8 3/4 Diff Dilemma

MJFUR

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69 Chrysler 300 with 8 3/4, 2.76 Open Axle, 489 Third Member that was converted to a 3.23 Sure Grip. Four rebuilds in 2800 miles. I'm not sure which way to go at this point.

First rebuild
Convert the 2.76 open axle to a 3.23 Sure Grip. All the typical conversion parts from Dr. Diff.. A recommended muscle car shop did the rebuild. Car drove with slight gear whine, nothing bad. "Give it a couple hundred miles to wear in they said." While 400 miles from home on a Saturday, rear end starts clunking, making noise. I take it to the local Chrysler Dealership that happened to be open.

Second rebuild
Chrysler Dealership says, "I'm in luck. They have an old school mechanic that drag races Mopars and the 8 3/4 will be easy." Mechanic finds that the driver side axle bearing has failed. (Axle bearings were not replaced by the shop above, but they did have to pull the axles out for 3.23 conversion.) I can't prove if above shop messed up or the axle bearing just went bad. Dealership pulls and replaces both axle bearings, removes third member to check for any debris and re-assembles. Still a slight whine in rear end. Whine steadily gets worse. Dealership work is warranted, but 400 miles away.

Third rebuild
Local mechanic recommended by the local Mopar Club. Pulls rear end apart and finds Dealership installed new axle bearings without grease. Presumes whine was from axle bearings. Dis-assembles rear end, repair, replace, re-assembles axle. Still slight gear whine. A hundred miles later, clunking when turning left. Mechanic guarantees his work. Take car back.

Fourth rebuild
Mechanic feels the Dr. Diff yoke, ring and pinion aren't cut properly (manufacturing tolerance) and that's why continuous whine. Swaps out my 489 third member with a 742 third member that has original Chrysler parts in it. Dr Diff. Powr-lok Sure Grip still installed. Car no longer whines, but still clunks when making left turns.

All bearings replaced multiple times. What is my fix? Every "Expert" can't seem to get it right. I could have bought a brand new Moser or Dana 60 at this point. Frickin junkyard 8 3/4's run without a problem.

Disgusted!!!

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Any whines from the rear axle would be gear adjustment, usually, not bearings. Once a rear axle gear whines, it will always whine, from what I have been told. Yes, we had some mid-'80s Chevy pickups which had a slight gear whine at highway speeds, for a few model years, usually heard with the windows were down, driving on hard pavement. GM said "normal" as light as it was.

How was "dealership installed bearings without lube" determined? All bearings have some sort of light assy lube on them, as I recall. Nothing major, but enough for rust prevention.

Aren't the outer wheel bearings press-fit to the axle shafts?

As for the clunks on lh turns, that might be the ltd slip unit causing that, needing some fresh lube with ltd slip additive? Reason I say that it that our '69 C-10 pickup with factory PTrac always popped on lh turns, on hard pavement. When it was new, which the dealership and others said was normal for a ltd slip axle (12B GM), but the 10B GM in my '77 Camaro never has popped. On those more vintage-era ltd slip units, they are "locked until they unlock", with 4 springs visible between the clutch packs. Not sure about the "Cone Style" units, though. Not sure why only on lh turns they make the popping sound?

Of all of the various shops involved, can't go back to a prior one with a possible warranty issue, unfortunately. ONLY the last one, if needed. Did anybody ever show you the gear pattern on the ring gear using yellow gear marking compound or similar? Or check the clearances with a dial indicator?

Now, I know that some aftermarket suppliers use aftermarket gear sets rather than OEM-spec items, which MIGHT be where the "whine" issues come from. On the other hand, I would hope the gears would be finished nice enough to be quiet upon installation. Might order up two bottles of GM PTrac Additive and see if that might help the lh turn noise.

Just some thoughts and observations,
CBODY67
 
Fourth rebuild
Mechanic feels the Dr. Diff yoke, ring and pinion aren't cut properly (manufacturing tolerance) and that's why continuous whine. Swaps out my 489 third member with a 742 third member that has original Chrysler parts in it. Dr Diff. Powr-lok Sure Grip still installed. Car no longer whines, but still clunks when making left turns.
If the car still clunks on left turns after rebuilding the center unit and then replacing the center unit, the clunking is most likely not the center section. Axle shaft outer bearings are done separate of rebuilding the center section. Clunking could be a bad axle bearing, but another possibility is a brake issue at any of your 4 wheels, front wheel bearings, suspension components like control arm bushings and ball joints, tie rod ends.

I personally don't think the clunk is an axle shaft bearing. They usually whine on turns when going bad in my experience. 8 3/4 axle bearing lube.
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20190430_132637 Left Side.jpg

Lube needle
20190430_131329 Left Side.jpg
 
I've had many 8 & 3/4 rear ends, 741, 742 and 489 housings and gears. The gears should not whine if adjusted properly. If they do from the start and they are new gears, then someone didn't get it right. As said above I would try to see if the clunking you here is from something else. Good luck and let us know what you find.
 
Ask the manufacturer if the gears you're buying are two angle or three angle cut. Three angle require a tighter backlash. I'm sure the numbers vary, I went through this with my Impala. We set the gears up to original spec. The gear sing like a bird, I called DTS they asked what the backlash was, I forget the numbers know but we set them slightly out of spec because the rear assumes the gears were from 1994 and 2 angle cut. Three angle cut gears are not the same. I say all that because your techs may be assuming they can use decades old specs. Also, and I'm sure stated above, once the ware pattern is established that's it. If you want the whine gone, you'll have to buy new gears and start over again, do not go by original specs.
 
