7 1/4 to 8 3/4 Newb Questions

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OttawaDartGT

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I got lucky today, I found an a body 8 3/4 rear from a 71 Dart pretty local to me at a fair price.

My cars a 67 Dart GT, was orignally a slant car that a previous owner swapped a 273 into. Everything else seems to be original. The 7 1/4 is humming and leaking and I want to swap it out.

I'm taking the whole rear from the 71. 355 SG, with the drums, lines, shock plates and u bolts. I know I will require the the later model brake lines.

The one thing I'm not sure on is driveshaft length. The rear is coming out of a roller with no driveshaft. Will I need to shorten my existing driveshaft or will it mount up to the a body 8 3/4? Any idea how much it needs to be shortened?

Is there anything else you guys can think of that I will need to make this a painless on the ground diy swap?
 
here's a thread for quick reference:

www.forabodiesonly.com

Add your abody driveshaft length for quick reference

A 108 wb with 904 & 7 1/4 to 8 1/4 swap the difference between 7.25 and 8.25 housings is 1.25 so what ever your original drive shaft measures minus 1.25. Here is some other tech I've found from here: 108"wb with 440/727/8 3/4=46.5 904/8 3/4=49.25 add 2.25 for 7 1/4 subtract 4" for 727...
www.forabodiesonly.com www.forabodiesonly.com

the correct and bestest way to do it is to measure with the new rear end installed and talk to the driveline shop doing the work and ask them how they want it measured.
 
I needed a new driveshaft when I did this swap on a 67 Barracuda, but my numbers won't help you (because Dart is 3" longer). I was fortunate in that my buddy pulled the correct driveshaft out of a 6ft high pile of driveshafts at a wrecking yard — first try. You probably won't be that lucky.
 
If you haven't already asked the seller about the driveshaft, look in the car to see if it is laying inside. If it isn't, the best thing to do is just measure. One thing to check, is to see if the 8 3/4 has a big or small yoke on the pinion. If it has the big one, and yours is the small one on the driveshaft, you can buy a conversion U joint to make it work.
 
Yes, you will need to shorten the drive shaft.
 
OttawaDartGT said:
I got lucky today, I found an a body 8 3/4 rear from a 71 Dart pretty local to me at a fair price.

My cars a 67 Dart GT, was orignally a slant car that a previous owner swapped a 273 into. Everything else seems to be original. The 7 1/4 is humming and leaking and I want to swap it out.

I'm taking the whole rear from the 71. 355 SG, with the drums, lines, shock plates and u bolts. I know I will require the the later model brake lines.

The one thing I'm not sure on is driveshaft length. The rear is coming out of a roller with no driveshaft. Will I need to shorten my existing driveshaft or will it mount up to the a body 8 3/4? Any idea how much it needs to be shortened?

Is there anything else you guys can think of that I will need to make this a painless on the ground diy swap?
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You should be good to go as long as you have the correct u-joint for the yoke in the rear differential .

Use the driveshaft you are presently using in your Dart.
RustyRatRod said:
Yes, you will need to shorten the drive shaft.
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Must be different with a Cuda as I had no problem doing this going from a 7 1/4 to an A-body 8 3/4 from a 71 Demon.

As far as I'm aware, that as long as you didn't change the transmission, the drive shaft would work as long as you had the correct u-joint and yoke.

Going from a 904 to a 727 you definitely need a different shaft.
 
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Super-Cuda said:
You should be good to go as long as you have the correct u-joint for the yoke in the rear differential .

Use the driveshaft you are presently using in your Dart.


Must be different with a Cuda as I had no problem doing this going from a 7 1/4 to an A-body 8 3/4 from a 71 Demon.

As far as I'm aware, that as long as you didn't change the transmission, the drive shaft would work as long as you had the correct u-joint and yoke.

Going from a 904 to a 727 you definitely need a different shaft.
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Really? All of them I've ever seen needed the drive shortened on the order of "around" 1.5". I guess it's possible I'm remembering something different.
 
i am doing the same thing to my duster my buddy has his drive shaft from center to center ( u joint) of drive shaft 49.5 in 904 8 3/4 combo ... i am going to measure mine to make sure...
 
RRR is right, you are going to have to shorten the drive shaft.
7 1/4 10.09 inches from center of ring gear to end of yoke
8 3/4 12.35 inches, that is a 2.26 inches difference in length.
So measure the drive shaft that is in the car now and either take 2 to 2 1/4 inches off, with 3/4 to an inch of slip yoke travel or find one that is that much shorter.
Other Mopar axles for reference,
9 3/4 Dana 13.47 inches
8 1/4 and 9 1/4 are both 11.69 inches
These measurements are for Mopar factory yokes if using aftermarket yokes measurements may very.
 
RustyRatRod said:
Really? All of them I've ever seen needed the drive shortened on the order of "around" 1.5". I guess it's possible I'm remembering something different.
Click to expand...

19DART66 said:
RRR is right, you are going to have to shorten the drive shaft.
7 1/4 10.09 inches from center of ring gear to end of yoke
8 3/4 12.35 inches, that is a 2.26 inches difference in length.
So measure the drive shaft that is in the car now and either take 2 to 2 1/4 inches off, with 3/4 to an inch of slip yoke travel or find one that is that much shorter.
Other Mopar axles for reference,
9 3/4 Dana 13.47 inches
8 1/4 and 9 1/4 are both 11.69 inches
These measurements are for Mopar factory yokes if using aftermarket yokes measurements may very.
Click to expand...

