Pulling the head of my 302 to replace stem seals. The head is sludgy and dirty so I'll give it a good cleaning. Thought I might as well give it hand valve lapping. Never done this before but looks like if I get the lapping tool and some lapping compound, it should be a simple procedure. What's the worse that could happen if I don't do it right. How can you tell if the seats and valves even need lapping.