Your engine’s valves are like the doors to a house. They open to let air and fuel in and close tight to keep the power inside. But sometimes, these "doors" get damaged. Understanding why is the first step to fixing them.
If you were building a door for a house, you would want the edge of the door to be perfectly flat, and the door frame to be perfectly flat. If they match exactly, they close tight.
In engine repair, you might think the same rule applies. If the valve seat is cut at...