inch increase in width on either side? Likely not.

Unless there's some specific reason why you just have to have a particular rear out of another body style/year, the cost of narrowing/widening the thing is usually well over the cost of just getting one that was made for your car.
 
I do not have a SSP but I have swapped 71-74 8 3/4" housings into two 73 Chargers that had 8.25" rears and reused the driveshafts in both cars. I have not had an issue, but maybe I missed something. One has a 440/727 and one has a 318/904. The 318 car actually ate two 8.25's before I but the 8 3/4" in. It took teeth off the ring gear and ate a pinion bearing and the 440 car smoked the pinion bearing so I disagree that it takes 700hp to tear them up. Neither was brutally abused or raced in my ownership. I checked the yokes to see if I needed a conversion U-joint, but other than that, the swap was straightforward. I see from the chart above that the 8.25" "C" dimension is an inch shorter. Don't worry about the housings and the mounts, if you get a B-body housing it will work with or without your ISO mounts, you will just need the correct shock plates, etc. Axles are available aftermarket and the adjusters may have been discontinued, but you can find them with bearing sets, I just bought two brand new ones. There used to be some on ebay a while ago if you want to retain the tapered bearings.
 
Keep in mind there are two dynamics in play here: an engine swap and a rear swap. Both of those will alter the distance between the trans and the rear. I would also say if you've blown up not one but two 8.25s, either you're the most unlucky guy on the planet or your "never been brutally abused" commentary requires some clarification because someone definitely abused them to cause that kind of damage. Taking teeth off a ring gear doesn't happen unless you're really getting on it. :)

What I would like to do, and am still considering, is getting a lightweight aluminum driveshaft out of a 1992-2010 Police Interceptor and seeing if I can adapt it for use in the Roadrunner.
 
73 RR w/440/727, swapped in 69 8_3/4 which is narrow enough to fit 13" wide meats in back gotta alter perches only.
 
Great comment and information with good history. Just a few more comment on the 81/4. If you elect to stay with 81/4, keep in mind 3.55 gears are common in Dakota trucks to include track lock or sur-grip setups. Also the 91/4 is used in Ram Chargers with a minor difference in width. These rear ends are strong and its very common to find one with track lock setup. Relocating the spring mounts is the only requirement. Regarding drive shaft length. The yoke take up some of the difference, but for my money, I will use the correct length to compensate for speed bumps and the constant changes with raising and lowering the car for service. The chart posted worth saving to your files

- - - Updated - - -

Just wanted to add the Ram Chargers and truck typically are geared about 3.54
 
Hiya,

Do not want to necro post but this is exactly what I am looking at next weekend, so instead of creating a new one of the same thing I'am posting in here. I have a 72 SSP 8 3/4 rear that I want to put in my 71 that has a 8 1/4 rear. When its said "bolts right up" how exact is that?
 
dasol7172 said:
Hiya,

Do not want to necro post but this is exactly what I am looking at next weekend, so instead of creating a new one of the same thing I'am posting in here. I have a 72 SSP 8 3/4 rear that I want to put in my 71 that has a 8 1/4 rear. When its said "bolts right up" how exact is that?
Click to expand...

Same mounting dimensions (spring perch distance) between the 71 and 72 and perches are the same (73 and 74 have large centering hols for the ISO setup)

PS:Canadian BBodies last year for the 8.75 being used was 74
 
Habib said:
I have a 8 3/4 Out of a 71 Charger and it has the Axels with it. I don't know if your interested.
Click to expand...

HABIB Do you still have the 8-3/4" for sale?? I'm very interested. I have a 72' RR with an 8-1/4 that I've been dying to swap. -Thanks Tyler
 
hey, @Bruzilla would you happen to know the overall width of a 71-4 axle housing? I've got a few 8 3/4's around, and I'd like to figure out which cars they belong to.
 
71-74 BBody 8.75" Specs:
Housing width flange to flange = 57 7/8"
Perches center to center = 47.3"

NB: Not the Wagon
NBB: Bruz included a link to Moparts listing many of the specs
 
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