8.75 Help

Ok my brain is smoking and I am at a total loss....

I purchased Nitro axles shafts with Green style bearings for my A body 8.75 rear end with a clutch type limited slip diff... I have 2 741 cases with a 3.91 clutch type diff one which was previously installed and driving with a howl but fully functional.

I purchased the second 741 case built the same way used but functional. I installed the newly purchased diff with the factory style axles and bearings to load the car on a trailer without any issue. I relocate the car and install the new axles with the green bearings which required me to remove the thrust bearing from the noted diff.

After installing the new axles with the green bearings I slowly drove the car in reverse 10 feet and heard a loud pop. At this point I had not movement as the driveshaft would spin but the differential would not so I assumed either the pinion popped or the axles did not fully engage the diff. I pulled the diff, swapped in the old diff and again the new axles shafts... Same results. Back up and a pop resulting in no movement though being driven by the driveshaft. When pulled the diff, no visible damage to either the axles or the diff.

I measured both axle sets of
shafts and they measure the same length so I am at a loss here.

What am I missing?

Are you sure the bearings are pressed on all the way?

Yes, I did verify the location of the bearings being correct.

You're saying after it went pop, the drive shaft turns but the wheels don't?

If so, the answer should be in the first diff. Either the pinion shaft broke which is highly unlikely given it happened to 2 different diifs, or the splines on the axles aren't engaging the side gears for whatever reason. Time to get the first one on the bench and see what's broke or what the side gear splines look like.

Kevin

I have them both on the bench with no visible damage to either one...... The axle shafts new and old are close to being the same length (nitro axles are .25 shorter in length) and the Nitro axles also have a shorter splined section, maybe 2 inches long where as the factory ones have a spline section that is 3 inches or so .

With the diffs on the bench they appear to function as intended. Good pinon engagement and no damage.

is it possible the axles are not engaging both sets of splines on the left/right side? I can see some engagement when I pull the shafts. They appear to engage about an inch.

Thank you

In the original Sure-Grip differential with the 2-piece side gear, there is only about 1/4" of the axle tip engaging the inner gear. Could be the problem, axles too short.

Pic below shows how little of the axle tip engages the inner gear.

ok... we are starting to get somewhere. Help me understand what the inner splined sets do in the differential. When I heard the pop in the rear, did I do any damage if the axle was not engaged into the inner set of splines?

Keep the info coming!

Thanks

You are loosing the transfer of power somewhere, The splined axle shafts go into the side gears. You said you measured the axle shafts but did you measure from the bearing to the end of the shaft? If the bearings are wider or not in the correct position it will move the shaft out(away from the side gears). You may want to lift one wheel off the ground and see if you can turn it.

Sounds like the new shorter axles are coming out the 3rd member.
BTW The clutch type are called Dana Power Lock

The pic below shows the side gear components. The axle splines first pass through the side gear retainer then through the side gear; the side gear makes contact with the spider gears. If the axles only pass through the retainer and don't engage the side gear, there is no power transferred to the axle shafts.





I'm guessing that the initial movement was made with the gear components stuck together from age and deposits which broke loose and made the pop. No damage to hard parts.

I'd put the old axle shafts back in it and see if it does the same thing.

I wouldn’t. I would want to know what the splines on the side gears look like. Been there done that.

that and if the side gear splines aren't lined up any more you'll never get them in. You have to have the axle in when bolting the case halves together to align them.

Put some grease on the splines and slide it into the third member all the way you get it to go and then remove it to see how much it went into the side gears and into the rest of the parts
I've broken, twisted off, more than one stock left side A body axles and they all went way more into the rear end ( 7/8 to 9/10 of the splines) than the one John showed in his picture
Yours should go that far in also

OK! UPDATE:

Here is what I learned on my issue. My diff has the 2 piece splined section in the differential and my aftermarket axles are just short enough to only engage in the outer set of splines and just barely touch the inner set. I spoke in Doctor diff on this as this is not unheard of with newer aftermarket axles. The recommendation from Doctor Diff was to remove the factory 2 piece splined section with their on piece.

[u][/u]

Yep…..that’s what you want.