Rebuilt Diff - Posi Growls only when HOT

[SmudgeServices]

long time lurker, first time post'r!

i'm having a perplexing situation i'm having with my 1970 dodge challenger and i'm looking for guidance/insight as to the problem and what direction i should go to fix it.

bearings went out in the third member a few months back so i pulled it out and had it rebuilt.
it is a mopar 8 3/4 rear (chrylser brand - not aftermarket), 489 case, 3:23 with sure grip 'cone' type posi. it was rebuilt by a local shop where they replaced the ring and pinion gear and used a bearing set to replace the bearings throughout the differential. since the car was apart, i replaced the axle bearings with the green style, replaced the rear shocks, and put on new tires.

i put the car back together and tried to read all i could about the best oil to use. there is so much conflicting information so i just used "Mobil 1 Synthetic Gear Lubricant LS 75W-90 (Limited Slip Friction Modifier Included)" from the local parts store. in attempting to break in the differential properly, i drove about 15 miles <40mph with no issues and let the car cool. since then, i've taken it out to drive about 20 miles each trip to get it hot and then let it cool. but that's where the problem arises:
once the differential gets HOT after i drive for a few miles at 55+mph the posi starts making nasty noise when i slow down and make turns. especially sharper neighborhood turns. it's a growling type sound and it feels like the axle/wheel is skipping, getting caught up, or grinding in there. when i straighten out and continue driving it goes away until i go to make another turn. seems like both sides make the noise and it happens both turning left & right (but again, only after it's hot). if i let the car cool down for a day and take it back out, it's the same story--- quiet until it gets hot from driving 55+mph.

as per Dr. Diff's site, i purchased some Ford Friction Modifier (assuming the mobile 1 limited slip premixed in the gear oil just wasn't good enough) and added it but the problem continues to happen. it's been a total of about 100 miles since rebuild. i've driven it in 20 mile sessions to heat up and cool down but the problem persists. i'd say it's "gotten slighly better" since adding the ford friction modifier but certainly is still blatantly there and nasty growling.

any ideas or suggestions? the shop which rebuilt it is not responding to my calls. any guidance would be extremely appreciated. i just want to be able to cruise again! it's a hobby car and i'm no mechanical expert.

 

[Rich G.]

Sounds like the clutches are chattering in the rear. I’d add another bottle of clutch additive and go in a parking lot and drive around in circles for a while in both directions. ( Missed the cone type )

 

[57fury440]

It sounds like the sure grip. How many miles are on the unit? You replaced the ring and pinion and bearings but reused the sure grip. You said it is the cone style which does not use clutches. I remember an article in High Performance Mopar from years age that showed how you can do some adjustments on the cone style units.

 

[Chryco Psycho]

The cones tend to wear down until they bottom out inside the case at which point the grip is lost & they can be a problem , this also allows the axle gears to move apart & not mesh properly , my bet is the sure grip is worn out & needs replacement , I would look at a Torson / Eaton Tru Trac unit to replace it , of course this means the whole center section has to come apart except the pinion & be reset properly .

 

[toolmanmike]

Rustyratrod on FABO can rebuild those cones. Try some standard 80/90 or 80/120 or whatever. The synthetic may be too thin.

 

[SmudgeServices]

It sounds like the sure grip. How many miles are on the unit? You replaced the ring and pinion and bearings but reused the sure grip. You said it is the cone style which does not use clutches. I remember an article in High Performance Mopar from years age that showed how you can do some adjustments on the cone style units.

thanks for the idea, i'll try and dig up information about doing adjustments to them.
yes, the ring-n-pinion were replaced but the sure grip was reused. no idea how many miles are on the sure grip.

The cones tend to wear down until they bottom out inside the case at which point the grip is lost & they can be a problem , this also allows the axle gears to move apart & not mesh properly , my bet is the sure grip is worn out & needs replacement , I would look at a Torson / Eaton Tru Trac unit to replace it , of course this means the whole center section has to come apart except the pinion & be reset properly .

oh man!! not the cheap, simple fix solution i was crossing my fingers for. makes sense though, absolutely appreciate the info.
@57fury440 mentioned possible adjustments. could just an adjustment possibly fix it without having to pull the entire pumpkin apart again, ya think? since it was rebuilt, could it be adjusted wrong such that it is not meshing properly with the new bearings and ring-n-pinion rather than worn down? it wasn't acting up in this way before the rebuild which slightly puzzles me.

when the guy who rebuilt it had it apart, i told him i wanted him to just go through it all and fix everything it needed so that "i didn't need to go through this again". good grief! i work with computers for my day job; not enough experience with this stuff. would it had been obvious or noticeable during the rebuild to see that the sure grip was worn down? is that the kind of thing typically visible or measured?

