7 1/4" to 8 3/4" rear end swap advice

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1968 Dart 270

1968 Dart 270

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Hi,I finally found a factory 8 3/4" A body rear end. Complete drum to drum for my 1968 dart 270. I Iwill be swapping out the bad 7 1/4 factory rear and thought I'd ask for any tips or advice from those who have done the swap before. It sounds pretty straightforward and should bolt in easily. I have the u bolts. I am still looking for the correct length driveshaft
I've also seen people use a furniture dolly to wheel the rear end underneath the car. For those who have done this,is there a certain size that is best? I can get an 18 by 12" one at home depot for $11.99 but they have larger sizes at harbor freight.
Will the 8 3/4" pretty much bolt straight in other than the different size u bolts and droevshaft? I'm looking forward to doing this and getting back on the road..and not having to worry about a 7 1/4" rear end failing again.
 
What he said. The dolly doesn't help you lift it, so is pointless.

Just make sure the rear has the same u-joint size as the driveshaft. You might need an adapter u-joint. And you can have a driveline shop cut down your driveshaft to length, if necessary.

Does your Dart have disc brakes? Because the 8-3/4 will have the 10 x 1.75 drums — that makes a bad combo with the base 9" front drums, though it's OK with the 10".

It's possible you might need a different e-brake cable. You'll just have to see if it works out.
 
Like they said, floor jack will make the job easy. Put 10x2.5 inch brakes on mine after install of housing. You did not mention shock plate for U bolts. This is different than one for 7 1/4. E brake cables not sure of. Appear to be same length. Give them a try. I purchased new ones. Drive shaft will be too long. As mentioned, you need to know what size yoke you have. Think 7290 comes to mind. I swapped my yoke to 1350 but not necessary at all.
 
I just did this a couple weeks ago. Used a floor jack.
You will need different shock plates, the 7-1/4 plates are different than the 8-3/4
My rear came with parking brake cables and hose. So I have no idea of differences there.
I just had a new drive shaft made and it was for a Barracuda with a 727 so the length will not be the same.


Alan
 
Cuda Al said:
I just did this a couple weeks ago. Used a floor jack.
You will need different shock plates, the 7-1/4 plates are different than the 8-3/4
My rear came with parking brake cables and hose. So I have no idea of differences there.
I just had a new drive shaft made and it was for a Barracuda with a 727 so the length will not be the same.


Alan
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Cuda Al, Do you recall your driveshaft length?
 
You can get drive shaft lengths from the parts book for your year and car.

BUT.... measure twice cut once.

Your stock drive shaft most likely is a 2 piece design with the rubber isolator between the inner and outer shaft. Those do not lend themselves to shortening very well.

If your shaft is a one piece then have it shortened.

The yokes are most likely 7260 on the driveshaft and the slip yoke. The 8 3/4 is an unknown. You will need to measure it to determine if it is a 7290 or 7260.

Your U bolts need to have the flattened section in the middle of the U or you will crush the housing tube.

Read and follow the torque specs or you will crush the housing tubes.

The stock e-brake cable will fit just fine assuming you had 10" rear brakes. I don't know if 9" rears used the same part. Check the parts book to see.

Put a 12" square piece of plywood between the housing and center section and the jack so the housing does not rotate as you lift.
 
mvh said:
What he said. The dolly doesn't help you lift it, so is pointless.

Just make sure the rear has the same u-joint size as the driveshaft. You might need an adapter u-joint. And you can have a driveline shop cut down your driveshaft to length, if necessary.

Does your Dart have disc brakes? Because the 8-3/4 will have the 10 x 1.75 drums — that makes a bad combo with the base 9" front drums, though it's OK with the 10".

It's possible you might need a different e-brake cable. You'll just have to see if it works out.
Click to expand...
Thanks, my car has the 10" brakes,but I also have factory front disc setup from another car I could swap into the front if I want too. Just curious but what would be the happen if someone put an 8 3/4" in their car with 9" front drums? Others here have done it in threads I've seen and nothing was said about. There is also a member (daves69) who said he had a barracuda that came from.the factory with 9" front and 10" rear (8 3/4"). As far as the E brake goes do certain years have different e brake cables? I'm new to A body rear ends
 
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Cuda Al said:
I just did this a couple weeks ago. Used a floor jack.
You will need different shock plates, the 7-1/4 plates are different than the 8-3/4
My rear came with parking brake cables and hose. So I have no idea of differences there.
I just had a new drive shaft made and it was for a Barracuda with a 727 so the length will not be the same.


Alan
Click to expand...
Yep have shock plates too,just forgot to mention them.
 
you'll likely need a new brake drop line from the frame to the T on the axle. depending on the year you may need one, the other or both.
 
