Hardened valve seats - do they really matter?
Seat recession happens because above a critical temperature, the exhaust valve, when it closes, micro-welds to the unhardened cast iron seat if the valve/seat junction is not buffered by residue from the likes of a lead, manganese, or phosphorus compound. The next time the valve opens, the micro-welds are broken; they pull apart like taffy, leaving a rough surface. The valve keeps opening and closing, and—remember, valves rotate—that rough surface works as an abrasive, grinding up (grinding down) the seat. See page 2 of the 1964 Chrysler paper attached to this post, and also see here (and the subsequent page), and here. And here's confirmation from Chrysler's many-decade chief engine engineer, Bill Weertman, on page 164 of his book, "Chrysler Engines 1922-1998":
So, blah-blah-blah. How much does this really matter, practically? On a stockish Slant-6, not a whole lot; the exhaust valves are small and well cooled, so in ordinary service they seldom reach the critical temperature required for the badness to happen. Seat recession just isn't high on the list of failure modes. And that generalizes to a lot of other engines, too; see the other PDF attached to this post.
Nobody reasonable would say "You can only get unleaded now, so you have to tear down your motor and put in hard seats or else you're DOOOOOOOOOOMED!". But adding hard exhaust valve seat inserts adds only a little to the cost of an ordinary head rebuild, and the rest of a Slant-6 is notoriously durable, so putting in hard seats can reasonably be seen as cheap insurance protecting the investment of money-time-effort in the project, so from that standpoint it's wise to do.
Since we're on the topic, the Slant-6, like the rest of the engines in the stable, was built with stainless steel exhaust valves right from the start. The standard-duty engines used 21-2N alloy; heavy-duty truck 225s used 21-4N, which is a bit harder; 32 versus 28 as you can see by comparing the data sheets for 21-2N and 21-4N. The heaviest-duty Slant-6s, the 225-3 motors put in the biggest and heaviest-duty trucks and buses, had positive exhaust valve rotators, which extend valve life on leaded gasoline, sharply reduce valve life with unleaded gasoline and unhardened seats, and are probably not beneficial with unleaded gas and hard seats.
(And all that said: yes, good air filtration is important to engine life, so use real air filters—don't fall for the likes of K&N's fraudulent claims)
People don't realize this because it isn't so. The mechanism behind valve seat recession is well understood, and it's nothing to do with a lack of air filtration (which will do a job on the rings long before it has any effect on the valves).
Seat recession happens because above a critical temperature, the exhaust valve, when it closes, micro-welds to the unhardened cast iron seat if the valve/seat junction is not buffered by residue from the likes of a lead, manganese, or phosphorus compound. The next time the valve opens, the micro-welds are broken; they pull apart like taffy, leaving a rough surface. The valve keeps opening and closing, and—remember, valves rotate—that rough surface works as an abrasive, grinding up (grinding down) the seat. See page 2 of the 1964 Chrysler paper attached to this post, and also see here (and the subsequent page), and here. And here's confirmation from Chrysler's many-decade chief engine engineer, Bill Weertman, on page 164 of his book, "Chrysler Engines 1922-1998":
So, blah-blah-blah. How much does this really matter, practically? On a stockish Slant-6, not a whole lot; the exhaust valves are small and well cooled, so in ordinary service they seldom reach the critical temperature required for the badness to happen. Seat recession just isn't high on the list of failure modes. And that generalizes to a lot of other engines, too; see the other PDF attached to this post.
Nobody reasonable would say "You can only get unleaded now, so you have to tear down your motor and put in hard seats or else you're DOOOOOOOOOOMED!". But adding hard exhaust valve seat inserts adds only a little to the cost of an ordinary head rebuild, and the rest of a Slant-6 is notoriously durable, so putting in hard seats can reasonably be seen as cheap insurance protecting the investment of money-time-effort in the project, so from that standpoint it's wise to do.
Since we're on the topic, the Slant-6, like the rest of the engines in the stable, was built with stainless steel exhaust valves right from the start. The standard-duty engines used 21-2N alloy; heavy-duty truck 225s used 21-4N, which is a bit harder; 32 versus 28 as you can see by comparing the data sheets for 21-2N and 21-4N. The heaviest-duty Slant-6s, the 225-3 motors put in the biggest and heaviest-duty trucks and buses, had positive exhaust valve rotators, which extend valve life on leaded gasoline, sharply reduce valve life with unleaded gasoline and unhardened seats, and are probably not beneficial with unleaded gas and hard seats.
(And all that said: yes, good air filtration is important to engine life, so use real air filters—don't fall for the likes of K&N's fraudulent claims)