.... What's the main reason for adjusting on a warm engine?
So they are already 'expanded' when you set the gap.
If you set the gap (without allowing for the expansion)they are cool 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.
That's why you see a cold and a hot setting aome times.
