Just for Information, my machinist with over 30 years experience, building engines told me that the originally Installed 1960's mopar seats are harder than any ford or chevy seats at the time. And, any new seats he would order for my engine today are no better than the original factory seats made back in the day.
Your machinist is probably just full of interesting fairytales like this.
don't waste your money unless the seats are bad.
It is foolhardy to build a pre-hard-seats head without installing hardened exhaust valve seats.
I remember reading a mopar Direct connection magazine article in 1988 where Chrysler tested all the mopar heads, and engines regarding seat wear with unleaded fuel. The results were, on Heads run during the 60's and early 70's, that hardened seats were not necessary unless you exceeded 85% of the RPM band, 85% of the time.
I think you might not be remembering quite as accurately as you'd like. To answer the OP's question: induction-hardened seats on the Slant-6 for '72, and the V8s for '73. See
here for it straight from Chrysler's mouth, and if you want (a lot) more detail, Chrysler put out an engineering paper on the subject, published through SAE, quantifying the exhaust valve recession which—despite your machinist's imagination—happened when unleaded gasoline was used in stock Chrysler engines without hardened seats.
Chrysler did put even harder seats in later,(1972) because of Increasing emissions standards.
No, they hardened the seats because it was necessary for adequate durability with unleaded gasoline. Hard seat or unhardened seat does not impact an engine's emissions characteristics.
In contrast,Ford didn't even put seats in their engines in those days
All engines have valve seats. That's the machined hole where the valve...seats. It's usually machined into the cast iron that makes the head (or block, on a flathead engine), and Chrysler stayed with that inexpensive and perfectly adequate method even once unleaded gasoline made a harder seat necessary -- they just began induction-hardening the exhaust seat.
lead was added to prevent pre- detonation in 1914 or so
1923.
and valve lubrication was a side benefit.
Lead has never provided a valve "lubrication" function. The only lubrication the valves need is where the stem goes through the guide. That's handled by engine oil, restricted to a small amount by the valve stem seals.
What lead did, as far as the exhaust valves were concerned, was provide a buffer/cushion effect. The exhaust valve gets superheated by the exhaust flowing through it. With an unhardened exhaust valve and seat, once the valve closes, it and the seat can micro-weld to each other. Then when the valve opens next, the metal pulls apart like taffy. This roughens the meeting surfaces, and they become quite abrasive. The pounding/turning of a valve with such "pulled" metal on it creates a nice grinding wheel effect on the seat, grinding it down so the valve sinks lower and lower. This is valve seat recession. In addition, the roughened surfaces no longer seal against each other properly, which eventually allows still-burning combustion gases to flow through the "closed" valve, causing a blowtorch effect on the poor valve and depriving it of any prayer of a chance to cool while it's on the seat. The blowtorch effect rapidly deteriorates the seal further, snowballing the seat recession. Lead halides act as a buffer (a physical "cushion" barrier) to prevent this happening.
Very little lead is required to prevent the localised welding and taffy-pull-apart effect that leads to the abrasive surface which, through incidental or positive rotation of the valve, eventually grinds down an unhardened seat. The majority of the lead was in the fuel as an octane booster, that's all. It was widely used because it was a very cheap and very effective octane booster. It also made a crapmess of the inside of an engine and (oh yeah) poisoned us and brain-damaged our kids. We're well rid of it. If that means having to spend an extra $100 in the course of a head rebuild to have hard seat rings installed, well, yup, I'm gonna do it.
