Inside a newly-acquired engine

[Bob Culver]

Leaded fuels produced deposits which tended to prop valves open and cause failure. Narrow seats were recommended to break up the deposits. From my experience with other makes, weak mixtures and heavy valve springs contribute to recession, with or without lead. For years I ran Javelin which pocketd severely even on lead. (The horizontal engine and alloy expansion leads to heavy valve closing) I have run a 1600cc ohv Hillman for decades. Used to use an additive but too expensive. With a lot of main road running around 60 mph I get very slow recession but of just one valve suggesting mixture a major factor.
Considering many Sevens had valve grinds evry 5,000 or so through much of life, and persons used the lapping to clean up the valve, the degree of recession is often remarkably little.