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Old 19-09-2008, 12:22 AM   #1
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W124 Front Wheel Bearing

While fitting new brakes at the weekend, I found a problem with one of the front wheel bearings.

I had jacked up the car, and felt the wheels prior to the MOT, and found them all OK. The car passed its MOT that morning just before I began to replace the brakes.

Even with the wheel off, and turning the disc, I felt nothing unusual, but, when I took the disc off, and began to wire-brush and clean the face of the mounting flange, I found the bearing felt really quite rough to turn. D'oh!, I found the problem just approching midday on Saturday, as every decent local bearing stockist was closing!

Attached (hopefully!) is a piccie of what I found in the smaller, outer bearing; the outer race is badly pitted, and the roller just to the right edge has failed quite badly. The main cause of the roughness was actually flakes off the failed roller wedging themselves under other rollers. The odd thing was, there was no noise in the car, and the bearing felt OK when felt by rocking and turning the wheel.

I'm really glad that I caught this before the bearing locked, and spun the inner race on the stub axle. The only indication that there was anything wrong was the roughness which was only felt at hub radius - you couldn't feel it with the disc in place.

Upon re-assembly, with new bearings in place, I set the end-float to MB's spec of 0.01mm using a dial gauge, and I can confirm that there's no way anyone can meet the spec by feel alone; without using the gauge. You can't feel 0.01mm of free play when there's grease in the bearing - no way! - but, it shows up on the dial gauge plainly enough.

The bearings themselves are quite easy to replace - there's a decent lip to get in with a drift to knock the old outer races out of the hub, and, when pressing the new ones in, you can use the old outer races as spacers, because the races don't sit deeply recessed in their bores. The only thing to be at all wary about is making sure that you don't damage the ABS teeth - oh, and, as ever, maintaining scrupulous cleanliness when dealing with the new bearings!

Attachment 11995

Last edited by Number_Cruncher; 28-05-2011 at 07:59 PM.
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Old 19-09-2008, 09:38 AM   #2
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Good post-should maybe go in the how to section?? Altho some more pics would have helped. Maybe difficult to take pics when your hands are covered in grease?
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Old 19-09-2008, 10:30 AM   #3
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Number Cruncher

Good post.

How long would it take to replace a front 124 bearing assuming starting from when the car is on the ground on all four wheels.

Job will be done on the ground just using Mercedes jack and the use of an axle stand.

Thanks
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Old 19-09-2008, 10:50 AM   #4
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Mine went almost the same way. Only a couple of days after passing the MOT the rumbling started suddenly. None of that faint whining, wondering whether a bearing might be on its way out. This started so quickly I thought a caliper had fallen off.

Replacement is straight forward enough and can be done on the ground and the only real problem is finding the right tools for knocking out the old races. I would guess at 2 hours start to finish, with a coffee break of course.
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Old 19-09-2008, 10:52 AM   #5
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Good - not long at all then.

Thanks for that.
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Old 19-09-2008, 12:52 PM   #6
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Yes, I think 2 hours is a reasonable estimate to do it on a driveway, without rushing - in a workshop, much faster, I would hope to have both sides done in that time.

I found that the brake disc came off the hub without putting up too much of a fight, so, it's reasonable to do the job without having to budget for new discs - I was replacing mine anyway.

You do need a good drift, say 6 or 8 inches long, 8 to 10 mm in diameter at the tip, with a good unbroken corner, something a bit like this;

http://buy1.snapon.com/catalog/item....uk&dir=catalog

You don't need to spend a fortune on the dial test indicator and stand - as long as the dial gauge can read down to 0.01mm, it should be OK.

I found that the washer behind the securing split nut had been damaged/worn (item 38 on the piccie on the link below), but, this was fairly superficial, and a few minutes flatting it on some fine abrasive paper spread over a flat sheet of glass cleaned it up. My concern was to make sure I had a good flat surface, so the end float measurement would be consistent, and not depend upon whether I was hitting a burr, or not.

http://www.detali.ru/cat/oem_mb2.asp...SGR=030&SGN=01
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Old 05-06-2010, 02:42 PM   #7
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I have just come in from replacing my front wheel bearing and thought i'd make a comment on adjusting bearings without a dial guage
The pitch of the axle thread is 1mm.
Assume we tighten the nut firmly finger tight. There may then be some preload on the bearing.
We do not know how much, there are a lot of parts and they are now all under a bit of tension (stretch) or compression (squeeze).
One full turn of the axle nut would give something like 1 mm of free play.
one half turn would be .5mm (half a millimetre)
To make an angular adjustment on the nut you will need to assume no free play (endfloat) or preload at all.
Now back off the nut 3.6 degrees (up to 14.4 degrees to stay within MB tolerance) to release the end float to 0.01mm (in my world that's 0.0004 or close to half a thou!)