My 68 Barracuda with a factory 7 1/4 rear end and a 904 automatic transmission.

Installed a 8 3/4 rear end from a 1971 Demon 340.

741 case , small yoke.

Installed with no modifications to the original driveshaft.

When I installed a 727 small block transmission from a 70 340 Duster, I had to use the driveshaft from that car.

No issues, worked fine.

I still have both driveshafts so I will see if I can dig them out for comparison.
 
Super Cuda, That would tell me that the slip yoke in the end of the trans was sticking out about 3 inches on the original 7 1/4 set up
 
19DART66 said:
Super Cuda, That would tell me that the slip yoke in the end of the trans was sticking out about 3 inches on the original 7 1/4 set up
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Yup! You may be correct if the measurements you have are the same as my driveshafts. I am going to pull out both driveshafts and check the measurements.

I did buy my car used and its possible the wrong shaft or a different shaft was in it.

Pics to follow.
 
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Appreciate all the responses guys, going to pick up the whole rear axel this weekend, very excited to have a sure grip 8 3/4 :). Sounds like some of you have got lucky with the driveshaft, I'm going to go into expecting to have to shorten it, I have a good driveshaft shop 5 minutes away.
 
I have done that 7.25>8.75 swap more than a few times, and it always required a different driveshaft.

However, having gone from an 8.25>8.75, in a 1984 D100, the same shaft did work for me. Not perfect, but drove the truck for many years that way.
 
AJ/FormS said:
I have done that 7.25>8.75 swap more than a few times, and it always required a different driveshaft.

However, having gone from an 8.25>8.75, in a 1984 D100, the same shaft did work for me. Not perfect, but drove the truck for many years that way.
Click to expand...

I'm looking forward to checking the lengths of the the two drive shafts I have and will compare them to 19Dart66 measurements as I'm sure they are correct.

I'll post pics when I measure them.
 
OttawaDartGT said:
Appreciate all the responses guys, going to pick up the whole rear axel this weekend, very excited to have a sure grip 8 3/4 :). Sounds like some of you have got lucky with the driveshaft, I'm going to go into expecting to have to shorten it, I have a good driveshaft shop 5 minutes away.
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that's an excellent attitude to have. trust but verify.

just because somebody else got away with their bobo *** fix don't mean it's gonna work for you.

and more importantly, that big old spinny bit is right there up under you, don't want that mess comin' apart and taking out big jim and the twins. or, of less concern wrecking out a trans or the floorboard and a set of shorts all at the same time.
 
junkyardhero said:
that's an excellent attitude to have. trust but verify.

just because somebody else got away with their bobo *** fix don't mean it's gonna work for you.

and more importantly, that big old spinny bit is right there up under you, don't want that mess comin' apart and taking out big jim and the twins. or, of less concern wrecking out a trans or the floorboard and a set of shorts all at the same time.
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Totally agree .

I'n definitely going to check my parts and verify the measurements .

I don't do half assed fixes and" close enough" is not in my playbook.
 
Super-Cuda said:
Totally agree .

I'n definitely going to check my parts and verify the measurements .

I don't do half assed fixes and" close enough" is not in my playbook.
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100%

we've all been young and dumb and clueless. believe it or not, even i paid the fine and did my time for crimes against automotive decency.

but these days, there's no excuse. the information is out there, the resources exist, and yet the collective ignorance still perseveres.

it's only temporary, until it isn't.

now if you'll excuse me i have some zip ties to go tighten up.
 
junkyardhero said:
100%

we've all been young and dumb and clueless. believe it or not, even i paid the fine and did my time for crimes against automotive decency.

but these days, there's no excuse. the information is out there, the resources exist, and yet the collective ignorance still perseveres.

it's only temporary, until it isn't.

now if you'll excuse me i have some zip ties to go tighten up.
Click to expand...

They're next to the duct Tape LOL !!

:lol::lol::lol:
 
Super-Cuda said:
They're next to the duct Tape LOL !!

:lol::lol::lol:
Click to expand...
what is this, amateur hour?

my tool kit starts with 100mph tape and progresses to stainless steel zip ties. the bailing wire and linesman pliers live in the glovebox.
 
So.. I checked the driveshafts.

Rusty Rat Rod and 19Dart66 are correct.

Although I don't have the driveshaft I used with the 7 1/4 rear end , just the shaft for the 904 and the 727, I can see the difference right away. ( Major egg on face !!! LOL !!)

As junkyardhero said...


"... the correct and bestest way to do it is to measure with the new rear end installed and talk to the driveline shop doing the work and ask them how they want it measured..."

I'll just go stand in the corner now....
 
it's all part of the process amigo.

it a collective effort and, at times, knowing what is wrong can be just as valuable as knowing what it right.

the reason i say to call the shop doing the work and verify with them how they wanted it measured is because i got jammed up on that deal. shop i dealt with tons of times had changed hands, had them do some other little piddly work like balance and swap out joints. then, like always just called them up and said i need a shaft spec'd out to this.

so i got it back to the shop and the damn thing didn't fit. turns out the new HDIC wanted all the measurements this way instead of the old way, which according to him was "wrong" and "inferior". so i got to pay for it again.
 
The drive shaft out of my slant 6 904 71 Plymouth duster... Going to an 8 and 3/4

IMG_20240512_172546424.jpg


IMG_20240512_172552631.jpg
 
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