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[GertrudesFifthAve]

Anybody know if the driveshaft has to be shortened when upgrading from a 7.25 to 8.25 rear axle?
Thanks -Fred

[Dippy.org]

[lsm360]

I don't recall by how much, but yes, unfortunately your driveshaft will have to be shortened.


If you've ever considered a 727 trans swap, now's the time to do that. That would require shortening of the driveshaft too.

[GertrudesFifthAve]

Thanks LSM - that's what I was thinking for the driveshaft.  Side-by-side, the pig is just a little longer on the 8.25, maybe an inch or so.  I actually have an OD trans to put in, but I was hoping to install the axle first and drive as is for a bit.  I don't want to cut the driveshaft twice, so I will probably just do everything at once.... 
Fred

[lsm360]

Yep, might as well all at once. Some will tell you that you must have it re-balanced. I've cut two so far, but couldn't find anyone that balanced them around here, but had no issues without having them re-balanced. Just my experiences.

[GertrudesFifthAve]

Found this page on dippy:
https://dippy.org/forum2/index.php/topic,189.0.html
Center-line of axle shafts to center-line of rear universal joint:
7 1/4 - 10.09"
8 1/4 - 11.69"
The space for the driveshaft is 1.60" shorter.  I don't *think* that can be absorbed by the spline joint.
Fred

[DobaMark]

No, the joint can't even absorb 3/4" inch. I had a friend who didn't shorten his driveshaft going from an 8.25" to an 8.75" which was only about 3/4" difference.
End result: busted transmission housing and a driveshaft broken in half.

[GertrudesFifthAve]

I can believe it....  There is no where for all that energy to go once that spline bottoms out in the trans!  I should have my engine/trans setup ready to go in soon.  Just have to get it running on the stand and cleaned up.  Oh, and I still have to modify the trans tunnel...that should be fun 

[chargincharles]

Anybody know if the driveshaft has to be shortened when upgrading from a 7.25 to 8.25 rear axle?
Thanks -Fred

Ok.  Lived this one.  Ended up having to have a whole new driveshaft made.
So that said learn from my misery and do this:
Put the slider into the trans.  Leave it out about a 1/4 or so.  Don't bottom it out on the trans.  Then tie string from the u joint on the slider to the joint pad on the rear end.  You would have removed the u joint to get the slider by itself.   Make sure the car is on the ground or on ramps so there is weight on the chassis.   Mark the flange location for the u joint on the trans side and on the rear end side.  Remove the string.
Then measure the length of the string between the two marks you made.  Add the width of 1 u joint cap. 
This is the length you need.
Now find a place locally that makes semi truck drivshafts.  Google is your friend.  I found a place in Cincinnati called Republic driveshaft.  Take them the shaft you have and the measurements.  Tell them you want a new shaft with the old ends and you want it balanced.  Do not have the old one cut...it's 30+ years old.  Not worth it.  They will use a new shaft.  Replace the u joint and slap it in.  Done. 

Good luck!
CC.

[chargincharles]

Found this page on dippy:
https://dippy.org/forum2/index.php/topic,189.0.html
Center-line of axle shafts to center-line of rear universal joint:
7 1/4 - 10.09"
8 1/4 - 11.69"
The space for the driveshaft is 1.60" shorter.  I don't *think* that can be absorbed by the spline joint.
Fred

Every car is different after 30+ years of use.
Those are good guidelines but don't take them as gospel. Measure for yourself on your car.  That's the only way to be sure.