anyone swapped a 70s diff into 81 Im sending a 75 diff to toms for beefing up ,toms told me I could modify my cover to fit the 75diff and I have to modify the front camber bracket ,doesnt sound to bad, im curios if anyone has done it .Im only running 475-500hp so he said i dont need to go whole hog for the 12 bolt conv. and in a few months ill have a rebuilt 81 diff w/373gears for sale. thanx Ive learned sooo much from this website.

 

Many of times. This is a 1480 kit in a 80. 650 HP Motor

http://www.vansteel.com/photos/index...thumbs&e=4&s=1

 

Do you have a manual or auto '81?

I'm half kinda in the process of doing this myself. What I've found is this:

The 1/2 shaft Ujoints of the auto are smaller than every other C3. The Manuals have the same size as the 70's. If you've got an auto, you'll need new halfshafts & flanges (Yoke & Spindle flanges - the 81 has a 1 piece unit here) to fit the bigger Ujoints - unless you can find a joint that's got a small side & a big side to mate up - Duntov may sell this but they advertise it for driveshafts.

The Driveshaft Ujoint is different also - It's the same size as an 81 auto halfshaft. You may need to lengthen your driveshaft. Should be able to have the Driveshaft place put the right yoke on it to match the pinion yoke.

You'll need the Strut rod bracket for the Iron Diff, it mounts to the case, where the 81 sandwiches the case & cover. The Strut rods are different as well, due to the differing position of the bracket - the 81 ends are angled.

The 81 rear cover "batwing" has 6 bolts that actually go into the case, the other 4 holes are for the strut rod bracket & sandwich the case. The Iron Diff has 8 holes around the perimeter.

I've read of people having the front differential mount be too close to the pinion, causing the diff to point down in the front. Others do not have this problem. You should be able to fab up a new bracket pretty easily if needed.

Other than that, it looks plug & Play. Like I said, I'm gathering parts... I've got an auto 81. Found a rear end from a 68-70 that has everything I need except the gear ratio I want... Anybody want to buy a good condition diff with 4.56s in it??

THe Spicer P/Ns for the U-joints that I know are as follows:

81 auto halfshaft & Iron Diff Driveshaft (may vary depending on tranny) - P/N 5-790x

81 manual & all 68-79 halfshafts - P/N 5-799x

Hope this helps.
Dan

 

Im running a automatic Toms told me my old cover w/modification will work and I would have to cut the front camber bracket and modify for mine, I am redoing t/a,s half shafts but i wont be going much over 500hp so I wont have to go with the 12bolt conversion,Just the 74 diff 30 spline axles and better half shafts and u-joints and new spring. thanx

 

You might want to add ARP bolts and a steel cap.