[QUOTE=c4racer;778028]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.... [QUOTE]

I believe you're right on the money with that. As much as I hate to admit it (preferring to rant on at BMW) engineering sense prevails here. I did wonder if it was something I had done wrong, or the previous owner(s), ie maintenance, for the guides to 'prematurely' fail. I wondered if it might be the spring-loaded chain tensioner that may have contributed in some way.

I mean there was always a slight knocking sound ('marbles in a can' as its been termed on several forums including this one) at start up from cold. Perhaps it had gotten worse than I originally noticed due to the slackness in the tensioner which caused the chain to slap about on the plastic more than it should have. But because I've had the car for 2 years now I suppose I didnt notice the gradual degradation and increased knocking, and length of time of knocking on start up increasing to about 3-4 seconds. Is this theory of the failed/failing spring-loaded chain tensioner being a contributor to the premature failing of the guides something that anyone else shares?