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Classic Mini Valve Stem seals
Valve stem seals: The o-ring style seals are used on the early small bore heads, most later heads use the umbrella style. As an engine wears, the clearance between the valve stem and guide increases; as this gets beyond tolerances the car will tend to smoke under deceleration (when you take your foot off the throttle). Good valve stem seals will allow for greater wear before the smoke becomes intolerable.