I did not visit the referenced website, but I can tell you that Cummins has
used "composite," or, "plastic" valve covers on their class eight L10 and M11
heavy duty diesel engines for years. The valve covers are designed to
withstand the weight of a mechanic standing on them. These engines typically
accumulate 800,000 to 1,000,000 miles between overhauls. When I was at
Cummins, (1992 through 1994) these covers had already been in service for
several years and the warranty rate on them was very low, basically a
non-issue.

Jon Lauter
Rotary Power Marine Corporation
www.rotarymarine.com

I don't doubt their strength when new. But I have seen a lot of old nylon 
and it is not good. It becomes brittle and it warps. Especially when subjected 
to long term heat. As far as I know most plastics do not age well. Something 
to do with out gassing I suspect. I will pass on them for now. Maybe in
twenty years I will change my mind :-) 

Paul Lamar
 
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