Rear end clunk?

Alright, so we are working on a car which drove perfectly fine last week. Now we go to test drive and it develops a knock. When the brakes are applied it stops. I check the brakes and all look good. This car just passed standard inspection. When turning the rear wheels you can feel a knock. The rear is from a 67 Pontiac with factory limited slip. We pulled the rear cover and the ring and pinion look fine. I can see what looks like good gears inside the housing on the drivers side but cannot see inside the pass side. What could be the problem in here or are we just barking up the wrong tree?

 

This went from dead silent to a little clunk so we should drive bad to the shop to HOLY SHIT that is loud we need to get back NOW! all within half a mile.

 

Stating the obvious, but it has the posi-additive in the diff fluid, right?

 

I have no idea. I just started working on the car and it may have never had any. The fluid coming out looked like standard gear oil. I never could see a difference in the special fluid though.

 

Maybe you may want to put in some fresh fluid with the proper limited-slip additive. I know that you need the additive on a posi rear. Hopefully your clunk problem is as simple as that.


Something like this:

http://www.summitracing.com/parts/nal-88900330?seid=srese1&gclid=CLTmiv7z77wCFVRo7AodrU4AVw

 

Hopefully. I really don't want to tell this guy "Yeah we just did a fine point quality resto on your engine now we have to do the rear which wasn't planned."

 

Ouch, damn hopefully your customer is an understanding guy. Good luck, bro!

 

If the noise happens when you're driving straight you may a bearing or gear problem.

If you're getting the clunking when you're turning the posi clutch plates may be glazed up with varnish. They can slip and grab when turning and make clunking and popping noises. It might help to add a bottle of GM detergent additive to the gear lube. It'll help deglaze the clutch plates and quiet things down but it may take a little time to work.

Ideally you'll want to drive the car enough to get the axle lube hot. Then find an empty parking lot and start cutting some slow figure eights to work the additive into the clutches.

GM part number is 88900330.

 

X2 What Clay Mart said !!

 

Could it be the GM trans yoke clunk.

 

Don't forget to inspect the u-joints. When in doubt, have 'em out.

 

Don't know about 67 Pontiac rear ends but 57 Pontiac rears have 2 short, 5/16 bolts with 1/2 inch heads that hold the rear drums on the axle flange. They stick out about 1/8th inch and the stock wheels have recesses for them to fit in. When using any aftermarket wheels, those bolts must come out because the wheel won't fit flush against the brake drum. My kid brother didn't know all this and bolted up a pair of American 5 spokes to his 57...made a lotta noise and ended up breaking off wheel studs before he caught it.
I dunno, maybe 67s have these same Brake drum attaching bolts?

 

I know that dumb things can get the best of us but..... Loose wheel nuts on one side? I v'e been caught and shoulda known better...

 

67 does not have the drum locking screws and I checked all the torque on the wheels as soon as we got back to the shop, that was my first thought. The clunking was constant not just around turns.

 

Double check U Joints. I had the same progression of noise right before I lost a driveshaft. That was a really loud CLUNK!

 

The wheel are Cragar SS and show no signs of damage on the hub side. The clunking seemed to go away or at least quiet a good deal with the brakes applied. Yet all the brakes look good making me think it has something to do with torque e.i. a bearing. the u joint look and feel fine, there is a little play in the output shaft of the trans but it seems marginal.

 

You say that you just worked on the engine not anything else.
Could it be something as simple as the exhaust hitting something.

 

Posi additive, then go to a parking lot and do tight figure eights. If that doesnt smooth it out, you have other issues.

 

Filled the rear with 85 140 limited slip fluid and pulled some tight figure 8s in my parking lot. Clunk is gone. Drove the car around the curvy hills and then 70+ on the highway with no problems. She's got some kick but definitely not a top speed car.

 

Love a happy ending!!!

 

I had the same problem in my 77 pontiac. Turned out if forgot to add the limited slip additive. It would do just fine when it was cold but as soon as it warmed up and i went to make a tight turn it would clunk and pop and sound like the rear end just fell out. You can get oil that already has it in there or add it to non-slip fluid ($5-$10). if that doesnt work check your u-joint or even your pinion nut