Some years ago, Brother Lon had Carl Holbrook do up a set of TP heads for the engine going back in his '67 Mustang. Because he had them, Lon gave Carl a brand new set of NOS valves. We put the engine back together and got it running and set off to go to Carl's place for a look-over. Lost a valve head during the trip---all of four or five miles. Carl said that it was a known fact that a batch of valves had got into the supply chain that had faulty welds between the stem and head. The damage was significant.
KS