O K head gurus can someone tell me when hardened valve seats were introduced and how far can you grind them before losing the hardness

consider this a bump for you. IIRC, hardened exhaust seats started "about" 1974, in preparation for cats that became mandatory in '76. It is not hard to cut through the hardening... oversized valves and you are through it, into untreated metal. That said (at least on street duty), mopar unhardened seats lived better than Brand X.

Thanks GoFish that's more than I had

From a well-known Cyl head guy who worked at Chrysler engineering and Ted Spehar .....

For the stuff we do here ... WOT for 8-13 seconds .. or cruising down the street in your RoadToad and hitting the throttle for 10 seconds or so.......

.... Hardened seats are uNnecessary.

But for a tow vehicle, RV, road course car, off-shore boat ....you BETTER have 'em .. ....