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Bikes: 748S, HM1100S, V4SP, Was: DD-A #111
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Join Date: Feb 2002
Location: Back in Essex
Mood: Passion Killer
It's in the ACU rule book - I got pulled up on it in my first season in 2005 so it's been there for a while.
I believe it's because plastic ones can split or wear the threads, exposing the valve.
...or it might just be in there as an easy check to see if you read the rule book!!