Another driveshaft length question!

[Suregrip391]

okay, currently a 904 trans with a 7 1/4 rearend in my 67 Barracuda Fastback. Looking to install an 8 3/4 from a 70 duster and still use the 904 trans. Is there anyone out there with correct info on what driveshaft to use? I still do not have a yoke for the 8 3/4. Will this be an issue as I know there are a couple different sizes as far as yokes go? Thanks group! :thumblef:

 

[Coyote Jack]

The best way to approach this is to install the 8 3/4. If the perchs were already on the rear then you are set to go. Next, get the yoke you are going to use and slide it all the way into the tranny. (be sure to lube it with tranny fluid so you don't ruin your tranny seal) Next pull the yoke back 3/4 of an inch. Then with the help of an assistant, it can be done alone but better if two are doing it, measure from the center of the u-joint holes in the yoke at the tranny to the center of the u-joint holes in the rear differential yoke. This distance is how long your driveshaft should be. Be as accurate as you can as you will probably be getting one cut down by a machine shop.

Jack

 

[Ace]

You have 7260 U-joints on the current setup. The 8 3/4 is nice for higher HP, but if you ever intend to ever use it for that, you might as well take this opportunity to move up to the large 7290 U-joint.

Somebody posted the rear dimensions chart here on another thread just the other day. You will be needing a shorter driveshaft by somewhere around an inch or two. Get a driveshaft with large joints made up at a driveline shop after you mock it up in the car and carefully measure it. They will normally be able to get all the joints and yokes you need for less than you can get them yourself, unless you manage scrape up some really good deals off eBay or something like that (unlikely). Ballpark figure a complete shaft with all joints and yokes from scratch will be about $200. And you'll know it is balanced and strong enough for your application.

 

[340mopar]

On the 7-1/4" the distance from the axle center line to the centerline of the u joint is 10.09". On the 8-3/4" the distance is 12.35". 12.35" - 10.09 = 2.26" so your driveline needs to be 2.26" shorter for the 8-3/4". 2-1/4" will be close enough.
The 8-1/4" distance is 11.69". 11.69" - 10.09" = 1.6" shorter driveline than the 7-1/4"