Just saw one of those big name, high dollar ($1700) modified cylinder heads for a 250F. It had the improved 1 piece valves, bronze seats, springs, etc. The seats hung out into the ports on the back sides. So much for "porting". One of the exhaust valves had broke and the piston tried to shove it thru an intake, then thru the head itself. It ran a total of 5 laps at a local track. Only good for a conversation piece now. I guess I will stick with stock parts a little longer!