I found this from an old flathead forum. this guy explains it better than I could. RUn what you want, hardened seats are not necessary but under the mose severe circumstances. I have about 10-15 sets of big block heads out here in my shop. only ONE set has ONE velve that has valve seat recession and that is on the intake. Valve seat recession is not caused by soft seats. It's caused from running a dirty air filter. It's funny, I argued FOR hardened valve seats in another thread around here somewhere because the circumstances were different, and everybody disagreed with me. They said hardened seats were NOT necessary, but now I'm wrong again. I can't be wrong both times. On anything but the most extreme service, don't waste your money. Here is the quote I found.
"Actually, tetraethyl lead was developed in the 20's by GM (Mr. Kettering, the designer of the famous '49 Oldsmobile 'Rocket' OHV engine, among others) and Standard Oil, only to allow the use of higher compression ratios, and NOT as a valve seat lubricant! Contrary to popular belief, the additive was actually very hard on cast iron exhaust valve seats, burning pits in the seats and valves, not to mention fouling s/plugs and creating deposits on the valve heads, etc. This was the reason for developing the hard alloys for use in valve seat inserts, as well as tougher alloys for exhaust valve heads, not the other way around. It was produced under the brand name 'Ethyl' by DuPont, added to gasoline and sold as leaded gasoline by the Ethyl Gasoline Corporation. In the 30's, it was finally sold as an additive to other gasoline companies. Since the re-introduction of unleaded fuel, much has been made of the disasterous results of unleaded fuel and 'soft' valve seats. Well, your cast iron seats would have a far shorter life if you were using leaded fuel! True, hard seat inserts will last longer than cast iron, but the question was it worth the expense of inserting a cast seat for unleaded fuel use in a normal use road engine? No, not really. Only if the cost is not a problem, or you're going to drive it hard, and put on a lot of miles. Otherwise, the cast seats will be fine."
Again, run what you want but remember this. the thinnest part of the cylinder head is in the valve seat area. That's because water needs to be close to the seats to promote good cooling. Cutting the seats out in this area for hardened seats weakens that area and can make it prone to crack. I'm not just blowin smoke up your skirt here. I've got a LOT of machine shop experience and SEEN it happen. Also, why take chances with something that can fall outta your engine and cause a major catastrophy? But hay, that's just my opinion. Lastly, it's a misconception to say that lead in fuel was used as a lubricant. That's just not true. That's one of those myths that's been repeated so much, people will kill you trying to argue you down about it. All it was ever add was to be able to run higher compressions as an anti knock agent. What happened as a by product with the lead was that it coated the valves and seats with an oxide coating that acted to coushion the valves and seats, not lubricate them. It was the big oil companies that said engines needed hard seats in the first place. I'm not ready to take stock in a damn thing they say. I prefer to see the evidence in the field. All of my evidence says hardened seats are not necessary. I've done work on literally hundreds of sets of heads without hard seats through the years. I've not seen many recessed valve seats. THAT'S what my opinion is based on. Not some BS hearsay.