I imaging what MB expect of us, is no free play and no preload, which some of us probably are happy to judge by feel alone.
A garage foreman I worked for 30 years ago used to say
"Tight's tight and too tight's f***ed"

I hope I dont appear to be disagreeing with anyone just sharing my thoughts aloud.
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Old 05-06-2010, 06:56 PM   #8
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Wink W124 wheel bearings

One of my wheel bearings completely failed and was replaced on my old 260E and that was fine a few months earlier. Chap at the Garage said they were prone to sudden failure at very high mileages.

Mind you it had done 183,000 miles that was on n/s/f , the o/s/f had to be replaced at 187,000 miles. Rears were still original when I sold it on.

At 190 k having replaced the steering box 12 months earlier I decided to call it day in our 7 year relationship.

One of the two best Mercs I have ever owned. Not fast, not very economical but stone age technology with overdesign everywhere - just look at those LT leads compared to a Ford or Vauxhall !
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Old 05-06-2010, 07:38 PM   #9
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Quote:
Originally Posted by bigshineybike View Post
you will need to assume no free play (endfloat) or preload at all.
That's the problem - once you've got grease and the friction from the oil seal, I think it's simply impossible to arrive at this happy position by feel alone.

Quote:
Originally Posted by bigshineybike View Post
I imaging what MB expect of us, is no free play and no preload,
I think you're right, and I think your point is very perceptive. I've since found some corroborating information in a book quoting German best practice for bearing assembly and setting.

Quote:
Originally Posted by bigshineybike View Post
which some of us probably are happy to judge by feel alone.
As a point of interest, did you adjust the bearings by feel or with a gauge?

I maintain that until you've tried to hit 0.01mm with a gauge, you can't appreciate how difficult and tight the specification actually is.

Quote:
Originally Posted by bigshineybike View Post
I hope I dont appear to be disagreeing with anyone just sharing my thoughts aloud.
This is one of those subjects that invariably causes differences of opinion. Most wheel bearings aren't specified with such a tight tolerance, and so, adjusting by feel is normal practice in most workshops, but incorrect practice with MB front wheel bearings.
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Old 05-06-2010, 07:47 PM   #10
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It's not that hard, nip it (the nut) up gently, rotate the hub/disk a few turns, back it off, then nip it up by hand (no spanner) again, et voila, no preload and no float.

No more difficult than a tappet.
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Old 05-06-2010, 08:54 PM   #11
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>>It's not that hard

I would be *amazed* if you could reliably hit the MB specification without a gauge.

Usually, people who say it isn't difficult are those who have never tried using a gauge to get down to 0.01mm, and so, they really don't know if it's difficult or not.

Tappet clearances are about 30 times larger than the values involved here - yes!, tappets are easy in comparison!
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Old 05-06-2010, 09:08 PM   #12
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Quote:
Originally Posted by Number_Cruncher View Post
>>It's not that hard

I would be *amazed* if you could reliably hit the MB specification without a gauge.

Usually, people who say it isn't difficult are those who have never tried using a gauge to get down to 0.01mm, and so, they really don't know if it's difficult or not.
Helps if you can read specifications.

MB absolutely will not spec "float 0.01 mm" there is always a tolerance.

In any event, "feeling" a thou is no big deal, just ask anyone trained on a manual lathe / mill / scraper / etc. Hell, every time you use a micrometer you are "feeling" something of the order of a fiftieth of a thou.

MB wheel bearings aren't unique on the planet either, "No preload and no float" describes 99.99% of wheel bearings on the planet.

They'll also slacken off a bit when the whole thing gets up to operating temperature, if you ever have to do these things in places with greater temperature ranges than we have you put in extra float to allow for the low temperatures and thermal contraction.
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Old 05-06-2010, 09:11 PM   #13
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Duck !!!!
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Old 05-06-2010, 09:19 PM   #14
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Here's the spec;

http://www.ps2cho.net/downloads/MB%2...is/33-0010.pdf

Yes, there's a tolerance, but, it really doesn't help much.

I guarantee that you can't feel it by hand.

By your reply, it's clear that you've never actually tried it.

>>MB wheel bearings aren't unique on the planet either

Maybe not, but, there's a clear spec, and a clear method to acheive that. Anything else is guesswork.
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Old 05-06-2010, 09:21 PM   #15
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Without taking either side on this, I do wonder just how many MBs are running out of 'spec'. I'd hazard a guess that the majority on the roads today will be.

Reason I suggest this is that I've had several MBs over the years, and the front wheel bearings have been slightly loose on a few of them (in that, there was a noticable level of free-play).

Despite full MBSH and a modest miles (60k odd at the time of servicing) my CLK55 had slightly loose front wheel bearings on both sides. As did previous W202s (same setup).

Surely if MB don't check and adjust this at a service (?), after a few thousand miles they'll all be out of spec.

Didn't have a dial gauge to hand (although I would consider using one if I did ), but if they're not currently to the nearest 0.00th of a mm they're a damn sight closer than they were, and they're not running tight either.

Has to be better than doing nothing, as MB seemed to have done.
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