Drive shaft length ???

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MR.7DUSTER3

MR.7DUSTER3

His money, My time
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Ok Guys , Here goes

I have done a search on this forumn and have gotten so many different opinions that its really confusing.

This is what I have : 1973 Duster, 8 1/4 rear end, 904 trans, 360 engine

I am putting in a 8 3/4 rear end, 741 casing.

Is there too many variables in this equation to tell me what I need to do with my current driveshaft. Do I need to shorten it and if so how much ?

I would like to get it shortened if needed before hand so it would be an easy swap.

What do you guys think. Is there one answer out there that every body agrees on or do I just have to be patient and measure it when I get the new rear end in ?

Bob
 
Crackedback,

Is that center to center between the ujoints ?

Thanks for replying so quickly

Bob
 
Not that it helps tremendously, but I swapped my 8 1/4 out for an 8 3/4 and the length was pretty much the same between the two. I ended up getting a new shorter shaft because the one that was in it was too long to begin with, but I'm pretty sure the fit was "equally bad" between the two rear ends with everything else the same before I got the new shaft. I did it on a 67 Dart, but the theory should still hold. If only the rear is changing (engine and trans the same) then the only thing that you need to check is axle centerline to the center of the pinion U joint on both rear ends as that's the only thing that is changing in the system.
 
I've got a drawing here somewhere that shows those housing differences 7.25 8.25 8.75 but it could take hours to find it.
If you have both housings on hand you could just measure them to determine the difference.
 
It's the POS chart that has incorrect widths.

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Only thing worthwhile on that chart is the c/l to pinion measurement! :)
 
So looking at the chart,if my 8 1/4 is 11.69 and the 8 3/4 is 12.35 than my drive shaft would have to be shortened roughly .66 inches or between 5/8-3/4 of an inch.

That measurement came up a lot when I was doing the search.


thanks guys, Bob
 
Or move the rear axle back. Some have used offset spring hangers to get back space and wheelbase changes ( height too) all at the same time.
as for the drive shaft shortening 3/8 to 1/2 inch more will put the rear seal on a new spot on the yoke which may be a benefit if the yoke is well worn. All just points to ponder.
 
MR.7DUSTER3 said:
So looking at the chart,if my 8 1/4 is 11.69 and the 8 3/4 is 12.35 than my drive shaft would have to be shortened roughly .66 inches or between 5/8-3/4 of an inch.

That measurement came up a lot when I was doing the search.


thanks guys, Bob
Click to expand...

Just to add some confusion, I had mine shortened when I switched to an 8 3/4 in my 74 Dart Sport w/904 and it measures 48 3/4" center of cap to center of cap.
 
I havent measured mine yet to even see how long mine is,I guess I will do that first and see.

Bob
 
I didn't second guess it, I measured after the rear was in and under load. It took my local driveline shop 2 days to shorten, balance and install my u joints along with a fresh coat of paint.
 
I just completed the 7.25 to 8.75 Swap in my Scamp, The best advice I could give would be to install the new rear, in your case the 8.25 and measure from center of rear end yoke to center of tranny yoke. Do this with the weight of the car on the rear end, car on the ground or at least have the jack stands under the rear axle housing.

IN my case the amount I needed to shorten was exactly the difference between the 7.25 and 8.75 center of axle to center of yoke = 2.26"
But, that may not always be the case as others will point out.
 
All depends on area, here it's 75 to 100 to shorten, or 85 to 125 for AL shafts.

Doesn't mean it's the same in your area, try calling the local driveline shop
 
Has anyone had to lengthen their drive shaft, I just put the new rear end in and under full load with the jack stands under the 8 3/4 rear end I can push the drive shaft all the way in until it bottoms out in the trans and then I can pull it back about 1 1/2 inches until it rest in the yoke of the rear end. Besides that I'm pretty sure I have to get the bigger u-joint in the rear it is way too small.

Bob
 
Before just shortening the shaft by chart measurements, measure what you have. On the ground, level, with motor, tranny and rear end installed, measure center to center of where u-joints are. Ta-da!
 
MR.7DUSTER3 said:
Has anyone had to lengthen their drive shaft, I just put the new rear end in and under full load with the jack stands under the 8 3/4 rear end I can push the drive shaft all the way in until it bottoms out in the trans and then I can pull it back about 1 1/2 inches until it rest in the yoke of the rear end. Besides that I'm pretty sure I have to get the bigger u-joint in the rear it is way too small.

Bob
Click to expand...


Depends on the length of the slip yoke, if it's 4" and you got 1.5 out that means you got 2.5 on the bushing, if you have a 5" yoke then you got 3.5" on the bushing and then you're ok
 
Hey Supershafts, you are right on the money . Took the Duster to my drive line guy here and all he had to do was put on the bigger u-joint on the rear, my yoke in the front was the long one and he said no problem at all with that. He did put a new ujoint in front since we had it out and a quick balance and its good to go.

Thanks every body who helped out with info, Damn I love this forum.

Bob
 
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