8 1/4 to 8 3/4 rear diff swap

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hawkgoalie31

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Guys,

My dad and I our swapping out a 8 1/4 to 8 3/4 rear diff and I was wonder is it going to line up the same with the leaf springs and the drive shaft what is your guys thoughts.
 
Shorten the driveshaft like said above and you might have to get different wheels if it has different bolt pattern then your wheels.
 
Depends on what bodies you are swapping to/from. A-Bodies can only use a stock length A or '66-'72 B body rear. All others are too long. U-Joints may be different size.
 
If both rears are out of a stock A-body, the only thing you need to change would be the wheels due to the different bolt circle diameter. Same driveshaft and u-bolts/spring plates. Depending on year, you may have to swap the e-brake cables or brake hose.
 
As cudamark says, the 8.25 was usually a 4.5" bolt circle, whereas the a-body 8.75 was typically a 4.0" bolt circle.
 
Ok guys here is my question.I am running a 904 trans with a 7 1/4 rear in my 70 duster.If I swap it out with a 8 3/4 do I have to change the drive shaft or is length and issue.Also thinking of going with 8 1/4 because it should be the same i think.Would greatly appreciate any info.Thanks,Eddie Aa rearend as well.
 
you can also have Moser set you up with a nice set of Large Bolt pattern axles and not have to run two diff bolt patterns on your wheels.

-RPM
 
mynumber38 said:
Ok guys here is my question.I am running a 904 trans with a 7 1/4 rear in my 70 duster.If I swap it out with a 8 3/4 do I have to change the drive shaft or is length and issue.Also thinking of going with 8 1/4 because it should be the same i think.Would greatly appreciate any info.Thanks,Eddie Aa rearend as well.
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Putting an 8 3/4 in place of a 7 1/4 requires a shorter driveshaft. You also need to get the bigger diameter u-bolts and shock plates.
 
mynumber38 said:
Ok guys here is my question.I am running a 904 trans with a 7 1/4 rear in my 70 duster.If I swap it out with a 8 3/4 do I have to change the drive shaft or is length and issue.Also thinking of going with 8 1/4 because it should be the same i think.Would greatly appreciate any info.Thanks,Eddie Aa rearend as well.
Click to expand...
If you install a 8.250 in place of the 7.250 You will need to shorten driveshaft as well
 
my 74 duster had a factory 8 1/4 i went to an 8 3/4 and used my factory driveshaft. no issues at all. after that i went to a ford 8.8 and the driveshaft length was good for that too, but i either had to put the ford driveshaft end on my driveshaft or get a conversion joint, i opted to just stick the ford end on, not change in length.
 
jamesdart said:
my 74 duster had a factory 8 1/4 i went to an 8 3/4 and used my factory driveshaft. no issues at all. after that i went to a ford 8.8 and the driveshaft length was good for that too, but i either had to put the ford driveshaft end on my driveshaft or get a conversion joint, i opted to just stick the ford end on, not change in length.
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if you do this you need to be careful, the 8 3/4 moves the driveshaft forward towards the trans 1/2" to 3/4" using up most if not all of the free play in the yoke possibly causing it to bottom out when you hit a bump, using higher lift rear leaf springs will ad some clearance
 
caferacer, i think you bought the 8 1/4 from me that i removed for the 8 3/4 it was in the car until the end of this summer. no issues and i checked the driveshaft freeplay. i dont have any high lift springs, stock height 6 leafs. from what i remember the driveshaft with the 8 1/4 had almost 1 1/2" of free play. i didnt take an exact measurement, beut i was counting on having to cut it for th e8 3/4. when i put the 8 3/4 in, it was damn close to 1". you dont need 1 1/2". you can get away with half that with no problem.
 
the important thing for anyone doing a rear swap is to check the free play before installing the drive shaft, on some cars (like yours) you will be fine, on others you may run into problems,
 
i didnt have to change length driveshafts on my 8 1/4 to 8 3/4 change. i am eventually going to get a larger diameter driveshaft made with 1350 u-joints
 
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