Engine respond to the actual movement of the valves. Since the valve cannot move until all the running clearance (valve lash) has been taken up, the amount of valve lash you use affects the engine’s performance.
The alloys of certain engine parts can be affected by thermal expansion in different ways, therefore the amount of correction factor to the lash setting depends on whether the cylinder heads and block is made out of cast iron or aluminum.
You can take the “hot” setting given to you in your cam manufacturer’s catalog or cam specification card and alter it by the following amount to get a “cold” lash setting:
• Iron Block Iron Heads – Add .002?
• Iron Block Aluminum Heads – Subtract .006?
• Aluminum Block Aluminum Heads – Subtract .012?
Remember this correction adjustment is approximate and is only meant to get you close for the initial start up of the engine. After the engine is warmed up to its proper operating temperature range, you must go back and reset all the valves to the proper “hot” valve lash settings.
Source: Crane Cams 
