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7 1/4" rear end handle 500 HP.

28925 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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If you had this engine behind a 7.25, I would expect you to be 95 years old, frail, have cataracts and have difficulty lifting a bag of peas. In short, I believe we all talked about this before, drive gentle (major emphasis) and she will be OK, but step on her or shift at high rpm, forget it. The purpose of the posi compared to a single legger is the third member can shift from one side to the other and in the process snap axles, especially under higher torque of this engine.
The 7.25 rear axle is marginal behind a 100 hp slant 6. I'd be surprised if it lasted long no matter how gentle you are with it.
So it looks like I'm going to start looking for a rear end. From reading some other post it looks like a 8 3/4" would work. Would you agree?
An automatic Hemi had an 8.75 behind it. That should be enough reason to figure it is strong enough.
Yep, the 8.75" should work fine. The only downfall is that these will all be small bolt pattern for the A body, I believe your 1975 Valiant is the larger bolt pattern (unless it has 4 wheel drum brakes which may still be the old small bolt pattern).
You can install new axles to get the larger bolt pattern or narrow a non-A body axle to fit.
So if I install a 8 3/4" rear end then I am going to have to find some new small bolt pattern axles. Is this so that the tires that I have now will fit?
Also on the valiant the rear tires are ressesed into the wheel wells and I would like to install the largest tires as possible. Should I take that into consideration when looking for the axles?
Will the 8 3/4 rear end just bolt into position with out any modifications?
Thanks again for all the help. I would not be able to even consider doing this project with out this forum.
There were 8¾ differentials in A-bodies. One of those axles '67+ should bolt right in. The A-body is also a popular drag car, and I would expect that there are aftermarket or Mopar-Performance-aftermarket 8¾ axle housings or complete axle assemblies available, though probably fairly pricey.

Just to confirm, you do know, for certain, what your wheel bolt pattern is, right? There were two patterns common on the A-body, the 5 on 4.5", and the 5 on 4". The smaller was just about ubiquitous early on, and the larger was common later.
Agree. The small bolt pattern 8.75 is easier to find and a direct bolt-in, most prefer to move up to the larger bolt pattern on the A bodies, so that would actually be an advantage you have. These days, there are a lot more tire/wheel selections available, you would want the rims to be offset to fit the wheelwells.
In my understanding, the 1975 Valiant has the 5x4.5" bolt pattern if it has front disc brakes. It has 5x4" bolt pattern if it has front drum brakes.
All A body 8.75" axles were from earlier years when all A bodies had 5x4" wheels.
valiant67 said:
In my understanding, the 1975 Valiant has the 5x4.5" bolt pattern if it has front drum brakes. It has 5x4" bolt pattern if it has front drum brakes.
All A body 8.75" axles were from earlier years when all A bodies had 5x4" wheels.
You meant 5 on 4.5 if disc, right?
You meant 5 on 4.5 if disc, right?
You are correct and I editted my post. That's what I get for posting past my bedtime. Thanks!
IF you can find an 8 3/4, it will be 3 inches too long for your car. It can be narrowed by a good drive line shop, and work very well.
The A body 8.75 small bolt pattern are easier to find than the large bolt pattern A body 8.75s. Once the housing is located, the third member can easily be swapped from any 8.75 or gear ratio changed without a problem, there are two pinion sizes, based on the case number, I think it is pre-69 and post 69 for the larger pinion.
I am having a hard time finding a 8 3/4 rear end. I might have a found one with a 10 bolt pattern, is that correct?
Are all 8 3/4 end all the same? Will it bolt right in or will modifications need to be made?
Once I find the rear end how can I tell what it has for gears? Would it be better to just suck it up and order a complete rear end from Mosers or is it worth the search for a true 8 3/4" rear end?
There are locations on the web that show which rear ends are direct bolt-ins, I think it is just about every A body after 1965 or something like that. Going Moser can be a little expensive, but if money isn't the issue, it is correct. An 8.75 has a pumpkin that bolts to the front of the case and the axles have to be pulled to remove it (others do too, but the gears remain part of the case), and I think it is 14 nuts that hold the pumpkin on, the others have a rear cover.
I found a 8.75 out of a 68 barracuda. The seller is telling me that it is a cone type. Should I stay away because from what I have been reading the cone types can not be rebuilt. The rear end is apart and to me that sends up a red flag. He is asking 600.00 is that a far price for something that might not be able to be rebuilt?
Honestly, the axle tubes/housing part and the axles shafts are probably the harder part for an A-body to find than the removable center section differential, which will be common with all of the same generation of the differential.

I would think that you would do well to buy this one, identify which generation of 8¾ the axle and differential is, and to use it as-is until it fails, then deal with a new center section or new parts for a more thorough replacement of the limited slip or locker in that center section after such a failure, or maybe if you proactively feel like changing it. The axle housing itself, and the axle shafts should be the same.
Agree, the third member gear set/limited slip portion is swappable, you need the housing right now and can deal with the rest later.
From the photo I was able to see 2070741 from everything that I have been reading has told to stay away. The forums are talking about the big nut. I think that it refers the axle? So if it is a 741 core type if the core section goes then I can just change it out of pumpkin ? Not sure on the axle type or length is there a certain size I should be looking for?
Thanks for the help.
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