8 3/4 with 904. Which driveshaft?

[4mulaSvaliant]

Hey all. Ok well Ill be getting my 8 3/4 this weekend and need to figure out the drive shaft sizing issues.
Currently I have a 904 with a 7 1/4.
Im keeping the 904 and dropping in the new rear.
I need to get some input on what I will encounter and what I need to look for.
I have heard that Ill need a shorter driveshaft and particular U joints.
Oh and this is a 69 valiant 2 door sedan.

Any help is much appreciated. :headbang:

 

[66dartman]

I was able to find a 904 slip yoke with 7290 u-joint and I used a 1330 rear yoke on my 8-3/4. A good driveline shop helped me out. If you can find a good shop that does race car work, they should be able to set you up with a driveshaft you won't have to worry about. 727 and 904 slip yokes are different, take it from me!

 

[Suregrip391]

I was able to find a 904 slip yoke with 7290 u-joint and I used a 1330 rear yoke on my 8-3/4. A good driveline shop helped me out. If you can find a good shop that does race car work, they should be able to set you up with a driveshaft you won't have to worry about. 727 and 904 slip yokes are different, take it from me!

I am going to be doing a simular swap only with a 69 barracuda, We should be able to use our old slip yokes providing we get our shafts shortened right? Its the pinion yoke that needs changed out due to the 7 1/4 vs. the 8 3/4 rear correct??.....

 

[4mulaSvaliant]

sounds like thats the situation to me. Ill keep you posted on what ever I find out. Lata'

 

[4mulaSvaliant]

Ok well I got my 8 3/4 this week end and 2 drive shafts as well. He didnt know the application to either of the shafts, so I compared lengths to my current DS and one is the same and the other is about 7 3/8 inches shorter.
Actual length is 45 3/16 with out slip yoke.
The shorter one is also bigger around as well. Im hopeing that this may work out for me but I think its too short.
How do you tell what U joint it takes with out checking a bunch of them?
MIC it? or?
Is there anywhere there may be a part # on these shafts? so I can atleast find the application?

Also does anyone know the actuall correct shortened shaft length?
U-joint center to U-joint center.
Anyhelp is much appreciated!

 

[Suregrip391]

Ok well I got my 8 3/4 this week end and 2 drive shafts as well. He didnt know the application to either of the shafts, so I compared lengths to my current DS and one is the same and the other is about 7 3/8 inches shorter.
Actual length is 45 3/16 with out slip yoke.
The shorter one is also bigger around as well. Im hopeing that this may work out for me but I think its too short.
How do you tell what U joint it takes with out checking a bunch of them?
MIC it? or?
Is there anywhere there may be a part # on these shafts? so I can atleast find the application?

Also does anyone know the actuall correct shortened shaft length?
U-joint center to U-joint center.
Anyhelp is much appreciated!

I was told when switching from the 7 1/4 to the 8 3/4 the driveshaft would only need to be shortened by roughly 2" total. Your short shaft sounds like it will be WAY to short........Keep us posted! Thanks for the feedback. I really dont know the answers to your other questions......sorry.

 

[4mulaSvaliant]

Hey 391

On the "my mopar" site I found the dimensions to compare from the center of the axel to the center of the U-joint. the difference from 63-72 A bodies with 7 1/4 rear to the A body 8 3/4 rear is 2.26" but Im just gonna go with 2 1/4" now I just need to find the correct set up for the rear u joint and Im in there! hope this helps.
http://www.mymopar.com/axledims.htm


\\/

 

[4mulaSvaliant]

Ok also on that same page I found the measurements for the several different u joint cap dia. that are used in 8 3/4s
Using that, I determined which Yoke I have. I have the 7290.
So to end this: I have to get my drive shaft shortened by 2.26" and have a 7290 yoke welded in it to bolt to the differential.
hope the research page will help you 391.
lata'

 

[Suregrip391]

Good job! Ill take a look. Thanks for the post!!! :thumbup:

 

[AdamR]

The drive shaft out of my 69 Barracuda is 49" from the center of one U joint to the center of the other. Thats with a 8-3/4 rear and 904 trans.

