Clunk From Rear When Shifting Into Gear

[ed1948]

86 SS, posi rear axle, 100 k miles, fairly new to me car.
With the idle speed at 600 rpm when shfting into D or R there is a jolt and clunk from the rear. The driveshaft joints seem tight although they do not have the grease nipples(possibly original).
The rear axle sideways play is close to 1/16th "

Should this require attention?

 

[MrSony]

Yes, it should. No car should "clunk" when doing anything. Check all mounts pertaining to the transmission and rear end. Check the shock mounts and spring perches. Check all the bushings, if nothing looks out of place or suspicious, pop the diff cover and check it out. Drag a magnet through the fluid that pours out in a container (duh) and see if it picks up anything. Check for wear on the ring and pinion or any excessive looking play on anything. You can have the car on jackstands with the rear end hanging freely then have a buddy start it and put it in gear (before you drain the fluid to avoid spatter and possible damage) and listen/look for the clunking sound. Sounds like a driveshaft thing to me but just being thorough.

 

[pontiacgp]

Have you ever driven a RWD from the 80's before? The axles do slide side to side and that is not a problem

 

[axisg]

I seem to recall they did that when new LOL. Wouldn't hurt to toss some new u-joints in it esp if there are no grease nipples. Its a pretty sloppy setup but with appx $100 in parts ( ujoints and trans mount ) you will likely see a noticeable improvement. Its not too tough to pull the cover and swap the fluid in the rear. Its worse in auto cars than stick as well. Even with new parts it will still have some movement and likely clunk with the right conditions. I don't even notice it anymore

 

[ed1948]

I plan to change the diff fluid, gasket and add a suggested additive. I'll do the suggested checks while it's off the ground.
I know the axles have some looseness but I'm not sure of the spec.

((Is a posi diff additive even recommended?)) Time to check the manual.

 

[GP403]

I seem to recall they did that when new LOL. Wouldn't hurt to toss some new u-joints in it....

That was my first thought. The clunk was always a u-joint thing, unless it wasn't.

 

[Bonnewagon]

It could be the U-joints or the rear. Before doing anything I would get the rear up in the air for a look see. Put the trans in neutral, parking brake on, and grab the drive shaft. Feel if anything is loose. Bad U-joints will be obvious, and if so, should be changed. If they are tight then see how much play is in the rear. Grab the drive shaft again. See how much play there is between the companion flange and the ring gear inside. Some play is acceptable but a lot is not. It is easy to get fooled because when you add up the gear backlash and the axle play it can seem pretty sloppy. But if it is loose as a goose, then it will need attention. On the other hand, if the clunk is the only noise and there are no other expensive noises, I would not be in a hurry to open the rear up. Aside from changing the oil, which is a good thing but won't fix anything, the rear end is very complex, expensive, and difficult to repair properly.