i have been told that an 8.25 rear out of an 80's dodge diplomat/chrysler new yorker will work well with my 71 duster. is this true? i want to step up from my 7.25, especially since the axle bearings are shot and i dont really want to put any money into it. i have the big bolt pattern wheels up front since i just did the disc swap, and i am under the impression that the wheels from a diplomat are the same bolt pattern, just 15". will this work? theres many old diplomats in the boneyards out here and it would be awesome if it did...

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"I don't like these seatbelts, they're too complicated." - my lovely girlfriend in reference to my 71 duster seatbelts

Measure the axle, A body size is 52 3/8 inches from flange to flange, I've gotten a B body rear to fit on my 70 before with the correct wheels, and it measured 55 inches flange to flange, so you have a little bit of wiggle room. Keep in mind, you'll have to get the shock plates and U bolts since the 8 1/4 has larger tubes, and you'll have to get your drive shaft shortend an inch or two.

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70 Dart, 360, 727

Not really, for several reasons. First off, the M-body rear axle you describe is not a direct fit in an A-body like your Duster. The M-body (same as F-body Aspen/Volaré) rear axle is wider, and would cause the rear springs to splay outward, affecting ride and handling and rapidly wearing out the bushings. So, the spring perches on the rear axle tubes would need to be cut off and rewelded in the A-body locations for a proper installation.

Secondly, the axle ratios used in most of the M-bodies are super tall, e.g. 2.45 and even 2.26, made with V8 engines and wide-ratio transmissions in mind, and even with those engines and transmissions, acceleration was sluggish in those cars. With an ordinary transmission and slant-6, you'd have 0-60 times measured in hours. The exception would be the 8¼" rears used in cop-spec Diplomat, Gran Fury, and (Canadian) Caravelle models, those got desireable 2.94 axle ratios.
Correct.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

be carefull, not all diplomats have 8.25's in 'em. my cop car has a suregrip and 2.91's in the rear, the only gearset deeper is the 3.07's (i think) and they are rare. It is a good rear end and took a beating with the stock motor (4bbl 318) and even more of one with the 360. also they use whats called an isoclamp(?) and not u bolts. kinda weird looking but seems to work, dont know if it will work on the A body