I know this has been hashed probably a million times already, and I have searched and read, so please indulge me. I have a '73 low-option swinger special, "no power anything", drums w/4" BP wheel mounts, 3 spd floor shift manual (a-250) and 7 1/4 rear end (don't know the ratio...need to check) that I run as a DD. I'm soon to receive practically all of the 4.5" BP front disc components from a '74 Duster donor.

Since the front will move to a 4.5" BP, I'd like to know my options for moving the rear to the same. From what I've read, I take it this isn't simple (even possible?) for a 7.25 rear. I'd really like to keep this as "bolt in" as possible (e.g., no housing/axle/drive shaft shortening, adaptors, perch welding, etc.) So, am I looking for an 8.25, 8.75 axle assembly from a '73-up disc equipped A body? Were disc equipped A bodies backed by 7.25 axles/housings? Basically, what are the simplest, least expensive options for moving the rear to 4.5 BP?

Many thanks.

Yes they built 7.25" Big Bolt pattern rears. That should be a bolt in if you find one out of a late A-Body. Rears are not that desirable so they are cheap

An 8.25, would be a bolt in except for having the drive shaft shortened. More desirable, not as cheap

8 3/4 won't be a cheap swap no matter what. There were no big bolt A-body rears. So you would need custom axles and the drive shaft shortened. Did I mention they aren't cheap!..

Good Luck, Rick

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12.70 @ 104.6

Exactly, your '74 duster Disc Brake donor would have come with this automatically with 10" drums...if it was a HD optioned car then it would have had the 8.25" rear.

Yeah, after waking up this morning, I realized it was a pretty ridiculous post, and confirmed by searching for rear axle assemblies on car-part.com. Guess it's time to scare up a 7 1/4 or 8 1/4 a-body rear (or at least 7 1/4 LBP axles). I'm not going to take this car in a direction that would require an 8 3/4, just DD use.

Thanks all!

One note - in the 7 1/4, lbp axles use a much larger bearing and corresponding 'stepped' shape that won't fit into a sbp housing, so you would have to swap the whole rear.

It's not that hard to do, so no worries.

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If you change out the entire rear axle assembly, don't forget you will also need new u-bolts and new lower shock absorber plates due to the increased diameter of the axle tubes. The donor Duster will have the shock plates and you can get new U bolts at a spring shop, hardware store, or online.

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Wow. Thanks. Lots of good, useful info, here. I'd actually prefer to swap the entire assembly opposed to doing axle swaps so, given that there are differences btwn the two (that I didn't know about), I now know my preference is actually a requirement. Didn't know about the shock-plate difference, either, so thanks for that info bit, too.

Unfortunately, the donor car is several hundreds of miles from me, so sourcing could get a little tricky. I don't know the details of the donor other than it was a disc-equipped '74 duster. Trans type and rear end are unknowns. I'll check w/the owner and, if all lines up, see if he's willing to sell/ship the rear at a doable price. I wonder, as that may be more effort than he's willing to go.

Question: if I find a 7 1/4 LBP unit and the ratio is different, is it technically possible to swap gears btwn the two, or are there differences within the differentials as well?

Again, my thanks!

Yes, the guts will swap.

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<nod>
Thanks. 'ppreciate all the help.

Be advised that depending on the gear ratios involved you MIGHT have to swap the rear axle gear carriers as well as the gears. the 2.7 and higher gear sets use a different carrier than the 2.9 and lower.

The 8 1/4 rear axle I installed in my brother's 74 Duster has a 2.2 gear ratio and I am going to swap in a set of 3.2 gears but I am going to have to swap out the gear carrier as well.

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