 

[SmudgeServices]

Rustyratrod on FABO can rebuild those cones. Try some standard 80/90 or 80/120 or whatever. The synthetic may be too thin.

that's good to know, gracias.

the oil being a culprit was my instinct considering the 'limited slip additive' was already in it and since she only acts up when hot. immediately i felt a heavier or dino oil might matter for that heat. it's the cheapest, easiest solution so it's totally worth a try swapping it out before getting into an expensive rebuild if i don't have to. i'll do that this week.

 

[Daves69]

Here's RRR's "how to" thread on the cone.............
How To Recondition a Sure Grip Cone

 

[Rich G.]

Interesting. I never knew there were 2 types of setups. Always room to learn.

 

[Chryco Psycho]

There is no Adjustment to the sure grip , if the cones are bottomed the sure grip & ring gear have to be removed & disassembled to try to repair them , , the cone type is a poor design & new cones are not available to my knowledge , once they bottom out & most are worn that far ,you can mill off the end of the cone so it will grip again But you lose pressure which requires some kind of shim to get the pressure back to normal . Personally I would not rebuild the cone type but replace it with a better design that can be rebuilt such as the Eaton Tru Trac or the clutch type sure grip .

 

[bossmike]

long time lurker, first time post'r! i'm having a perplexing situation i'm having with my 1970 dodge challenger and i'm looking for guidance/insight as to the problem and what direction i should go to fix it. bearings went out in the third member a few months back so i pulled it out and had it rebuilt. it is a mopar 8 3/4 rear (chrylser brand - not aftermarket), 489 case, 3:23 with sure grip 'cone' type posi. it was rebuilt by a local shop where they replaced the ring and pinion gear and used a bearing set to replace the bearings throughout the differential. since the car was apart, i replaced the axle bearings with the green style, replaced the rear shocks, and put on new tires. i put the car back together and tried to read all i could about the best oil to use. there is so much conflicting information so i just used "Mobil 1 Synthetic Gear Lubricant LS 75W-90 (Limited Slip Friction Modifier Included)" from the local parts store. in attempting to break in the differential properly, i drove about 15 miles wheel is skipping, getting caught up, or grinding in there. when i straighten out and continue driving it goes away until i go to make another turn. seems like both sides make the noise and it happens both turning left & right (but again, only after it's hot). if i let the car cool down for a day and take it back out, it's the same story--- quiet until it gets hot from driving 55+mph. as per Dr. Diff's site, i purchased some Ford Friction Modifier (assuming the mobile 1 limited slip premixed in the gear oil just wasn't good enough) and added it but the problem continues to happen. it's been a total of about 100 miles since rebuild. i've driven it in 20 mile sessions to heat up and cool down but the problem persists. i'd say it's "gotten slighly better" since adding the ford friction modifier but certainly is still blatantly there and nasty growling. any ideas or suggestions? the shop which rebuilt it is not responding to my calls. any guidance would be extremely appreciated. i just want to be able to cruise again! it's a hobby car and i'm no mechanical expert.

 

[bossmike]

Smudge, I have not seen these two options mentioned. First- check that you have enough axle end play using the end play adjuster. Second- Assuming everything mentioned has been checked or tried, look to the rebuild guy. Any company that will not honor their work should be met harshly with a final call mentioning the next step is the BBB, then follow up with a claim. I have gotten slow or non responding people to wake up using the BBB. The least they can do is help you diagnose to confirm it is not their work.

 

[i_taz]

Don't assume synthetic is inherently better than dino. Metallurgy along with design clearances all come into play...
I had Mobil 1 smoke a pinion bearing in a Dodge van. When's the last time a pinion bearing went bad at 30k...? Within a few thousand miles of changing the gear oil
it did...

 

[Chryco Psycho]

Synthetic in diffs seems to be a problem at least in older vehicles .

 

[Mopar Nut]

First- check that you have enough axle end play using the end play adjuster.

He’s using green bearings, no adj. necessary.

