I believe they use plastic chain guides to keep it quiet, since the chain is metal. Metal guides against a metal chain would be rather noisy. Also you want the guides to fail before the chain. So the use of plastic guides is pretty common and done on purpose. Now, as to what plastic they use - that's another matter....

Audi V8's have the same issue and some very small % of motors have this failure for seemingly no reason. It can't really be related to maint or use, just some low level failure mechanism.

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2003 4.6is - SOLD
2003 M5 Imola
2008 Mini Cooper S
2011 Mazda CX9