Cuda Al said:
I just did this a couple weeks ago. Used a floor jack.
You will need different shock plates, the 7-1/4 plates are different than the 8-3/4
My rear came with parking brake cables and hose. So I have no idea of differences there.
I just had a new drive shaft made and it was for a Barracuda with a 727 so the length will not be the same.


Alan
Click to expand...
Thanks Al,glad to get advice from someone who just did it and knows what's involved. As I said in my first post,I don't plan on messing around with cutting my driveshaft. I'll try and find one that's the correct length to work. I do have the correct shock plates. They came with the rear end. I also.have the u bolts.
 
Dana67Dart said:
You can get drive shaft lengths from the parts book for your year and car.

BUT.... measure twice cut once.

Your stock drive shaft most likely is a 2 piece design with the rubber isolator between the inner and outer shaft. Those do not lend themselves to shortening very well.

If your shaft is a one piece then have it shortened.

The yokes are most likely 7260 on the driveshaft and the slip yoke. The 8 3/4 is an unknown. You will need to measure it to determine if it is a 7290 or 7260.

Your U bolts need to have the flattened section in the middle of the U or you will crush the housing tube.

Read and follow the torque specs or you will crush the housing tubes.

The stock e-brake cable will fit just fine assuming you had 10" rear brakes. I don't know if 9" rears used the same part. Check the parts book to see.

Put a 12" square piece of plywood between the housing and center section and the jack so the housing does not rotate as you lift.
Click to expand...
Here's a photo of the u joint yoke on mine.

20240914_113441.jpg


20240914_113338.jpg
 
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junkyardhero said:
yooo. what's going on here?

View attachment 1716303386
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When I showed up to pick it up I saw that. Seems sellers always leave something out of the description. Part of the metal fork where the bolt goes through split a little bit so the guy ran a longer bolt through it with a nut behind. He was driving it that way. Yeah,wasnt happy about that. Everything else is fine though. Have a replacement yoke?
 
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Put the leaf springs on the rear end first. Then mount the front perches first, so it hinges up when you jack up the rear end. It will help keep the rear end located. The driveshaft for a 8 3/4”/904 combo in a Dart is 48 5/8”, but I would have the rear installed and under load and measure cup to cup to confirm that. And I would consider having your existing driveshaft shortened. Now, as mentioned above, there are 2 different A body 8 3/4” housings, a 66-69 and a 70 and later. The only difference is that the vent tube and line splitter block was moved from almost on the round section of the housing, down to on the drivers side tube. Depending on which housing you have, you may need a rubber line from a C body car, or a hydraulic extension for your existing line. I used an extension thst I got from a guy who runs a mobile hydraulic hose maker that makes lines for semi trucks, they’re trucks are usually well equipped with adapters of all kinds due to their roadside service nature.

So, you have everything pretty much covered, although I would consider using at least 10” drums up front, or going with those disc brakes you have, if the bolt pattern matches your new rear end. Otherwise, 2 spare tires may be needed.
 
cosgig said:
Put the leaf springs on the rear end first. Then mount the front perches first, so it hinges up when you jack up the rear end. It will help keep the rear end located. The driveshaft for a 8 3/4”/904 combo in a Dart is 48 5/8”, but I would have the rear installed and under load and measure cup to cup to confirm that. And I would consider having your existing driveshaft shortened. Now, as mentioned above, there are 2 different A body 8 3/4” housings, a 66-69 and a 70 and later. The only difference is that the vent tube and line splitter block was moved from almost on the round section of the housing, down to on the drivers side tube. Depending on which housing you have, you may need a rubber line from a C body car, or a hydraulic extension for your existing line. I used an extension thst I got from a guy who runs a mobile hydraulic hose maker that makes lines for semi trucks, they’re trucks are usually well equipped with adapters of all kinds due to their roadside service nature.

So, you have everything pretty much covered, although I would consider using at least 10” drums up front, or going with those disc brakes you have, if the bolt pattern matches your new rear end. Otherwise, 2 spare tires may be needed.
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Hi,My car came from the factory with 10" brakes. The rear has the line and vent down on the drivers side axle tube (70 style). It came with u bolts and the shock plates. Will I need the c body rubber line for this style? The rear did not come with leaf springs but mine are still in my car,along with the 7 1/4" rear. My car is small bolt pattern,this rear end is small bolt pattern so no need for new wheels.
Are you sure On those measurements? Dana67dart posted a factory parts sheet saying the measurement for a 1967 with 273 an a904 and 8 3/4" was 52 1/2" with 3" diameter and I've had several other people contact me saying 52" as well. Just want to be sure so I can hunt for the right driveshaft. Don't want to cut mine down if I don't have too. Buying a factory correct one takes all the guesswork out of it and want to go under the car with everything I need to tackle it in one fell swoop.
There are some.other remembers who have posted that their cars came from.the factory with an 8 3/4" rear (10" brakes) but their car had 9" fronts. My car has 10" brakes,but im.curious as to big of a deal thar is to have 9" on front and 10" on back if Chrysler made some.cars that way?

