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7 1/4" rear end handle 500 HP.

28933 Views 39 Replies 6 Participants Last post by  core
I am installing a 500 HP motor into my 1975 valiant. I believe the rear end is 7 1/4". Will it handle the horsepower or should I start looking for an 8 3/4" rear end. Also is posi something that should be considered.
Thanks for the help
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It *appears* from what I've cursorially read that the axle housing with your 741 center section will accommodate 489 or a 742 center sections as a replacements, and that the limited slip module is the same in all three. The difference between the three is in the pinion gear and ring gear and how the pinion slots into the casing.
Yes, the 741 case and the other is the 489.Both are fine, the 489 just has a larger pinion but is still very strong. From there you can change this 741 or 489 case and gears to anything you want, right now it is the housing and axles (axles can be changed if you want), but there is nothing wrong with the 741, yours just happens to be the cone style posi which will still hold up to the 500hp engine a lot better than the 7.25 you have.
I bought the 741 and the seller had changed out the pinion with a 1 3/4. Does that change make rear end into a 742?
Also the gear is 3.73 and they are brand new, so I was wondering if these gears will work. I am just going to use my valiant for street use and to go to shows. Will this gear be ok for highway use or will the RPM clime to high?

The axles are 4.5 and my valiant has drum brakes, so will I need new axles? If I get new axles I can keep the drum brakes or should I just switch over to disk?
I need to figure out if these axles will even work with my car. The seller told me that the set up was 55" over all and 44" center of perches.
Well, 3.73 is a little high for the street/highway but from show to show you could live with it easily enough, better than 4.10s. You are going to have to compare and measure what you have with what is there, from the backing plates out, not sure what you have or whether or not it will work, Verify the width center to center of your springs, the outside of one side and the inside of the other at the rear of the car will make it easier, see if it is the same, the extra 11 inches is minus 2.5 for the spring width which gives you 8.5 inches, divided by two, which should be 4.25 inches clearance between the backing plate to the inside of the wheel stock, which is about right. Now that I think about it, your rear/axle setup is the 7.25 and the rear ends are completely different from each other, not even sure if the brakes are the same size. What components did you actually end up getting?
The 8.75 came with the casing, gears and two axles, no brakes. I did not end up buying the cone style rear end because I found a clutch 8.75 with posi that was just completely rebuilt. The seller did recommend to replace the bearing with tapered ones even though the bearings are new. The axles have the 4.5" pattern, my valiant has 4" what does this mean?
Thank you all the help.
I would get some disc brakes for the front, with the 5 on 4.5" pattern, and get wheels for 5 on 4.5" as well, which are REALLY common since Ford and AMC also used 5 on 4.5 in addition to virtually all Mopars...
Agree, it is a more common pattern and since the cost of the axles themselves would more than be greater than the front disc upgrade, go that direction. The disc brake setup on the A bodies is the larger pattern to begin with, so most end up changing the axles to the larger pattern on the rear axles to match, not the other way around.
I final had a chance to do some measuring. The housing is 55" over all. Are all housing the same? I was told this rear end came from a 1970 duster. That is an A- body isn't it? I measured the axles from end of spline to the flange and it was almost 29". That's an axle for a B- body I think? I'm concerned that I bought a B- body 8 3/4" rear end and now I will not be able to use it.

If I can use the housing and the gears, then I can just order new axles and disc brakes from Mosor, I hope.
The '63 through '72 A-Body 7 1/4 axle measures 53.2 inches from axle flange to axle flange, while the '62 through '70 B-Body 8 3/4 axle measures 55.0 inches from axle flange to axle flange. In addition, the leaf-spring center to center spacing of the B-Body axle is 44 inches, one inch wider than the 43-inch A-Body dimension.

Read more:

