I have recently replaced the heads on my 302 F150. The reason I had to do that is because the #8 exhaust valve had a 1/4 inch hole in it. What causes an exhaust valve to burn out? Ignition timing? I have always used the stock timing to get the best fuel economy. I have always used 87 octane fuel. I'm concerned that I have fixed the affected component rather than fixed the problem.

 

could be a number of things but if you only lost one I'd think it got a piece of crud on the seat or something to cause it to leak and once they start to leak they burn out very fast. Could have been a faulty valve and a piece chipped out. Not likely to reoccur if the valve job was well done.