.... What's the main reason for adjusting on a warm engine?
...but then they expanded an extra few thousands the gap would be smaller by that much... meaning more pressure and heat trying to squeeze through each time it cycles and.... oops the valve burnt.
Not exactly. The main cause of burnt valves as it relates to "tight" valve adjustment is that once the gap (your feeler gauge setting) disappears, the valve is no longer seating completely, or seating for as long a period during the cycle. And since the valve sheds most of its heat during this period while it is seated, it begins to overheat and eventually deteriorate and burn, or break, away.