 

[4mulaSvaliant]

Perfect man I just went out and measured the drive shaft!
If I do as my research has told me, then My draive shaft will also be 49".

Thanks for the info, though I am glad I was able to get the facts and then also have a real life proof situation to back it up!

49" center to center with a 7290 rear Yoke.......... glad to have that worked out.

Now Ill call the shops tomorrow and see what I can get it done for. $$$$ wise

 

[Suregrip391]

Hey 4mulaSvaliant...What is the length of your driveshaft now, 51.25" roughly? Did the math work out??......Also, how did you determine what yoke you needed? Do different cases 741,742,and 489's use different yokes? :thumbup: Thanks.

Aaron

 

[4mulaSvaliant]

Yes the current drive shaft is 51 1/4" cent. - cent. of the u-joint.

Yes they do differ. The 742 has a 10 spline yoke in the differential, the 489 has a 29 spline yoke in the diff.
As for determineing which one I have I measured across the area that the U-joint cap will fit in. According to "My Mopar" the spec are as follows.


7260 Yoke : cap dia. = 1.078" (This would be about 1 1/16" across the cap.)

7290 Yoke : cap dia. = 1.126" (This as it seems, is the most common for the 8 3/4. Also 1.126" is basicly the same as 1 1/8".)

1330 Yoke : cap dia. = 1.063" (this yoke is easily distinguished by having the "c" clips that hold the U-joint in place on the outside rather than the inside like most U-joints.)


If you can rule out the 1330 by looking for the grooves where the c clip fits at the outside of the yoke, then you can move on to measuring the yoke to determine which one you have.


try this link to anothere page on the same site

http://www.mymopar.com/Mopar8_75RearEndGuide.htm


Oh on that same page there is also a measurement that goes across the entire yoke to help determine as well.


Hope this helps! Def. go check that page I was able to get all of my info from that page and one other one on the same site. :thumbup:

 

[4mulaSvaliant]

Well the driveshaft is at the shop right now. I ran into an unfortunate situation when I got under the car to get it out. I just happen to notice that it steps down quite a bit.
It goes from 3 to 2 1/2" and my concern was in the right area. I was going to have to order a new yoke (7290) to put in the shortened shaft. Well the smallest the 7290 yoke comes is 2 3/4".
So what I've had to do is get a U-joint that is a combo. It has the smaller original size yoke and also the bigger 7290 to bolt up to the diff.
At first I wasnt to keen on this idea but the guy had a good point.
Im building this car slowly and adding HP here and there, so it may be best to have the U-joint be the weak link rather than break the trans or whatever else.
I plan on making a driveshaft hoop, so I should still be safe anyhow.
Ill have my DS back tomorrow and will be putting in the rear maybe this weekend.
Just thought Id let a few people know that there is one more option to this swap.

lata'

 

[Suregrip391]

Well the driveshaft is at the shop right now. I ran into an unfortunate situation when I got under the car to get it out. I just happen to notice that it steps down quite a bit.
It goes from 3 to 2 1/2" and my concern was in the right area. I was going to have to order a new yoke (7290) to put in the shortened shaft. Well the smallest the 7290 yoke comes is 2 3/4".
So what I've had to do is get a U-joint that is a combo. It has the smaller original size yoke and also the bigger 7290 to bolt up to the diff.
At first I wasnt to keen on this idea but the guy had a good point.
Im building this car slowly and adding HP here and there, so it may be best to have the U-joint be the weak link rather than break the trans or whatever else.
I plan on making a driveshaft hoop, so I should still be safe anyhow.
Ill have my DS back tomorrow and will be putting in the rear maybe this weekend.
Just thought Id let a few people know that there is one more option to this swap.

lata'

How did you make out with the driveshaft install?