8 3/4 rear

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jack colv

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i have a 8 3/4 in rear out of a 1971 ply. duster 340 will it fit in a 1973 ply duster are they the same specs. I do not have original rear out of car
 
thanks for the info I do have extra axles with the bigger bolt pattern When I pull old ones out see if they are the same thanks again
 
One thing to remember is the big bolt axles use different backing plates, Brakes, Park brake struts and drums. Unless you drill the axles and drums you have. If you drill your small bolt axles you will need a 3/8 spacer behind the wheels.
 
If the "other" axles you have are factory axles, they will not fit. The A body 8 3/4 never came with big bolt pattern axles.
 
What rear do you have now? If its a 7 1/4 you will need to swap the driveshaft as well. Im not sure if an 8 1/4 uses the same driveshaft as 8 3/4 but I seem to recall there being a slight difference in length.
 
I have an 8 3/4 out of a 1971 duster with small bolt pattern / I also have an 8 1/4 rear with large bolt pattern not sure what that came out of am hoping to use the 8 3/4 rear but would like to change to large bolt pattern
 
as far as driveshaft the one I have is for an auto my car is a 4speed the yoke for the auto is smaller so i am looking for either a drive shaft or a 4 speed yoke
 
jack colv said:
I have an 8 3/4 out of a 1971 duster with small bolt pattern / I also have an 8 1/4 rear with large bolt pattern not sure what that came out of am hoping to use the 8 3/4 rear but would like to change to large bolt pattern
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You can either redrill the drums and axles or get custom axles and redrill the drums or you can get new drums. The 8 3/4 was only installed in the A body up to 1972. After that, they got the 8 1/4 if they were the large bolt pattern or the 7 1/4 with the small bot pattern. The 8 3/4 was dropped. It never had the small bolt pattern, so you must choose one of the above choices if you want to still use the 8 3/4.
 
You can use the 8 1/4” backing plates and brake set up on the 8 3/4” for the easier to get parts and works with the large bolt patern. If you use green bearings you will have to take the thrust button out the center section for large bolt pattern after market axles to seat all the way. Get everything you need off Dr. Diff and you can also call him and he will tell you everything you need to know. Really nice guy.
 
thanks for all the info so I take it I could use the 8 1/4 rear is the 8 3/4 that much better or is it easier to change gears because of the design
 
jack colv said:
thanks for all the info so I take it I could use the 8 1/4 rear is the 8 3/4 that much better or is it easier to change gears because of the design
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It's not that much stronger. I have a local friend who used an 8 1/4 in a Demon that ran 11s hangin the front tires in the air every pass and it never broke. I think he still has that rear end.
 
jack colv said:
thanks for all the info so I take it I could use the 8 1/4 rear is the 8 3/4 that much better or is it easier to change gears because of the design
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8 3/4” is a lot stronger design and easier to work on. The 8 1/4” with a suregrip I think is rated for up to 350 Hp. So would be good with a healthy 340, the info is on Dr. Diff’s websight. I’d use the 8 3/4” rear end if you can. Stock 4 speed would of been 3:55 gears but you could go into the 3:90s and still be good on street with a half built up 340. Just use the backing plates and all the brake parts from the 8 1/4” on your 8 3/4” then you can use the stock 1973 duster parts, a lot easier to get. Order new axles and sell the small bolt ones on here. There is a guy looking for a set he just posted in the wanted forum. Use money towards the ones you want. I think I sold one of my extra sets of small bolt axles on here for $100 and $35 shipping.
 
72SwingerJosh said:
8 3/4” is a lot stronger design and easier to work on. The 8 1/4” with a suregrip I think is rated for up to 350 Hp. So would be good with a healthy 340, the info is on Dr. Diff’s websight. I’d use the 8 3/4” rear end if you can. Stock 4 speed would of been 3:55 gears but you could go into the 3:90s and still be good on street with a half built up 340. Just use the backing plates and all the brake parts from the 8 1/4” on your 8 3/4” then you can use the stock 1973 duster parts, a lot easier to get. Order new axles and sell the small bolt ones on here. There is a guy looking for a set he just posted in the wanted forum. Use money towards the ones you want. I think I sold one of my extra sets of small bolt axles on here for $100 and $35 shipping.
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I think @Bighead440 would disagree. He's the friend I was talking about in my previous post. I tagged him so maybe he'll chime in. There are strength differences, but the 8 1/4 is stronger than you quote. He has pictures to prove it. 350HP won't hang the tires in the air.
 
The 8.25 has it's limitations for sure, but I believe it's closer in strength to say, a Chevy 12-bolt, than people think. It's not an 8.8 Ford, or even a 9.25" Chrysler by a longshot, but I have used several, especially one, beyond expectations with success. My '71 Demon started out as a 318/904/7.25" car and I built a budget street/strip car when I was going through a divorce, as a cheap distraction. The car came with a set of fresh 2.02 J-heads with double springs, a Cam Dynamics solid camshaft (and jumbled lifters), and Crane adjustable iron rockers in the trunk, and a 2.45 geared 8.25 loose on the side. I have a friend that works for the Dodge dealer that "appropriated" a Dakota Sure Grip and some 3.92 gears for me, so I went with the 8.25". I built a KB107 .030" '71 block 360 with the J-heads, a windage tray, Torker II intake/750 AFB and a swap-meet new .528" solid Purpleshaft. The car ran instant high 12s on drag radials, and with slicks ran mid 12s after adding Crane 1.6 rockers and a AED 750 carb. I used to hot lap it and it was very consistent at 109mph, I put hundreds of runs in that configuration, killing one 904 converter and inspecting the 999 trans I built for it (was fine) while it was out. Then, about 3yrs later I built a 416 pump gas, dished piston, Edelbrock headed motor with about 525hp and ran the same converter/trans/rear and went 11.20s/30s @119-120mph with the same rear, still street driving it. It would pull the tires as Rob stated, just a little, but the SS springs were tired by this point and it was inconsistent, even with 10.5 slicks, so I ladder-bar/mini-tubbed it with an 8.75/Moser axles/coilovers etc. I still have the rear and just moved the springs over 1" with the old MP offset shackle/hanger kit in a 4dr '71 Valiant I plan to turbo 318 someday, using the whole original Demon suspension setup.
 
I would NOT recommend an 8.25" rear for 4spd drag or transbrake automatic cars due to shock loading and the crush sleeve and c-clip axles (27spl.) primarily.
 
RustyRatRod said:
I think @Bighead440 would disagree. He's the friend I was talking about in my previous post. I tagged him so maybe he'll chime in. There are strength differences, but the 8 1/4 is stronger than you quote. He has pictures to prove it. 350HP won't hang the tires in the air.
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I’m just going off of what the suregrip unit is rated for that Dr. Diff sells. I’m sure you could always build a stronger one but you will probably have alot more money and customs parts involved.
 
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