Increasing amounts of "fuzz" may well be an indication of early crownwheel bearing failure. I have done a number of preemptive FD rebuilds where the owner noted increasing magnetic fuzz and on inspection of the bearing, wear of the races was present. Note well that some of these bearings did not have spalling/pitting, but rather a wallowing out of the races. Some of my findings were written up here: http://www.bmwlt.com/forums/showthread.php?t=63109
Repeat oil analysis over time would give earlier warning than visual evaluation of fuzz in the lube, but both are an indication of the same thing. i.e. carbon steel parts shedding increasing amounts of metal indicating something wearing out, in this case most likely the crown wheel bearing.
Opening "the FD casing to see what shims exist" won't tell you anything unless you compare the existing shims to shim thickness calculated for a new bearing. That comparison would provide another data point regarding suspected cause of premature crownwheel bearing failure. that is, excess preload.
Outward signs like: "measureable wheel movement, no oil leakage, smooth rotation" have never been reported as indicators of early crownwheel bearing failure. As a matter of fact, even a worn bearing, removed from the final drive, and held in the hand will likely feel smooth on rotation. In my experience, the only way to really assess the status of a bearing is to cut it open for a direct view of the races. The wallowing out of the races that I have seen in early stages of bearing degradation are not obvious even on direct inspection unless other bearing races are available for comparison.
My suggestion is that since you have increasing fuzz, and increasing metal in lube as determined by oil analysis, and you already have a bearing in hand, is to rebuild the final drive with your new bearing using proper technique, carefully measuring for preload of that bearing.
(Once you have done that, comparing your calculated shim thickness to the original shim thickness will give you an indication as to whether the original setup was over-shimmed, but again that is the only reason to be interested in what the original shim thickness is.)
Originally Posted by K100Dennis
Recently I changed the FD oil and sent a sample off for analysis (Castrol 75W90). Results returned a reading of 69 ppm for Fe (Iron). When the megnetic plug was inspected at the time it had nothing except the "fuzz" on it, but when I left some oil in another clear plastic sample bottle, after standing for a week or so there appeared to be dark deposits suspended throughout the oil. So I took a very strong magnet and taped it to the side of the sample bottle, waited for approximately 2 hours to check and guess what ?...... the oil was crystal clear with all the fuzz over at the magnet. So the fuzz is not aluminium, it is iron based (magnetic) which means of course it must be from gear surfaces or bearings. I have just completed 10,000 Km's in the last month and dropped out the FD oil again. It came out darker than the previous sample which had done 3000 Km's. Sent off a sample to the lab, still awaiting the results, but I have repeated the magnet test and again the oil has cleared off but this time there is a larger amount of fuzz to the side of the bottle (same size bottle and sample volume). I believe I may have discovered a very early failing of the crown bearing and am now tempted to open the FD casing to see what shims exist. I have no measureable wheel movement, no oil leakage, smooth rotation and a new FAG 61917 c3 bearing in my tool box. Thoughts please gentlemen ?