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  • Differential Help

    My lovely S54 has passed its MOT but sadly the garage has said the diff is badly corroded and fears it could just give way such is the corrosion on the casing.

    The replacement part is £2k! from BMW and is on back order until Oct.

    Grateful if any of you good people know of any second hand diffs or another way to approach this issue.

    Thanks in advance

  • #2
    Welcome to the forum

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    • #3
      We had a similar post recently

      http://www.z3mcoupe.com/forum/showth...highlight=diff

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      • #4
        Dave has attached the previous post on this. My indy is not concerned by this at all, seemingly it looks much worse than it actually is. Perhaps the MOD station are over pessimistic? I would get a second opinion.

        Do let us know how it goes.

        Ian
        S54 Titan Silver metallic. Black leather, most factory options including sunroof, Becker Cascade and Becker Silverstone changer.

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        • #5
          Thanks

          Huge thanks for the swift replies, the garage are very reputable and he is an excellent ex main dealer who I trust but I'll get my REME lads to have a look to get their views.

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          • #6
            The diff casing is v thick indeed, you say yours is looking very corroded, is it leaking at all? If not then it's prob just surface corrosion (even if it looks very bad).

            The best way to deal with it is to remove the diff off the car, clean it and remove as much of the surface rust as possible. If at this point you have got all the corrosion off then hurrah! Apply lots of rust killer to the case, then Paint the diff case in anti rust primer and then a good few coats of a thick, oil based paint, finish it off with a surface coat of high temp silver. The other option is to have the diff case powder coated once all the corrosion has been removed.

            If the cleaning process doesn't remove all the surface corrosion, send the diff case to be sand/media blasted, then repeat the steps above.


            Your garage will be able to do all that for you if you can leave the car with them. I know of a car (not mine) that had the diff case sand blasted and powder coated, looked fantastic when it was done. Cost a lot less than £2k!


            Hope that helps, cheers,



            Jam

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            • #7
              Rusted diff case is only a visual issue. It will NEVER fail because of surface rust. The rear cover which holds the rear bushing is aluminium so it will NEVER fail because it simply cannot rust :)

              A rusted diff case is living proof that it does not leak oil

              So, it should be all good.

              Here is mine after fixing the oil leak and bearings that gave some slight resonance sound at a steady 55 mph speed : https://v2tre.wordpress.com/2016/03/...d-and-updated/

              Now runs perfect and looks also fairly good.

              br, Juha
              The older I get, the faster I was ...
              BMW HP2 Sport
              ex- BMW Z3 M Coupe - Cosmosschwartz Metallic

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              • #8
                Juhav; did you do the rebuild by yourself? When you changed the pinion bearing did you have to recheck pinion/crownwheel engagement?

                Lol at this garage! Unless they mean the subframe which it hangs off? It's normal for the diffs to go a bit rusty, there's some proper blocks of rust on ebay from e24s and e30s which will be absoliutely fine internally, it's cast iron and will get surface rust easily but it's so thick it will never be a structural issue in our lifetime.
                Last edited by c_w; 02-05-2016, 04:23 PM.

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                • #9
                  Originally posted by c_w View Post
                  Juhav; did you do the rebuild by yourself? When you changed the pinion bearing did you have to recheck pinion/crownwheel engagement?
                  I gave the work to a local pro who has serviced hundreds of BMW diff units (both for road and racing). He commented that BMW many times seem to have used slightly too low pretension for the pinion bearing and therefore these units typically start to give a slight noise. Thus, mine was given a slightly higher pretension per his experience after being fitted with new bearings. I think that this is the only challenging part of the job : to have some idea and experience in your hands when the proper pretension has been achieved.

                  The engament of the teeths of the pinion/crown wheel were checked with paint and adjusted.

                  The friction plates in the lock were also replaced because the original ones had taken some beating. The service took place at 120 000 kms.

                  Now the unit runs more silently and without any noticeable backslash. Good for next 100 000 kms I hope.
                  Last edited by JuhaV; 02-05-2016, 04:40 PM.
                  The older I get, the faster I was ...
                  BMW HP2 Sport
                  ex- BMW Z3 M Coupe - Cosmosschwartz Metallic

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                  • #10
                    Interesting, I wonder if this is the cause of mainly peoples' complaints about noise/clunking?

                    I rebuilt the LSD in my diff a few years (when I say my diff, it was an E30 unit off ebay!) and the plates weren't particularly worn, unless they have been replaced before. The reason I replaced them was because the diff felt open, and with one wheel in the air on the jack you could turn the wheel fairly easily where with the original Z3M diff it takes some force to overcome the plates.

                    I fitted a 3rd clutch and dog plate in place of the thick spacer/washer which is supposed to give 40% lock, but I think I'm going to take it out as I can't decide if actually it makes it understeer on initial power.

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                    • #11
                      I hope MSL197 does not feel that we are hijacking his thread, however, this is still diff related :)

                      In my case the noise from the diff was more like a whining noise when driving constant speed at about 55 mph. If deaccelerating or accelerating the whine went away. This is typical noise coming from the pinion bearing not anymore having good enough pretension. If it becomes worse, then it starts to affect also the teeth engament but then it needs already to be pretty bad and close to have a bearing failure.

                      The noticeable clonk in the driveline (or most of it anyway) seems not to be coming from the diff. According to my experience the gearbox itself has much more free play to produce that clonk than the diff. In my car I am pretty sure that the clonk comes primarily from the gearbox and it is emphasized due to the two-mass flywheel where the springs have already lost some of their properties. This clonk is then transmitted through the driveline to the rear and perhaps further amplified by any small amount of backslash in the diff itself.

                      In my case the clonk was reduced perhaps 90 % simply by changing the clutch rubber hose (with the inbuild restrictor) to a free flowing braided line
                      ( https://v2tre.wordpress.com/2016/03/...d-clutch-line/ ). Now when the clutch fully and more rapidly disengages, the gearbox seems not to create any significant clunk any more. At the same time I renewed that rubber joint behind the gearbox which might also have introduced some attenuation into the driveline.

                      Having now serviced all these parts once, I have stop worrying about these and just drive the car without mercy. If something gives up, I just fix it and if needed, strenghten it where necessary.

                      I would also think that 40 % lock might be a bit too much. If the rear suspension is ok and one uses good enough rear tyres, there should not be a significant problem due to lack of traction out from the corners. Mine at least does not usually have any unintended wheel spin unless dropping down into second gear in a very slow and very tight corner and instaneously hammering it at or just before apex when the front wheels are still turned in.

                      br, Juha
                      The older I get, the faster I was ...
                      BMW HP2 Sport
                      ex- BMW Z3 M Coupe - Cosmosschwartz Metallic

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