Spike66

For years I've noticed a slight "clunk" sound coming from the rear end whenever I'd accelerate from a stop while turning. I'd feel a little lateral movement at the rear wheels as well. Almost like the rear of the car was moving left to right or right to left depending on the direction of the turn. I had my '66 coupe out for the first time the other day since rebuilding the engine and trans. While driving on a hilly and twisty 2 lane road, the clunk and chunk seemed more prominent than ever before. I doubt the rebuild and this condition are related since it's been there for years. Years ago I added Lucas differential lube and it seemed to help a bit. Before I jump whole hog into this, I'd be interested in anyone's thought here.
During the rebuild, I had the driveshaft balanced and 2 new Spicer U-joints installed. I'm thinking the issue is behind the driveshaft at the 3.08 differential or half-shafts.

Shultzie

Did you check the U-joints on the axles?
I had a '75 do that and the U-joints on the axles was the problem.
Maybe that's why I went with a solid axle car now. LOL

Spike66

No I haven't. The car goes up on a lift today. The half-shaft U-joints are original. What's the best way to check them?

plaidside

I would check:
The front differential cushion.
The bolts that hold the 1/2 shaft to the wheels, had many back off.
The strut bar bushings and eccentrics, make sure these are tight.
Trailing arm bushings
Joe

rtruman

Its hard to see check stubber rubber bushing on front of rear. Use mirror

watson

My 65 did that same clunk when accelerating and turning when I first bought the car. I was told to add some of the GM posi additive to the rear end. When I did that, the problem went away and the car was fine for ten years. It's worth a try for a simple fix. Be sure to get the real GM additive and not a knock off.

Doc

rtruman

Spike66

Hey Doc...I'll try the additive first. The summer slacker in me wants the easy way out. I'll get back on this.

watson

I found the additive at my local Chevy dealer, but the Corvette parts dealers sell it as well. I don't understand the chemistry involved, but the real GM stuff seems to be the key. Good luck.

Doc

leif.anderson93

And, after you add the additive...take your car to a big, empty parking lot and do figure 8's slowly. Do several figure 8's turning left and right then several more turning right and left. This works the additive into the clutches

bcwaller

I had a mysterious clunk as well. The "donuts" that hold the rear crossmember had aged and allowed for movement. Replaced the donuts and mounting hardware and it went away.

That was after having the differential, yokes, driveshafts, trailing arms, and more checked out.

Spike66

It's a noise that could have several different sources I guess. All the rubber bushings are original and looked tired. The half-shaft U-joints are original but seem OK.Don't know but I'll put the additive in first and see what happens. If it doesn't do the job, then it tail-chasing time.

bcwaller

Start with the cheap and easy things. You might be able to check the cross member if you can find ways to support the car and then load and unload the suspension with a jack to see if you can detect motion.

I don't recall how they figured it out, unless it was the shop (Dick Guldstrand's) doing trial and error. They did find a bent yoke, and thought that was it. But I could replicate the clunk in the alley behind their shop by hitting the throttle and lifting as I drove by them listening and postulating what it might be.

The upside was that after they put the gas tank back, it was still a bit loose and I got to help Dick Guldstrand tighten the tank straps.

irwiny

I had same issue and hand pumped out the oil fluid and added two bottles of GM additive and filled with Redline. This fixed the problem. Previous owner had the rear end rebuilt and didn't change the posi clutch plates. The shop that did the rebuilt said it was normal to slip and I almost had to rebuild the rear end again.

Irwin

emccomas

Get as much of the old gear oil out as possible (I used a suction pump), then put in two bottles of GM posi additive (get the GM stuff) and then fill to capacity with new 90 weight gear oil.

Then find a large open area and do lots of figure 8's. No need to go fast, just doing the figure 8's will circulate the new oil throughout the posi clutches.

Trkdrvr

worn yokes (inside ends) will cause clunks and slight swerving as you turn side-to-side. happened on my77...........chuck

Spike66

Today I sucked out the old gear oil with that manual oil pump from Harbor Freight. Worked slick. I bought a longer piece of vinyl hose to make it easier as I'm working on my back. Replaced it with Brad Penn Classic 80W GL-4 gear oil and a 4oz. bottle of the GM diff additive using a cheapo hand pump that screws on the quart top. I can't say it was fun but it's done. I put several wraps of blue Teflon tape around the plug, slipped the red diff tag on and carefully threaded it into the case. To say it is a rooster-fish to tighten this plug would be an understatement. I can't seat the plug as I can't get any more cranking leverage on it. No gear oil is coming out, the red tag is loose and I can see maybe 3 sets of threads on the plug. It appears tight as can't get anymore ticks out of it. Is the Teflon the issue? Am I ok?

emccomas

Not sure why the teflon was needed, but if it is not leaking, and you filled it up to the point where it just started to run back out when sitting level, you are good. Take a short drive, then check for leaks.

Spike66

I finally found the time to take the car out for a road test. It's like magic. The clunking and chunking are gone. The new gear oil and GM additive was the cure. As I said earlier, I've been dealing with this condition for years but lately it got worse; maybe from sitting idle for a year and a half. But it's now quiet and smooth as silk. Thanks to everybody who chimed in. I appreciate it.

rtruman

Every time I get a new midyear I do this just to be sure .I had to lay back there and suck all that outa there but it works and quiets the rear for sure.