Heat is the real enemy of any valve. Materials have changed, aluminum will need an insert. Iron? not so much. When deciding on inserts, consider the gain (Longer life) and the risk (Insert fail). When the materials for valve faces went to a hardened material the need for an insert was limited to softer casting material like aluminum. In an engine with modest valve spring pressure, and good castings, the cooling of the valve is what makes it last. Intakes are cooled rather well by the incoming air fuel mix. Exhaust valves, only through contact with the head and guide. Adding an insert at this point won't help cool it at all. The only good reason to install an insert would be to repair an already badly eroded valve pocket. The last land speed engine I was able to see up close had an iron head with the inserts removed and the biggest valve seat possible cut into the casting. And it was an air cooled engine..go figure. Welding of the valve face to the valve seat under extreme loads has been taken care of by changing the material at the valve face. That was the real reason that old valves were so fragile back in the day. Flat heads run very high valve face temps, the valve sits on an uncool ed port. We have good cooling in our castings, and fairly long valve guides. Our valves are in a pretty good place. So unless you need to repair a badly worn valve seat I don't think you need an insert. Seat pressure can make a difference, but temperature will be the real monster.
