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. These were an improved material with little springs in the neck to help create a better seal than the original
AEG327 seal. But for the best available see
LJQ101160 made in Dupont Viton material as used by Rover on the MPI engine.
Sold individually.