That diff is a piece of work! Hermetically sealed against rust, forever. What I don't understand is how could the diff not be leaking when the proof of the leak can be found all over the outside of it. Or perhaps he meant to say that it no longer leaks because the half-inch thick layer of goop has gotten to a point where no fluid can leak out anymore. I think it's safe to say that it has in fact leaked in the past since the proof is right there in front of you. I wonder if the diff oil has ever been changed. To change it however one has to be able to locate the drain and fill plugs without resorting to using an x-ray machine.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)