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When you say "comes loose" do you mean the U-shaped nut has moved - or are you saying it has not, yet there is play in the bearing that wasn't there when you last tightened it?

What is your procedure for tightening the bearing? (do you use a gauge to measure pre-load?)

 

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McAlpine identified the problem. Has the OP implemented it yet?

 

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I see that the washer is in fact there, Thanks for pointing that out. What I noticed while doing that is that the wall thickness of the bearing is different between the two photos. Could there be a POS bearing on one side?

If it were me I would replace the bearings and races and see what happens. Bearings are probably the cheapest finely engineered item you can buy today.

 

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Is the threads on bolt holding retaining nut loose/stripped? That is the only way the thing could come loose.

OP - are you saying that entire locking nut unwinds itself? if so the clamp is bad.
if something other than please clarify.

alt theory is the bearings are incorrect.

OP are you adjusting bearing with a dial indicator or just guessing? .01mm end play is very small....

Good to know I'm not crazy. I asked all the same questions in post 5.

 

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If the OP wants to solve his problem he only needs this:

Magnetic Base with Fine Adjustment and SAE Dial Test Indicator with 0.0005: Resolution (half a thousandth), 1" Travel, Accuracy 0.001" per 1" Mag Base MBDI: Amazon.com: Industrial & Scientific

Magnetic Base with Fine Adjustment and SAE Dial Test Indicator with 0.0005: Resolution (half a thousandth), 1" Travel, Accuracy 0.001" per 1" Mag Base MBDI: Amazon.com: Industrial & Scientific

www.amazon.com

and this:

Mercedes Wheel Bearing Service Kit - Timken 1409810305

Mercedes Wheel Bearing Service Kit - Timken 1409810305

www.fcpeuro.com

You can get cheaper dial indicators than the one shown but bottom line if the OP wants to resolve this for good all he needs to spend is about $100.

Buying only bearings for the one, problematic, side would be even less but would be the minimum replacement parts. None of us have seen the inner bearing on this car so that is why it would be dumb to think of this all in terms of just the bearing we can see in the photos, right?

 

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I have a dial indicator that has 0.001 inch increments. I use it to do the preload adjustment. The spec is something like 0.0005 IIRC and that is essentially half way between two 0.001 increments. So a dial indicator that displays to four decimals isn't really needed. But I don't want to guess on the preload since these cars can run 100s of thousands of miles on bearings that are set to factory spec. My '88 ran to 330k on the original bearings until it started rusting in places I didn't want to spend time on.

 

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Just put new bearings in the old hub.

Agreed. Plus a new axle seal.