Many older engines had hardened valve seats as an original feature.
What is difficult to determine without research is whether those seats were
hard enough and burn-resistant enough to stand up to unleaded fuels. Some older
engines seem to fare quite well with unleaded fuels, while others tend to
suffer valve/seat erosion.
Again, to be sure, I'd have NEW hardened valve seats installed. If you have
integral valve seats (part of the head casting itself), I'd consider the option
of having the seat area induction hardened, even though hardened steel valve
seat inserts are probably still best.
Alan
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