# Rear "Clunk..."



## curtis.smith68 (May 9, 2018)

I've searched and read many articles and threads and have not found anything that would point me in the right direction with my particular issue... This post could easily be started in the suspension/Brakes section depending on the source of the problem but I'm at a loss...

My setup relative to the issue...
-BMR Stage 2 suspension.
-Stock rear drum brakes
-Stock rear end and internals (I believe)
-Fresh gear oil and additive in rear end
-Fresh gear oil in trans
-Cleaned and greased trans yoke, verified moves freely in and out
-E-brake not connected to cable
-New solid (non greasable, sealed) U-joints

There may be more relevant information but I'll wait for questions since this is what is coming to mind. A lot of new parts and a lot of variables... 

The issue...

_ Typically _after sitting for a day or two I will get a heavy "Clunk" from the rear of the car. The car will visibly move with this clunk like someone bumped into it. It sounds like it resonates from the drive-train but I have yet to be close enough to the rear when it happens. It will happen, mostly, when going from a long standstill to a reverse direction and it will happen under no load, meaning when pushed by hand. It has happened in a forward direction too but typically when being pushed rear. It has also happened when sitting for a long period, I've moved it forward with no clunk then immediately moved it rear and there is a single clunk again. It's always one single clunk, when it happens. I've only driven it a few times, only around the block, since all this work. 
When I opened up the pumpkin it had about 1/2" thick layer of solid sludge in it, with black-strap molasses... Mmmmm mmmm! That was all cleaned out and replaced with new oil and additive for the posi. I had the axles out to install larger wheel studs but it was doing this before this work. I do not believe it's the brakes hanging up but could be wrong because the tires will almost always get a half rotation to little over a full rotation before this happens. I've gone over the suspension and verified everything is tight. I have yet to look closely for and indications of suspension components moving or shifting out of the ordinary... I'm not sure what else to say... If needed I will try to get a video of it happening but it is not a constant thing. I haven't noticed any clunking in a turn or when driving. It mostly happens in the driveway when being moved after having sat for a while.


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## BLK69JUDGE (Jun 10, 2010)

When I opened up the pumpkin it had about 1/2" thick layer of solid sludge in it 

hmmmmm


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## PontiacJim (Dec 29, 2012)

Since the problem was there before you did all the work, my guess is the rear end is in need of some possible repairs. Sounds like it may be something badly worn out in the gearing/carrier and it is binding, ie not meshing correctly.

It could be something higher up as well in the transmission? It is hard to diagnose such a sound or situation. Make sure the driveshaft is not at a steep angle and hitting the pinion yoke. I went and put a set of those aftermarket U-bolt caps to replace the factory straps on my brother's 1973 Plymouth rear axle. The replacement caps were indeed stronger, but were also too long and when the driveshaft rotated, the U-cap bolts struck the driveshaft. Had to go back with the factory U-bolt straps/nuts.

Just going to have to really look around for things and get the car up in the air so you can rotate things by hand and feel for the interference and then stop and see if you can identify it.


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## curtis.smith68 (May 9, 2018)

Thanks Jim. I’ll keep looking.
To clarify this issue is new. It was never there prior to any work I did. When I cleaned the sludge out of the diff I did inspect the ring and pinion and all else and didn’t notice anything obvious. I wasn’t looking for much either because at them at time I was just opening it up to clean and have a general look. The sludge was concerning but not terribly as the car had been sitting for almost 10 years and who knows how long before that had the oil been changed. I’ll keep investing.

I’m no expert and when it comes to differentials, I have had virtually no experience, I’ve been lucky and have never had any problems with one. I’ll look at the brakes again too but I’m completely stumped.

Could this possibly be something..... I completely cleaned out the diff, completely! I then added new oil/additive but I have only driven around the block maybe 4-5 times and never getting above 15mph. Could it be that the diff hasn’t really had a chance to mix the oil and additive and splash it around where needed? Could this cause binding? Just a thought I had today... I’m grasping at straws.
I’m going to look at the brakes, suspension and everything to look for contacts or slipping or something of a smoking gun.


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## O52 (Jan 27, 2019)

Did your use regular 'Dino' oil for the differential? My axle guy says never use synthetics for a Posi.


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## 67lemans (Oct 30, 2009)

Have you've jacked it up and with the back wheels off the ground tried to replicate it just by moving the wheels by hand? It sound to me like possibly a brake thing. Obviously be very safe and careful doing this.


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## PontiacJim (Dec 29, 2012)

curtis.smith68 said:


> Thanks Jim. I’ll keep looking.
> To clarify this issue is new. It was never there prior to any work I did. When I cleaned the sludge out of the diff I did inspect the ring and pinion and all else and didn’t notice anything obvious. I wasn’t looking for much either because at them at time I was just opening it up to clean and have a general look. The sludge was concerning but not terribly as the car had been sitting for almost 10 years and who knows how long before that had the oil been changed. I’ll keep investing.
> 
> I’m no expert and when it comes to differentials, I have had virtually no experience, I’ve been lucky and have never had any problems with one. I’ll look at the brakes again too but I’m completely stumped.
> ...


Assuming posi because you added posi fluid to the 80-90WT gear oil. I am also no expert, but some posi units use clutches. That sludge may have been all the clutch disc materials. So like an automatic trans going bad, you change the fluid and ad fresh and the trans quits because the only thing keeping the trans alive was all the clutch dust/particles in the gunky fluid. Now that it's all clean, there is no longer ANY friction material left to grab on the discs, and the trans slips and will no longer go.

So check to see what type of posi you have and maybe you need new clutches/posi rebuild or a new posi unit.


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## PontiacJim (Dec 29, 2012)

Just thought of one more thing. You want to ID the posi unit. Picture? Is it possible it is a "detroit locker" type? These can clunk when they engage and disengage as is their nature. Eaton makes one of these types. I have a Power Trax no-slip locker in my 9" Ford that I went with. Power Trax also offers these for 10-bolts and does not require complete rear-end disassembly, just replace the spider gears with the Power Trax. These can also make clunking noise as the engage and disengage.

So maybe you have some form of a "locker"?


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## curtis.smith68 (May 9, 2018)

Thank you all for the responses. I fiddled with it a bit this weekend and was unable to recreate the noise. A friend of mine who is a senior mechanic and lifelong gear head has me snooping around my suspension.

I did not use synthetic oil. I do agree with Jim too that the sludge could have been the clutch material that has now been flushed out. It is a posi carrier and I believe it is the original carrier unit, if not the original from the birth of the car an OEM carrier. 

I took her out this weekend for the first actual drive. City driving and some 55-60 mph driving with no issues. I'm going to keep investigating as this is somewhat unsettling.

I have some other work to do with her off the ground so I will take that opportunity to inspect the brakes just for peace of mind.


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## PontiacJim (Dec 29, 2012)

Hate to say it, but sometimes the best thing is to ignore it and wait until it gets real bad so you can identify the problem. But, sometimes the problem clears up and is never a problem again. Go figure.


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