Stupid like a fox! Plastic has always been the material of choice for tensioners/guides...
With chains it takes less power for the chain to skim over than metal of any kind and doesnt impart metal shavings into the oil system as it wears. Additionally it creates less heat/friction as the chain rolls over it than any metal could, is VERY gentle on chains and it dampens and quiets noise that would make an engine sound like a diesel truck idling at every RPM.
Crack open any Audi twin-cam 1.8T, 2.8 30V, 2.7T and V8 engines and you'll find plastic tensioner slides. Same with Porsche, Mercedes, BMW, Nissan, Toyota, Honda etc etc.
If one breaks its the most innocuous material to have floating around inside the engine as its softer than any/all other metal it comes into contact with, ie; it just crumbles or compresses on contact with any metal part. My old SCCA B-Sedan 1969 510 National car used plastic guides/tensioners with its timing chain (not belt) and never had any problems. Nor did/do the millions of other cars with them on the streets then or now. Sure there were instances of them failing/crumbling but it was VERY rare and no damage occured when it happened. The engine internals just ground the plastic into fine powder where it became suspended in oil and trapped in the oil strainer pick-up or the oil filter. When it comes to material choices its hard to imagine any material more desirable than plastic if choosing something to let go in an engine.
And the plastics used today are MUCH more technologically advanced than 30-40 years ago. Plastic is far and away the best/only material for the job PERIOD!
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