I just went through the same excercise on my W126. Yes, 0.01 to 0.02 millimeters are not really preceptible.

Now here is a question: How did all of your dial-gaugers keep the rotor from turning while measuring the play? I had a hard time keeping the disk from turning as I tried to wiggle out that .01 - .02 millimeter reading. We aremeasuring play here, not runout at the shaft's end.

Thanks.

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

I hope you mean hub and not rotor, or this would introduce another unnecessary variable. The rotor should be off for this procedure.

I don't see how rotation could be a problem. Once it's set up as previously described, you simply grab the hub and push-pull to get a reading. Magnetic base is on the hub.... dial indicator is on the spindle end, measuring in the axial direction. Are you sure you're wiggling correctly? Wiggling should be in the axial direction, not radial.

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95 E320 Cabriolet, 159K

I have always overtightened to get the "crush" eliminated and then loosen until I just get the rotation resistance I want. Hard to describe other than 43years of doing it to arrive at the feel.

Tim

(I hate to re-open this, but I can answer with what I did)

If you can accept my originally posted method for mounting the dial indicator base, look at the picture and you'll see a small spring clamp, one leg shoved slightly into one of the cooling spaces, holding the brake disk/rotor to the backing plate.

Even if you don't accept my mounting method (on the disk/rotor instead of the hub itself), the clamp should still work since the pressure to overcome the tiny spring would be negligible and, IMO, wouldn't affect the reading.

Kestas posted "I hope you mean hub and not rotor, or this would introduce another unnecessary variable. The rotor should be off for this procedure."

Sorry, I don't follow. The disk/rotor part of the assembly can be detached from the hub, but only when off the spindle. Did you mean "caliper" should be off?

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1986 560SL
2002 Toyota Camry
1993 Lexus

Thanks for the replies.

Right about the rotor and the hug being one unit at this point, since we are re-installing the disk assembly.

I measured using the magnetic base on the hub (so I'm ok on that), and yes, what I was looking for was play when PUSHING and PULLING the disk towards and away from me, i.e. direction of the spindle.

I just found it hard to NOT to slightly rotate the disk at the same time. If I had gotten ahold of the approx. $300 Mercedes Benz holder for the gauge as photographed in the manual (and I don't think any one of us amateurs has one of those), I am sure it would have taken care of the problem.

donbryce - I will look at your setup more carefully and see how you did it. Seems you may have addressed the problem well.

What really gets me is how sensitive that adjustment nut is in terms of creating the right tension on the bearing. I mean we are talking very fine turnign movements, and even tightening down the hex lock screw can alter the reading!

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Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

I found it very difficult to get the magnet to hold to the hub part while doing the push/pull, due to the rough casting surface, hence the change to the smooth disk surface instead. And as you found out, this is a tedious process, requiring several tighten/loosten/check iterations.
In the end, I'm sure I got it much closer than I would have with the no-dial-indicator technique, but maybe not as certain as the experienced ones among us can get it with the 'feel' approach.
Anyway, it's very doubtful that your best dial-measured effort would be that far off as to damage anything other than the bearing/race itself, which is a relatively cheap consumable, and the damage, if any, would likely not show up for many many miles either...

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1986 560SL
2002 Toyota Camry
1993 Lexus