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As I was doing my valve servicing, one question came to mind: As valves undergo millions of cycles of reciprocating motion, do they actually rotate or is their position relative to the seal fixed? Or is any possible rotation undesirable and could cause a loss of seal?
While the spring keeps the valve tight, there isn't really any mechanism that locks its position.
The simple answer to your question is: They rotate all the time. This happens during the open/close cycling and not while closed. This is the reason why we have to lap the valves so the entire face of the valve will seal with the seat and not just one little part.
Yes, the valves do rotate under normal operation albeit slowly. Some valves have rotate mechanisms to ensure they rotate at a predictable rate.
http://www.cdxetextbook.com/engines/comp/vlves/valverotation.html
http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Valve%20Train.htm
The rotation action helps keep the valve and seat clear of debris to ensure good sealing.
Valves rotate slowly to scrape off the unburnt fuel particles settled on the valve seat. This is very much necessary when engine uses heavy fuel oil. Moreover rotating ensures that valves have uniform wear
