The pushrods and lifter etc. being steel will expand but very little. Aluminum heads however grow 5 times as much or more. So if you have aluminum heads it will cause the valve lash preload to be reduced.
My Dad's Brad Anderson Hemi Motor is all aluminum...From pan to hat and its has a 14.71 blower on alcohol. The blower belt when the engine is cold is slack and loose. After warm up of the engine the belt is tight. The engine grows between 1/4-1/2" in height after its warmed up. Its amazing. Also makes 3,000 horsepower without Nitro.