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Manu, lots of controversial discussions on whether our 71-73 heads need hardened valve seats or lead additive when running on unleaded. My take away from it all is, the earlier the heads and the more the car is driven, the more reason to be concerned.
The machine shop that redid my heads advised me if they were his, he would install hardened valve seats since my heads were made in 1970, he advised depending on where the heads were made and the year, they may not need hardened valve seats or lead additive on the 71-73's.
Since my 71 is just a weekend cruiser and not a daily driver I probably could have driven it for a long time on unleaded without the hardened seats before there was any wear but since the shop had them tore down I had them put in, I did it for peace of mind more than anything.
I'm sure others can add their experiences concerning using a lead additive or need for hardened valve seats on the various years.
Jim