Screenshot_20240913-125533.png
 
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1968 Dart 270 said:
Hi,My car came from the factory with 10" brakes. The rear has the line and vent down on the drivers side axle tube (70 style). It came with u bolts and the shock plates. Will I need the c body rubber line for this style? The rear did not come with leaf springs but mine are still in my car,along with the 7 1/4" rear. My car is small bolt pattern,this rear end is small bolt pattern so no need for new wheels.
Are you sure On those measurements? Dana67dart posted a factory parts sheet saying the measurement for a 1967 with 273 an a904 and 8 3/4" was 52 1/2" with 3" diameter and I've had several other people contact me saying 52" as well. Just want to be sure so I can hunt for the right driveshaft. Don't want to cut mine down if I don't have too. Buying a factory correct one takes all the guesswork out of it and want to go under the car with everything I need to tackle it in one fell swoop.
There are some.other remembers who have posted that their cars came from.the factory with an 8 3/4" rear (10" brakes) but their car had 9" fronts. My car has 10" brakes,but im.curious as to big of a deal thar is to have 9" on front and 10" on back if Chrysler made some.cars that way?

View attachment 1716303608
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I think what I would do is install the rear end and see if the rubber line will reach the splitter block. It being on the drivers side tube will be better than on the hump of the center housing. Same with the brakes, I think what you have will work, and since you have small bolt pattern, there is no concern about a mismatch. Now, for the driveshaft, if it’s a 52” driveshaft, that is a really common size, and once you get the rear end in, you can confirm that is the size you need. I wouldn’t buy one until you do. Maybe it’s a Dart thing, because the Dart has a 111” wheelbase, and Plymouth cars are 108”, and the difference is all in the rear floorpan. I probably have 3-4 52” driveshafts here, which is the common size for a 318/904/7 1/4” drivetrain. Again, you have the parts to install the rear end, I would get it in there before spending any money on parts until you confirm that you need them. It may take a little longer, but it saves from buying parts you may otherwise not need.
 
cosgig said:
I think what I would do is install the rear end and see if the rubber line will reach the splitter block. It being on the drivers side tube will be better than on the hump of the center housing. Same with the brakes, I think what you have will work, and since you have small bolt pattern, there is no concern about a mismatch. Now, for the driveshaft, if it’s a 52” driveshaft, that is a really common size, and once you get the rear end in, you can confirm that is the size you need. I wouldn’t buy one until you do. Maybe it’s a Dart thing, because the Dart has a 111” wheelbase, and Plymouth cars are 108”, and the difference is all in the rear floorpan. I probably have 3-4 52” driveshafts here, which is the common size for a 318/904/7 1/4” drivetrain. Again, you have the parts to install the rear end, I would get it in there before spending any money on parts until you confirm that you need them. It may take a little longer, but it saves from buying parts you may otherwise not need.
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A 318/904/7 1/4" uses the same 3" diameter 52.58" driveshaft that the 8 3/4" uses? How can that be? The 7 1/4 rear is smaller and uses an even longer driveshaft that is 54.61" long. (Again,these numbers are for the Dart and from the 67 parts catalog Dana67Dart posted.
 
cosgig said:
, there are 2 different A body 8 3/4” housings, a 66-69 and a 70 and later
Click to expand...
buy the flex line for the year of the housing and the car.

If you have the "up to 69" housing buy the hose for a 67/68 dart with 8 3/4

If you have the "70 up" housing, get a 71/72 dart with 8 3/4 hose.
 
This is from the 68 parts catalog. Despite commonly agreed on that the 904/833 are the same length, there’s a .75 difference in measurement. Wonder why that is and if that .75 difference is detrimental.. ride height is also a variable depending on application

IMG_9892.jpeg
 
When I had my replacement driveshaft cut down I measured with the slip yoke pulled out the same distance as when the 7 1/4 was in. And I got a dimension about 3/4" shorter than the parts book called for. So when i specified a length I split the difference between parts book and my measurements

The trans originally had a rear seal that was flat, it did not have the extension bellows. But the replacement seal has the bellows.

The resulting length makes the bellows ride on the part of the slip yoke that starts to sweep up. Had I gone with my measurement it would have been on the flat part of the slip yoke.

I forgot to mention that the OEM stock driveshaft was not the length the parts book said either
 
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