Swapped a 8 3/4 into my Doba. Bought a 3:91 sure grip in a 657 case. Clutch style sure grip unit. Got it on the ground and it won't move. I looked under the car and the driveshaft spins. I jacked up the rear end enough to get the tires off theground and put it drive and it works fine. What's goin on? It was supposed to be rebuilt but I have no proof and the seller claims it worked when pulled. I'm upset and annoyed. Please help.

Take it apart, pay attention & you will find a problem...

Possible causes:

1. Axles are too short to reach inner side gear splines
2. Axle splines and/or side gear splines are stripped
3. Side gear teeth and/or spider gear teeth are broken

Are you running aftermarket axles?

Running stock axles. Axles match the housing. Are sure grip axles any different than standard open axles? I don't think they are but I'm looking into any possible reason.

All 8.75" axle lengths were standardized after 1964.

Your axle is probably not engaging the innermost side gear. This could be due to stripped splines, or a severely bent bearing retainer.

Are you running OEM wheel bearings, or Green wheel bearings?

Did both axles install easily?

Green bearings. And yes they popped in fairly easy. Got home from work a little early, gonna go and pull an axle and see whats going on.

maybe put white lithium grease on the axle shaft splines so you can "read" what's gong on when you slip them into the diff?

Pulled the axles. All looked normal. One side was tapered a little bit and the bearings arent pressed all the way down. (1/16th of an inch from being seated) So I got them down as far as I could. Put it all back together and still nothing. This Pumpkin is shot. Thats all i can think of. I used a white grease marker and checked to make sure the axles were going all the way in and sure enough they are making contact. Anybody else got any ideas. Next up is to put the 3:23 open back in and see if it does the same thing.

sounds like you got took,just pull the punkin an get it over with,GOOD LUCK.

If the Green wheel bearings are seated, the distance from the snap-ring to the outisde face of the axle flange will measure 2.4".

The axles will always engage the first set of clutch hub splines, but the hidden inner side gear splines are often stripped. These inner splines drive the vehicle.

Open differentials do not have this problem.

And here's a picture to show the double splines the good Dr is describing... BTW Doc has new gears...

My guess: side gears in diff. are sheared. DAMHIK.

Could be that the axles are not engaging the inner splines of the side gears ?

It's a 657 chuck which is the pre 741 , does it have the early clutch type suregrip in it from a rear with tapered axles ? Which may require a longer axle ???? I remember john K. posting something about the early rears having a different thrust width ????

If the axle isn't engaging the inner splines then the problem the OP is having can happen , doesn't necessarily mean the chuck is bad. A change of the side gear and the correct thrust spacing would fix it ?

I vote for a broken pinion.

R.

If it has the clutch type sure grip unit, the clutches are toast. You can buy rebuild kits for them for around $70.

If the clutches are shot it will still move.