The dimensions are online, so it looks like you ended up with a B body rear end, so maybe you can take it back. The article here shows a lot of information, and a search will give all the different models and rear end swaps available. Time to do some more research.
I can get the correct housing and axles from Dr.Diff for 775.00. Do you no if he carries a good product? The pumpkin that I bought with the b- body looks to be. In good shape and the seller told me that it had just been rebuilt. Will the b- body pumpkin fit into an A-body housing? If yes then should I just keep the hole set up and order new housing and axles. Or return the 8 34 b-body and start over?
Thank for your help this is very difficult to do when you have no clue what you are doing.
If you are familiar with some brand of cars, it is a matter of researching and reading what you know but need to apply to a different brand. I did a simple mopar rear end dimension search and found this information and picked it because I have seen it from the magazines themselves. There is a ton of information with the magazine, by the way, including all the K member information, disc brake swap stuff, you name it. There are lots of people that like to show you the information they have or found and compiled, so suggest, as an idea, do some internet searches and comparison articles to verify (not everything on the internet is good), but two or three articles talking about the same thing is usually pretty reliable. As far as Dr. Diff goes, I think shopping around a little bit would be a good idea. Granted it is a new rear end housing and all, but you should be able to save some money on a used one. The pumpkin does fit all the 8.75 housings no matter what car they came out of. I would hit a few junkyards in the area, craigslist, ebay if you like that, used cars (ones with no title can have a lot of good parts, so get a good bill of sale and scrap what you can't use or sell), be creative.
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Thank for the advise. I just looked on Craigslist and some one is selling an a- body housing. I need to make sure it is the one that I need this time. I think that I will keep the pumpkin and sell the B- body housing and axles on Craigslist my self with any luck maybe I can break even.
Should be able to, B body stuff isn't cheap any more. Do the measurement verification stuff, listed above for you. Don't worry about the axle bolt pattern at this time, get that housing, complete if possible.
I'm onto another lead, this time I have the knowledge to make sure I get the right rear end.

In my 65 B-Cuda I am running a 8 1/4 from an Aspin R/T with the "Sure Grip" out of a Dodge van's 8 3/8's. These rearend carriers are the same but 8 3/8's sounds stronger than 8 1/4. I went with 3.55's and the RPMs are a bit high for commuting @ 60 - 70 MPH IMHO. We had a 425 Hp 360 in it launching hard. My Cuda was swapped to Road Runner front disc's so I have LBP all around. I am also able to run a 275/60 HR15 on some 78 Cop car Rally wheels on my Cuda. 64-66 Cudas have big wheelwells.

Another option is to get a 9 1/4 from a D150 and have it cut down. They are called "Baby 9 inch" but Mopar Action said its stronger.
limited gear slections but if 2.71 - 3.91 gears fit your app's......

I post some, as does Dr Diff over at
The first generation Dakota also had a 9¼ in some years, but I'm not 100% sure which years. The trouble with the Dakota as a source is that they switched from the five lug 5 on 4.5" hubs to a six lug pattern when they went to the rounded front snout and available V8 engines, so it's possible that they never used the 9¼ in the Dakota until the six lug pattern became standard. Since a lot of engineering in the Dakota was borrowed from the F/J/M body (Aspen, Volaré, and the like) it's likely that a Dakota rear end could be adapted to the power required, and with modern improvements in brakes, might have better factory brakes than anything factory from the earlier years.
I have been looking on line for a way to use the B- body 8 3/4 rear and the 4.5" axles that I just bought. I pulled off one of the variant's tires and the rim fit onto the newly perched axle. So I believe that the valiant has the larger bolt pattern.
The different between the a body perch and the b- body perch is only 1". I have been reading about solutions for installing the b- body rear into the A- body frame . One method was just to spread the springs a 1/2" per side. It seams like a simple solution , but is it the right one? The other more reasonable on is to remove the perches and move them out the 1/2" and weld them back on.
I would like to use the b- body rear end because it should work with the brakes and wheels that I have.
Do you think this is a good road to go down or should I still hound the guy with the true a- body rear end? The a-body rear end is a small bolt pattern I believe , so would i have get new axles to fit the brakes I already have?
Sounds like you read the link I added to my last post, it does describe how to make what you said work, I have moved perches before, just have to be careful a bit, don't want warping or anything, so take it slow, good tack welds, add a little more, little at a time. The only thing that needs to be addressed, which the article notes, is the offset to the rear wheels so the whole wheel well can be utilized, which the article does talk about and show examples. These days, custom wheel backsetting can be ordered without a problem.
Just for info, my 78 Dodge D150 Little Red Express cam with a 9 1/4 as did my 77 D150 Sweptline with the 400Ci engine. Not sure what years came with them, mine did. I was told the later year B and C body Big Blocks also got 9 1/4 rears too. Theres a Tech Archive over on that has a rear end chart.

Most truck axels are not tapered so having them shortened and resplined is easy.
I'm going to have the welder at work handle this task. He has been welding for years. I just need to make sure everything goes back in the correct locations. Once the perches are set I can figure out the rim sizes.
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