When hardened valve seats production??

nm9stheham said
When did the hardened valve seats go into production in the slant 6's? I assume around 1970/71 as no lead was becoming available then. But I am not 100% sure.

I'm asking so to know when to worry about ugrading the seats in a rebuild versus not. Tnx!

im thinking in 73 like the small blocks and big blocks..

i always put hardened seats in over the years. cheap insurance in my opinion...