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They had their day in the sun back before valve spring materials were as refined as they are today. Now they represent added complexity and unnecessary weight. They actually limit RPM's compared to conventional valve springs. The Ducs actaully still use a spring, albeit a lighter one, to fully close the valve. There is some lash that just cannot be removed, that is required to account for varying operating temps and differing CTE's of the various materials in the head and valvetrain.
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Jeff
'72 911T 3.0 MFI
'93 Ducati 900 Super Sport
"God invented whiskey so the Irish wouldn't rule the world"
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