[eschaider]

The experience Patrickt is sharing with you is directly related to the differences in the coefficient of thermal expansion for the two metals. The coefficient of thermal expansion simply gives the amount of growth per degree of temperature increase that a particular metal will exhibit.

The increase and then subsequent decrease in valve lash is attributable to the differences in the thermal conductivity of the two metals. Aluminum has a higher thermal conductivity than cast iron which means it will expand more and initially faster than cast iron. The lower thermal conductivity and lower coefficient of thermal expansion for the cast iron means the cast iron will expand less (but it will still expand) and it will take longer to see this expansion because it's thermal conductivity is less than that of aluminum.

Eventually when both metals have had sufficient soak time to normalize their temperatures and realize their maximum expansion for the heat energy absorbed, the cam and valvetrain will have quieted down again. The reduced lash that has quieted the valve train is directly attributable to the cast iron block's expansion after it's operating temperature has normalized with that of the cylinder head.


Ed