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.
The engine build I recently covered in previous Mini Magazine issues developed into a bit of an ongoing 'project'. During a discourse with the editorial staff at Mini Mag, it was decided the build feature presented an ideal opportunity to demonstrate just exactly what such an engine build is capable of in days where it's generally believed you have to have an all-singing, all-dancing 1380cc engine to have an enjoyable road burner - leaving those with very limited budgets a little depressed.