8 3/4 rear end conversion

[belve]

Hi i just bought a 83/4 rear end with 489 casing and wanted to put it in my 1970 plymouth belvedere that has a 318 with 904 transmission with a 8 1/4 rear end. I was wondering if it was going to be basically a bolt it sort of deal. I know that it will bolt right up but will i have to cut the drive shaft or change the yoke ?


Thanks

[70AARCuda]

there is a conversion u joint availabe to go from the big to little or the other way ...

[ehostler]

The snout of the 8¾ is longer than the snout of the 8¼. The drive shaft will need to be shorter.

Take a look at this image from the tech archives

[peg leg]

The location of the spring perches. The body type that the axle came from will determine whether you have to move them. That simply means removing the old perches and welding on new ones.
If you tell me what the center to center dimension of the perches is, I can tell you what the axle came from, and what you need to have.

[ehostler]

of course the above posted diagram has that information as well.

And don't forget, if end-to-end is too different, it won't work, regardless of the spring perches and drive shaft length.

[DJM]

Depending on what the 8 3/4 is out of the spring perches may well be in the right location. As for the u/joint yoke chance are that it will take the same u/joint as the 8 1/4 does. The vast majority of the joints were the small joints - even on the pickups including 4X4's.

[belve]

the rear end came out of a 69 charger.

[belve]

so im going to need to cut the driveshaft ?

[DJM]

Yes the drive shaft will have to be shortened by a couple of inches. Can't remember exactly how much but it isn't much but it has to be shortened some.

[ehostler]

The width and spring perch locations will be fine. The drive shaft will need to be 2/3 of an inch shorter. It doesn't seem like much, but the splines of the slip yoke will be that much closer to the end. It would be possible for suspensions travel to shove the slip yoke far enough into the tranny to bind on the output shaft and cause damage to the transmission.

[Tazdevilish]

Not sure where the info came from about shortening the drive shaft came from. The 8 3/4 has the same dimensions as the 8 1/4 as far as tranny tailshaft to rear yoke. Trust me, I just did this swap on my Dart. If you look at application charts, they list the same length for both rears

[Tazdevilish]

sorry, he is right about the 8 3/4 and 8 1/4 dimension. I found it in the same reference chart that shows a 71 dart having a driveshaft 1/4 longer than a 72 dart....[lol] These charts are for guidelines only. I have done many rear swaps, tranny swaps. The best way to do it is, to mock the yoke into the driveshaft at zero in and out travel then take a center to center measurment. Mopar allowed 3 to 4 inches for spline travel. Now that I know that my driveshaft should be 2/3" shorter, I am glad I havent hit any dips in the road over the last 3 yrs or I would have blown my tail shaft to pieces. All sarcasm aside, ehostler, if you advise someone to run out and pay a couple of hundred bucks to shorten and re balance a drive shaft for a mere 2/3" by trusting an application chart? So to make a long story short belve, do your conversion, mock up the driveshaft and trust what is infront of you

[ehostler]

Speaking from experience, I did it once, without measuring. Guess what. The slip yoke slammed the output shaft and caused internal damage to the transmission.

The fact is, the number that are listed are accurate. Maybe mine was on the long side of the tolerances and yours was on the short side. The best bet is to install it and measure it.

[ehostler]

Speaking from experience, I did it once, without measuring. Guess what. The slip yoke slammed the output shaft and caused internal damage to the transmission.

The fact is, the number that are listed are accurate. Maybe mine was on the long side of the tolerances and yours was on the short side. The best bet is to install it and measure it.