I have a 235 motor that I built in 85 sitting in the back of my garage. I beat on that motor unmercifully for many many years. Unleaded gas, no hardened seats. I have had that head off and on a couple of times over the years and not one hint of the valves pounding themselves into the seat.
On my 261, same deal, no hardened seats and that motor will rev to 6000 rpm. No issues. I have always been of the opinion that unless you are going to put 100,000 highway miles on your motor, probably don’t reallly need them.
Now my biggest reason for not using them, when unleaded became the only fuel available I rebuilt a couple of heads with hardened seats. Right up to the point that I discovered accidentally that one of my heads had core shifted really really bad. Did not affect the usefulness of the head at all. But when we went to machine the head for hardened seats, we unexpectedly hit water. Turned a perfectly good 848 head into an instant boat anchor. That was my last, has to be 25 years or better.
