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8.75 axle diagnosis - sages?
https://www.slantsix.org/forum/viewtopic.php?t=57932		 Page 1 of 2
Author:  Dart270 [ Mon Jun 22, 2015 10:12 am ]
Post subject:  8.75 axle diagnosis - sages?
Hi folks,

Well after another 200 miles and 25 burnouts and 1/8th mile passes with the 68 Dart this weekend, it developed a clunking noise in the rear. I am not a diff expert, and am wondering if folks want to hazard estimates of the problem before I tear into it. It is a 3.23 clutch Sure Grip in a 741 case.

My guess is spider gears or cross shaft. I have never had a rear axle failure except clutch packs and wheel bearings, so this has not come up for me.

Symptoms and tests/observations:

1) I first noticed the noise as a periodic whine that becomes a clunk/whine at <40 mph. I pulled over at the first exit, jacked it up and ran the car in gear and it is in the rear axle assembly for sure. The period of the noise goes with the wheel/axle speed, not the pinion d-shaft speed. I could not hear, or feel by wiggling the wheel, that it was a wheel bearing. The U-joints and pinion bearing are tight (no PB seal leak).
2) I went ahead and drove on it at 50-60 MPH another 70 miles to my aunt and uncle's house in Hendersonville, NC and the noise got better. I noticed it was worse on deceleration while in gear, and loudest between 40-50 MPH. If I did not do any deceleration loading (keep foot on gas or put trans in neutral), then the noise got better and almost went away. So, this morning I drove back to Blacksburg at 60 MPH (242 miles) and the noise stayed the same or got a bit better. I was careful not to apply any deceleration loading (big hills up and down).
3) Cornering does not seem to affect this (clutch-type Sure Grip), so I doubt it is clutch packs. Additional load on accel (up to about 130-150 HP) seemed to have no effect, so I suspect is not a pinion gear tooth.

Thanks to Brian Mimken for loaning me a 3.55 open diff to swap in, had I needed it on the way. We had a nice dinner last night with Brian and Anna Mimken, and my aunt and uncle Mark and Jane Madsen. Luckily I did not need to do a roadside swap and the car is safely in the garage. 25 MPG at 60 MPH average speed.

Let the games begin!

Lou
Author:  Joshie225 [ Mon Jun 22, 2015 10:39 am ]
Post subject: 
It sounds as though the pinion nut has backed off. This can be dangerous! I got a ride in a Barracuda about 10 years ago and we had the tires lock up under engine braking.
Author:  Dart270 [ Mon Jun 22, 2015 11:01 am ]
Post subject: 
Wouldn't that make the pinion shaft wiggly or cause a pinion seal leak? I tested that on my first stop. Note that this makes noise periodic with the ring gear rotation.

I will certainly check that again tonight. That could be an easy fix.

Lou
Author:  Rick Covalt [ Mon Jun 22, 2015 11:40 am ]
Post subject: 
Quote:
The period of the noise goes with the wheel/axle speed, not the pinion d-shaft speed
Not sure I follow this. :?
Author:  sandy in BC [ Mon Jun 22, 2015 8:11 pm ]
Post subject: 
the physical symtoms sound like pinion bearing.......
Author:  sixinthehead [ Mon Jun 22, 2015 9:04 pm ]
Post subject: 
The differential 'guts' (spiders, clutches) are just along for the ride unless you are going around a corner and therefore not likely to make noise running in the air, so I would say that part is ok.

Since your noise occurs at axle speed instead of driveshaft speed, and is drive/coast sensitive, my guess is chipped tooth on the ring gear.

Carrier bearings are another possibility, but they tend to make a roar/rumble at a higher frequency than an every-revolution clunk/thump.
Author:  Rick Covalt [ Tue Jun 23, 2015 3:22 am ]
Post subject: 
Quote:
Since your noise occurs at axle speed instead of driveshaft speed,
Still trying to understand this? The drive shaft is connected through the ring and pinion to the axles. If one speeds up they both speed up, correct? Sorry I'm so dense! :lol:
Author:  sixinthehead [ Tue Jun 23, 2015 4:51 am ]
Post subject: 
It's the ring & pinion ratio that makes the difference.

In this example, the driveshaft and pinion are rotating 3.23 times faster than the axles. Your ear can definitely tell the difference!
Author:  Dart270 [ Tue Jun 23, 2015 5:30 am ]
Post subject: 
The clunk or variation seems to occur once per axle/wheel rotation, so it must be in the axle. Ring gear chip was my first thought, Joe, so we'll see on that when I pull the pumpkin. The pinion bearing would create a periodic noise once per driveshaft rotation.

I don't see how a pinion bearing could cause this kind of noise, but I'm open minded. I did remove the driveshaft last night and the pinion nut is at least not loose with a socket on it applying 40 ft-lbs or so. There is a little seepage in the pinion seal, but no side play I can detect and not enough to cause a drip on the floor.

There is some increased leakage in the right axle bearing, so it could be that after all? Hard to see why drive/coast would matter, though.

More disassembly to come... Fun keeping us all in suspense, eh?? I love puzzles.

Lou
Author:  sandy in BC [ Tue Jun 23, 2015 6:33 am ]
Post subject: 
If the pinion bearing has enough wear it will throw things out of whack on decel. On accel the pinion "climbs " itself into alignment.

Any ring gear runout will be amplified on decel.
Author:  Dart270 [ Tue Jun 23, 2015 8:47 am ]
Post subject: 
Great. Makes sense. I might have time to pull it tonight, but might have to wait for Fri-Sun...

Lou
Author:  Dart270 [ Sun Jun 28, 2015 6:44 am ]
Post subject:  ALL SG case bolts backed out!
Wow, I would NEVER have guessed this one.

Apparently, whoever assembled this did not torque the bolts holding the Sure Grip unit together. Every single one has backed out and a few heads of these have been slowly grinding into the main case, after backing out by more than 1/4". This also explains why my car was spinning one tire sometimes on the track. The halves of the SG are now about 1/16" separated. This a used rear I bought a while back - WOW.

I guess I will just drop the diff out of the case, torque the bolts, clean it up, and put it back together (and set backlash, of course). It appears the gears are still perfect and everything else is OK. I may consider upgrading to a truetrac diff. In any case, I'll drop my 3.55 SG unit back in for the rest of this season.

During loading/drive this would push the carrier bolt heads away from the case, and on decal this would push bolts into the case - the noises make sense.

Thanks for everyone's ideas!

Lou
Author:  sandy in BC [ Sun Jun 28, 2015 9:23 am ]
Post subject: 
A perfect example of message board guesswork.

Even if someone had suggested that......It would have seemed unlikely.

pig in a poke....
Author:  Joshie225 [ Sun Jun 28, 2015 2:24 pm ]
Post subject: 
That's a new one for me too! :shock:
Author:  sixinthehead [ Sun Jun 28, 2015 3:01 pm ]
Post subject: 
One more for the realm of "possible".

Glad it was a pretty easy (and cheap!) fix!
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