In what year were hardened valve seats for use with unleaded gasoline first used on stock engines from the factory? Is there any way to tell other than from the date of manufacture of the head? I have an integral head on my 250 right now and I want to lose it. I found 3 heads in my stuff but I can't remember how or when I came by them. I expect I could I.D. them by the casting numbers but would still need the date of first use of hardened seats. Thanks in advance. Ray
