Why do engines need valve clearance?

Answer 1

because without it, your engine valves would hit the top of the piston. the clearance called for takes into account thermal expansion of the metal valve train components to assure that a gap still remains from the top of the piston to the valve

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thanks . . tafe gave me over 1500 pages of books, and theres nothing in there that tells me why . . yet they gave me the question

thanks again :Dy vlave

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Wats579 is correct about the thermal expansion of valve train components. However, thermal expansion is not enough to create piston/valve interference in an engine with a properly timed valve train. The purpose is to ensure that the valve is completely closed (when it is suppose to be closed). A valve that does not close completely not only makes for a less powerful engine, but during the power stroke (the time at which the fuel/air mixture is ignited and burns) burning air/fuel mix can pass between the valve and valve seat having a "blowtorch effect" actually cutting a groove in the valve and/or valve seat resulting in what is commonly known as a "burnt valve". This one of several causes of a burnt valve.

Fredrock