 

[fasjac]

I just use 75/90 gear oil and BG lsd additive. Has been fine for me.

 

[SmudgeServices]

it's gonna cost me a few nickels to try a few other options before following @Chryco Psycho 's advice and having it rebuilt with a Eaton Tru Trac or the clutch type sure grip but i'm really really really praying it's something less dire

so an update...
i was skeptical from the start about using the synthetic. the old man had some original mopar brand oil in the garage (pictures attached) so he handed me over 2 quarts to try. the label reads as if it already is pre-mixed for limited slip although it doesn't designate a specific oil weight/viscosity. regardless, i pumped all the gear oil i could out (~2 quarts) and then poured in this original mopar oil. i also poured in a single 4oz bottle of Kendall Limited Slip Additive. took her out for a test drive and put about 15-20 miles on it. was silent and normal even after driving up in the 55-60mph range periodically. eventually though, after cruising @ 60mph for 2+ miles, the first several turns the noise returned. but then it oddly seemed to go away after driving through the neighborhood with sharp turns (it had never gone away before unless cooling down). i took another spin around and repeated the same path only for it to behave the same (first several turns she was noisy but seemed to go away after a couple minutes of sharp neighborhood turns).
so in summary... it seems, better? but not perfect. i let her cool down (still breaking in the rebuilt diff) so i plan to take her back out and repeat again with hopes of an improved result.

a few of the other options i'm thinking...

1 --- since she seems to be getting better, could any of it be related to the simple fact that the diff is still being broke-in since the rebuild? she's only got 140ish miles on it since re-installing
2 --- this mopar oil seems to be pre-mixed with limited slip additive. i put in a 4oz bottle of additional additive regardless. should i put in a 2nd 4oz bottle of additional additive? will it hurt? how many bottles (or ounces) of limited slip additive do y'all typically put in?
3 --- something that has been making me dizzy thinking about, is the green bearings. Doctor Diff wrote this blog all about the green style bearings. i read it after i had purchased these MRERP400 bearings from mancini racing so i never knew there was multiple styles of green bearings.

First generation (RP-400) Green bearings, still sold by Mopar Performance and others, are problematic because the crimped-on flange will not allow the bearing to wiggle around inside a housing that is not perfectly straight (none are).

In addition, the design causes the axle to be inserted DEEPER into the housing than necessary. This results in pre-loading against the differential thrust block and early bearing failure.

Second generation (MO-400) snap-ring style Green bearings are forgiving because they can move around inside the housing and they do not preload the differential thrust block in a stock application.

Most guys who have problems with Green bearings are running the RP-400 first generation version or incorrectly made aftermarket axles or housings or poorly designed rear disc brake kits, all of which cause pre-loading and premature bearing failure.

i am running stock axles, stock housing, and stock drum brakes.
looking at the mancini racing part #, reading this info from doctor diff, and mixing them with my complete absence of knowledge, i'm feeling very certain i installed the RP-400 1st gen green bearings.
1) could it be the axle bearings? is it likely?
2) could the bearings be getting warmed up from the higher speed & rpm of the wheels causing them to get noisy on the sharp turn side load?
3) could the sound i'm hearing be the pre-loading against the diff thrust block? (i don't know if the warmed up, higher speed circumstances would contribute to this). what sound would preloading make?

 

[Chryco Psycho]

Change is a good thing , there is hope it may get quiet , not sure how well the cone sure grip will work though .
I personally hate Green brgs , they are not great & poor for side loading usually you have to remove the center thrust spacer to even fit the axles with green brgs , , the green brgs could be the cause of the noise but shouldn't be with new brgs installed . Green brgs do have a short life span .
I would not get in a panic yet , I would put a lot more miles on first , run at least 1000 miles & see what happens , of course if it gets a lot worse stop sooner of course .

 

[bossmike]

I have not seen axle end play mentioned. If you do not have a the axle adjuster set with a little play 1/8” or so, axles will bind up as things get hot and that could eventually hurt the rear. It actually sounds like what you are describing. Hope that helps.

 

[Mopar Nut]

I have not seen axle end play mentioned. If you do not have a the axle adjuster set with a little play 1/8” or so, axles will bind up as things get hot and that could eventually hurt the rear. It actually sounds like what you are describing. Hope that helps.

He’s using Green bearings. You can’t adj. them.