I have a 8 3/4 with a 657 pig in it. it is a posi unit with 3.91's tiny little u joint.

I have a stripped d60 with 4.10s and its open, big u joint (1350?)

Plan A: have driveshaft made for 8 3/4 run it till it breaks.

Plan B: buy posi for dana 60 at 30 spline, swap over 8 3/4" axles and cut d60 to fit current axles have driveshaft made for this

Plan C: Buy axles, posi and new gear set for dana along with new driveshaft. ( not in budget for car right now)

Any chance the 8 3/4" will live on the street with sticky tires a manual trans and a 540? at lease for a little while?


I need a driveshaft for any combination rear end I do. if I have one made for the 8 3/4" will I be able to reuse for the d60? if i have to swap ujoint sizes from the small 8 3/4 to the big d60 one?

Are your 30 spline axles "good " aftermarket ones? If so have the get a Power Lock for the Dana with 30 spline internals. If you sell the 3.91 pig while it's still worth something and that should cover some of the cost of the new venture.
Give Doctordiff a call and Cass will get you setup with all the right parts even the drive shaft.

Gus

Cut the ends off the 8-3/4 to use on the dana. Cut the dana so the tubes are the same length to fit your 8-3/4 axels, use open rear till you find a posi (any Dana truck rear will most likely have one in the bone yard) The axels will be the weak link till you buy the 35 splines and spool or posi. At lest you’ll be going up in the right direction money wise. 8-3/4 and Dana driveshafts will be different lengths so don’t buy twice. If you haven’t got the Dana yet scrounge the yards till you find a truck rear with the posi and 4.10 gears usually $200 around here. Install ends,axels, done. Get the driveshaft too.

Do you feel lucky? Lots of driveline failures only take out the drive part that breaks.

Or maybe the 8-3/4 breaks, damages the sprague, maybe you grenade a drum in the trans? Or maybe you bust the little U-joint and tear up the bottom of the car? Or maybe the rear end locks up solid at speed and you loose it and hit a tree? There's a lot that can go wrong.

So it all depends on how lucky you feel. Me, I don't like to trust luck. I'm not a worry wart at all. But some damage can't be repaired. So I always get the good stuff for the driveline. Do C, or at least B, but not A.

Words of wisdom. I try to follow this logic when i can afford the coin. After breaking the 3rd or 4th part on the car, i got tired of the carnage....

The standard Mopar U joints sizes are the 7260 and the little larger 7290, I'm not sure of the name or number of the U joint Mopar started using with the Dana 60 rear ends in the H.D. 4 speed cars in either 1966 or 1967, I'm thinking it is a 1330, maybe not
I think of how strong the U joint pins that the cups and needle bearing ride on by their diameter and length, the 7260 is around 3/8 to 1/2 shorter overall than the 7290 are, the longer the length on those pins the more mechanical advantage the breaking force has to break them, me thinks narrower U joints and the end of the pins being closer to the closer to the center of the pinion gear is stronger

I'd sell the 8.75 centersection while it's still good. A 3.91 suregrip center should sell for $600+.

The driveshaft will only need to be shortened when switching from the 8.75 to the Dana.

cab, i used to think about the small joint like you.
however, the cap size of the 7260 is 1.062 vs 1.125 for the 7290.
the width between the ears [snap rings] of the 7260 is 2.060 [maybe 2.125 ?] vs the 2.625 of the 7290, so the 7290 is quite a bit bigger than the 7260, thus stronger.
so ?
the mechanical advantage is something to think about, but comparing it to the joint size increase and strength differences between the joint pin and cross sizes is WAY beyond my pay grade.
like the saying goes : "bigger is ALWAYS better !"

That's not what all my old girlfriends use to say
I weighed 200 lbs. back then also

well i spoke with cass (email)

I will be getting a driveshaft from him, a powr lok, 35 spline axles and some odds and ends, plus I am going to re-gear my dads 3.23 posi unit to a 3.73.

Recently bought a Satellite-68 with a 741 8 3/4 without sure grip. Found a Dana 60 3,73 with a Trac lock for 150usd. Ended up buying an all new Powr lock and 35 splines axles from Doctor Diff.
Building the Dana one inch shorter than the 8 3/4. 59 1/8 total.