What would be considered an acceptible amount of rear axle/differential play?

My test with the transmission in Park, parking brake off, is to put one rear wheel in the air and try rotating it. Before I get any movement in the differential drive flange, I get about 1/2 to 3/4" of rotation in the rear wheel, measured at the outside circumference of the tire tread. At the point where the differential drive flange does start rotating, there seems to be very little further play, so I believe that everything from the U-joints forward is good.

I believe this play in the rear axle is responsible for the clunking I'm getting when I begin gently accelerating from a stop, especially when reversing directions. I'm not sure it's worth replacing the rear axle (I'd go the wrecker route if I did this); is there any serious problems with just driving it the way it is?

There will always be looseness at the rear wheel. A better check of driveline looseness would be to try rotating the driveshaft with the rear wheels locked (in gear or with the parking brake on. I'd be looking for looseness in the u-joints or in the flexible coupling at the front of the driveshaft.

The u-joints seem fine, and I believe the flexible coupling is for manual transmissions only (mine's automatic). If there is looseness at the rear wheel, wouldn't there always be a bit of a clunk when going from reverse to forward, as the drive "slams" against either end of the play?

If rear axle play is not the source of my clunking when going from reverse to forward, and everything forward of the rear axle/differential pinion gear flange seems tight, is there another likely explanation?

Several things could cause a clunk. My old 245 gave a clunk when reversing because the rear bushings were shot - torque rod and trailing arm. After they were replaced all was quiet, but when clunking started again it turned out the Panhard rod bushings were gone.

If your engine mounts and/or trans mount are worn, a driveline misalignment could occur which might give a clunk when you change direction, as well. Time to get under the ol' buggy and have a look around.

If another posting in this thread is accurate:
"locking the back wheels and then turning the input flange. About 1/8 turn is normal, 1/4 is getting worn."
- I would have to say my rear axle is about the same, and therefore fine.

This would then point to what you've mentioned - torque rod, trailing arm, panhard rod, engine and transmission mounts. I can see how rotational torque would stress engine/transmission mounts and cause them to clunk, but I wouldn't have thought that rotational pressures on the drive train & wheels would stress bushing contact points. I would have thought that vehicle motion would be necessary to stress these.

I certainly have no problem "getting under the ol' buggy", but I'm not sure I have the techniques for testing the various parts and assessing which ones need replacing. Any tips?

Thanks for your help.

If you lift the back of the car up and support it by the aft jack points, the diff and axle and trailing arms should not be under any load other than their own weight. If you can rotate the axle more than a few degrees (about the axle centerline), than it's a safe bet that trailing arm bushings are shot. Beyond that, your eayes will tell you most of what you need to know. If you have 100K on the car and the torque rods haven't been rebushed, then do them as well since they're easy, especially if you replace stock bushings with urethane. Also check the Panhard rod for bushing wear.

Thanks a lot for your input, Smitty.

Excuse my inexperience, but can you clarify what you mean by "rotate the axle more than a few degrees (about the axle centerline)". Would the test be to grab one end of the axle in each hand and try to rock the entire shaft back and forth in a front to back direction (sort of a teeter-totter around the pinion flange connection to the driveshaft)?

I will likely do all the rear bushings. Are you saying that installing urethane bushings is easier than standard bushings? (My understanding is that urethane bushings are stiffer and give a slightly harsher ride).

What tools are needed for the torque rod and panhard rod bushings? (I intend to use the homebrew tool for the trailing arm bushings).

OK, sorry - guilty as charged. Speaking engineer-speak.

If you had the car on a lift, supported by something other than the tires, imagine fopr a moment that you are under the rear of the car, facing forward. The differential is right in front of you, so the rear axle goes left and right out of the differential casing, outboard to the wheels. Now imagine that you reach forward and grab the axle (actually the axle housing... the axle per se runs inside the housing) with one hand, and attempt to rotate the axle in the same sense as the wheel rotates. Think of an imaginary line which runs from the center of the left rear wheel thru the differential to the center of the right rear wheel... this is the "axle centerline" to which I referred. Now as you attempt to rotate the axle as I described, and you see the differential move more than a small fraction of an inch, than it's safe to say that rear bushings need replacement. It really is a lot easier to understand what I mean when the car is on a lift, than to leave it to your imagination!

Hope this helps. Let me know if it doesn't and I'll come up with a clearer explanation.

Your description to "rotate the axle (housing) in the same sense as the wheel rotates" is very clear. Thanks.

Can you comment on the urethane bushings and tools questions?:

I will likely do all the rear bushings. Are you saying that installing urethane bushings is easier than standard bushings? (My understanding is that urethane bushings are stiffer and give a slightly harsher ride).

What tools are needed for the torque rod and panhard rod bushings? (I intend to use the homebrew tool for the trailing arm bushings).

You can check if the play really is inside the axle by locking the back wheels and then turning the input flange. About 1/8 turn is normal, 1/4 is getting worn.
Wear in the axle could be the crownwheel and pinion, the actual differential, the half-shaft splines, bearings, hubs, or end float somewhere. Worth checking that the flange nut is tight. If it has come loose then its splines will have worn.
The axles themselves seem to last for ever, if kept full of oil of course. The play is much more likely to be somewhere else in the drive chain.

I just had a guy replace the rear pads on my car. I did the fronts myself, and normally would have done this easy job too, but time and workload neccesatated bringing the car to a shop. The rotors were in good shape, one piston on each caliper needs replacing (I knew this beforehand - uneven pad wear).
Since the brake job I now have the same "clunk" sound under the same circumstances you describe. My thought is the "slider" bushing-thingy (technical term) :) is worn or missing, since this was exactly the cause of the same noise on my 89 Saab.

I don't think the 240 calipers slide. I would check to make sure the caliper itself is bolted up tight.