I've had many 8 & 3/4 rear ends, 741, 742 and 489 housings and gears. The gears should not whine if adjusted properly. If they do from the start and they are new gears, then someone didn't get it right. As said above I would try to see if the clunking you here is from something else. Good luck and let us know what you find.

The only other thing I would add is to make sure end play (at the passenger side axle) is set thusly:

img_20220731_120413730_hdr-jpg.jpg


Set at .018 on my 8.75 2.94 Sure Grip.
 
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For the people that mis-read - Axle bearings have been replaced twice and are brand new. The third member was replaced with Chrysler OE parts and rear end no longer whines. The only remaining Dr. Diff part is the Powr-Lok Sure Grip. It didn't clunk after rebuild one or two.

All other suspension/brake components are new. Gear oil and Friction modifier have been replaced four times.
 
I've had many 8 & 3/4 rear ends, 741, 742 and 489 housings and gears. The gears should not whine if adjusted properly. If they do from the start and they are new gears, then someone didn't get it right. As said above I would try to see if the clunking you here is from something else. Good luck and let us know what you find.

Agree with all of this. Whine is typically the gear pattern. The clunk you're hearing while turning may be from the brakes, wheel bearing or suspension or something. It could be the diff but I'd really be surprised as I've broken a lot of diff components and I've never had one clunk turning in only one direction. If it's something with the spider gears it will clunk at random times going straight and also turning in either direction.
 
The clunking is probably the locker/limited slip
 
If this was my car I would start by calling Dr. Diff and see if that type of locker is known to make noise when cornering. If not I would put jack stands under the rear axle. Put the car in park and try rotating one rear tire and see what it feels/sounds like. I don't know how the locker you have works so if the wheel will not rotate put the trans in neutral and then rotate the wheel and see how it feels/sounds. If it all feels/sounds good, block the front tires, start the car and put in drive and see what it sounds like at idle then give it a little throttle and see what it sounds like. See if you can recreate the symptom in the shop where you can get closer to the sound. I used to teach a class on vehicle NVH (noise vibration and harshness) and it can be one of the hardest issue to solve sometimes.

FYI I had a locking diff in a Dodge truck that worked really good but while cornering under no or light throttle it would click, under medium throttle it would clunk.

Good luck.
 
Ask the manufacturer if the gears you're buying are two angle or three angle cut. Three angle require a tighter backlash. I'm sure the numbers vary, I went through this with my Impala. We set the gears up to original spec. The gear sing like a bird, I called DTS they asked what the backlash was, I forget the numbers know but we set them slightly out of spec because the rear assumes the gears were from 1994 and 2 angle cut. Three angle cut gears are not the same. I say all that because your techs may be assuming they can use decades old specs. Also, and I'm sure stated above, once the ware pattern is established that's it. If you want the whine gone, you'll have to buy new gears and start over again, do not go by original specs.
I never knew this. Thanks for the info. That's why I read almost every post on this site, always learning new things. I have set up many rear gears and always use the factory specs so the next time I will check the gear cut.
 
Since you replaced the bearings, I assume that you replaced them with Green bearings. Did he replace both sides with Green bearings? He should. Green bearings will often push the axles too far inward so that they collide with the center thrust block. If that is the case, you won't be able to secure the axles properly, and it could cause similar problems to what you are describing. The fix is to remove the thrust block from the differential. Did he do that when the sure grip was installed?
 
Since you replaced the bearings, I assume that you replaced them with Green bearings. Did he replace both sides with Green bearings? He should. Green bearings will often push the axles too far inward so that they collide with the center thrust block. If that is the case, you won't be able to secure the axles properly, and it could cause similar problems to what you are describing. The fix is to remove the thrust block from the differential. Did he do that when the sure grip was installed?
Why do you say he should replace with green bearings? I like the original taper bearings as I can clean and repack them. As long as they are adjusted properly they last forever. The only time i switched to green bearing are on drag cars with a spool as the spool will not allow you to adjust the end play. IMO the taper bearing handles cornering better that the green roller bearing.
 
Why do you say he should replace with green bearings? I like the original taper bearings as I can clean and repack them. As long as they are adjusted properly they last forever. The only time i switched to green bearing are on drag cars with a spool as the spool will not allow you to adjust the end play. IMO the taper bearing handles cornering better that the green roller bearing.
I'm with you on the adjustable bearings, but I look at this from @MJFUR 's first post and wonder if green bearings were used or not. It looks like Green bearings were spec'd on the order.

It would seem to me that a suregrip unit set up for green bearings won't have the spacer that the axles butt up against. If he has used green bearings, then the suggestions about setting the backlash aren't applicable. If he has used the tapered bearings without the spacer, that could be a problem.... Possibly even this problem.

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Replacement taper bearings no longer exist. So if he replaced the axle bearings, they put in Green bearings.
 
Good to know, thanks. Looks like I was lied to by a shop that pressed my axle bearings for me. They insisted that green bearings were the only option.
 
Good to know, thanks. Looks like I was lied to by a shop that pressed my axle bearings for me. They insisted that green bearings were the only option.
I get upset that you can't trust people doing work for you. It makes it harder for everyone, especially the next time you have to trust someone with your stuff.
 
I get upset that you can't trust people doing work for you. It makes it harder for everyone, especially the next time you have to trust someone with your stuff.
This is compounded that most shops don't let you supply parts.
This is understandable, as they lose revenue on selling parts, and also they want to use their source so they can stand behind their warranty.

But it stinks when they force you to use something else because they don't know OER parts area still available.
Which they would've known if they spent 10 mins of research.
 
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