Yeah, and when you use aluminum rods in a race engine, you have to allow more deck clearance for that as well. But the valve lash has more to do with it than just thermal expansion. I posted on the HAMB where you asked this also and told you where to have it explained better for you, so I wont repeat it here.....
A hydraulic lifter can collapse some because it has a plunger inside of it, a solid lifter has no give and requires the lash to do that instead. Other than referencing numerous sites that explain the dynamics of how these components were designed and engineered, there really isn't going to be much that I can expand on that is going to help you "get it"....
